(— 7 N) «4" _UBRARY Michigan State University This is to certify that the thesis entitled CHEMICAL AND CULTURAL MANAGEMENT STRATEGIES FOR RHIZOCTONIA SOLANI KUHN ON POTATOES IN MICHIGAN presented by DEVAN ROPPOSCH BERRY has been accepted towards fulfillment of the requirements for the Master of Science degree in Plant PathologL NW ll \Lwlk Major Professor’s Signature ”-l / 10 I 100‘] ' I Date MSU is an Affinnative Action/Equal Opportunity Employer PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 5I08 K:IProjIAcc&Pres/ClRCIDateDue.indd CHEMICAL AND CULTURAL MANAGEMENT STRATEGIES FOR RHIZOCTONIA SOLANI KUHN ON POTATOES IN MICHIGAN By Devan Ropposch Berry A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Plant Pathology 2009 ABSTRACT CHEMICAL AND CULTURAL MANAGEMENT STRATEGIES FOR RHIZOCTONIA SOLANI KUHN ON POTATOES IN MICHIGAN By Devan Ropposch Berry Rhizoctonia solani Kiilm causes stem and stolon canker as well as black scurf on potato tubers, which can reduce plant health as well as yield quality and quantity. Standard planting and fiingicide application decisions were examined, as modifications may impact disease management. Current recommendations are for planting into warm soil (around 15°C) and for both a seed and early-season flingicide treatment. Three field experiments were established to examine these crop management options for multiple cultivars: planting based upon soil temperature (2004-2005, 3 temperatures), timing of seed and early-season fungicide applications (2004-2005, 1 seed treatment, 3 early- season fungicides, 3 applications), and interaction of both planting time and fungicide application (2006, 2 timings, 1 seed treatment, 2 early-season fungicides, 2 applications). In 2004 (inoculated) and 2005 (non-inoculated) plots that were planted early (8°C at 15cm) had a significantly higher percentage of stems and stolons with cankers than plots that were planted later (either 14 or 20°C at 15cm). Also in 2004 and 2005, the use of the seed treatment, fludioxonil, regardless of additional treatments reduced the percentage of stem and stolon canker. However, the use of additional early-season fungicide treatments, regardless of chemical or application timing, was not found to be consistently effective. In 2006, a combined experiment found contradictory results for planting based upon soil temperature, but similar results for fungicide applications. DEDICATION I would like to dedicate this work to all of my family and friends that have continuously provided support for this, and indeed all of my endeavors. Thank you especially to my wife Karen and son Joe, my parents Jim and Karolyn Berry, my second parents Wes and Mary Summers, my mentor Dr. Willie Kirk, and my friends Rob Schafer and Donny Nguyen. iii ACKNOWLEDGEMENTS I would like to acknowledge the extremely important help that I received on these studies: Rob Schafer, plot setup, management and harvest; Dr. Willie Kirk, expertise, guidance, and editing; Pavani Tumbalam, inoculurn production; undergraduates Justin DeMaagd, Josh Nichols, and Ryan Nowlin for assistance with data collection; Ronald Gnagy (Muck Soils Research F arm manager), CIifi‘ Zehr (Plant Pathology Farm manager), and Bill Kitchen (Kitchen Farms), for plot preparation and maintenance; and Michigan Potato Industry Commission (MPIC) for funding this research. iv TABLE OF CONTENTS LIST OF TABLES ............................................................................................................ vii LIST OF FIGURES .......................................................................................................... xii CHAPTER 1 INTRODUCTION ............................................................................................................... 1 Host: Potato (Solanum tuberosum) ......................................................................... 1 Importance .................................................................................................. 1 Basic potato plant anatomy .......................................................................... 1 Pathogen problems ....................................................................................... 2 Pathogen: Rhizoctonia solani Kiihn ........................................................................ 2 Introduction .................................................................................................. 2 Importance ................................................................................................... 3 Separation by Anastomosis Groups ............................................................. 4 Host range .................................................................................................... 5 Disease cycle and infection process ............................................................. 6 Environmental factors .................................................................................. 9 Management strategies ........................................................................................... 12 Biological control agents ........................................................................... 12 Fungicides .................................................................................................. 12 Cultural practices ....................................................................................... 14 Research goals ....................................................................................................... 15 CHAPTER 2 EFFECT OF FUNGICIDE AND APPLICATION TIMING ON THE INCIDENCE AND SEVERITY OF RHIZOCTONIA CAN KER AND BLACK SCURF ON POTATOES ..18 Introduction ............................................................................................................ 1 8 Materials and methods ........................................................................................... 19 Data Analyses ........................................................................................................ 23 Results .................................................................................................................... 24 Year ............................................................................................................ 24 Seed and early season treatments ............................................................... 25 Results of similar studies ....................................................................................... 36 Discussion and conclusions ................................................................................... 37 CHAPTER 3 EFFECT OF SOIL TEMPERATURE ON VARIETY SUSCEPTIBILITY TO THE INCIDENCE AND SEVERITY OF RHIZOCTONIA CANKER AND BLACK SCURF ON POTATOES ................................................................................................................ 42 Introduction ............................................................................................................ 42 Materials and methods ........................................................................................... 43 Results .................................................................................................................... 48 Discussion and conclusions ................................................................................... 58 CHAPTER 4 EFFECT OF FUNGICIDE APPLICATION TIMING AND SOIL TEMPERATURE AT PLANTING ON VARIETY SUSCEPTIBILITY TO THE INCIDENCE AND SEVERITY OF RHIZOCTONIA CANKER AND BLACK SCURF ON POTATOES ..61 Introduction ............................................................................................................ 61 Materials and methods ........................................................................................... 62 Results .................................................................................................................... 65 Discussion and conclusions ................................................................................... 76 Future investigations .............................................................................................. 78 BIBLIOGRAPHY .............................................................................................................. 80 vi LIST OF TABLES Table 2.1 Treatment matrices for seed and early-season fiingicides and application timings for field experiments that occurred during the 2004 and 2005 growing seasons ............................ 20 Table 2.2 Main effects analyses (probability of difference between years: p=0.05) of growing season for cultivars 'Russet Norkotah' and 'Superior' on agronomic factors and disease symptoms caused by Rhizoctom'a solani in field trials conducted at two sites in MI between 2004 and 2005. .................................................................................................... 25 Table 2.3 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted near Bath, MI during the 2004 growing season on cultivar ‘F L1 879’. ..................................... 28 Table 2.4 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted near Bath, MI during the 2004 growing season on cultivar 'Russet Norkotah'. ........................ 29 Table 2.5 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatments and application timing agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted in Antrim Co., MI during the 2004 growing season on cultivar 'Superior'. ....................................... 30 Table 2.6 Effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted in Antrim Co., MI during the 2004 growing season on cultivar 'Superior' ............................ 31 Table 2.7 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatment and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted near Bath, MI during the 2005 growing season on cultivar 'Russet Norkotah'. .................................. 32 vii Table 2.8 Effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctom‘a solani in field trials conducted near Bath, MI during the 2005 growing season on cultivar 'Russet Norkotah' ......................... 33 Table 2.9 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted in Antrim Co., MI during the 2005 growing season on cultivar 'Superior'. ........................... 34 Table 2.10 Effect of seed and early—season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted in Antrim Co., MI during the 2005 growing season on cultivar 'Superior' ............................ 35 Table 3.1 Cultivars planted during the 2004 and 2005 experiments evaluating the effect of planting time, based upon soil temperature, on disease incidence and severity caused by Rhizoctonia solani; Plant Pathology Farm, Michigan State University, East Lansing, M147 Table 3.2 Important dates and relative number of days after planting (DAP) for each planting time of the 2004 and 2005 experiments evaluating the effect of planting time, based upon soil temperature, on disease incidence and severity caused by Rhizoctonia solani. Plant Pathology farm, Michigan State University, East Lansing, MI. ........................................ 47 Table 3.3 Main effects analyses (probability of difference between planting times: p=0.05) of the effect of planting time and cultivar planted on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2004 ................................................................................ 50 Table 3.4 Main effects analyses (probability of difference between planting times: [I p=0.05) of the effect of planting time and cultivar planted on agronomic factors and disease symptoms caused by Rhizoctom'a solani in field trials at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2005. ............................................................................... 51 Table 3.5 Effect of cultivar planted on agronomic factors in field studies at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2004. ............................................ 52 viii Table 3.6 Effect of cultivar planted on agronomic factors and disease symptoms caused by Rhizoctonia solani in field studies at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2005. ............................................................................... 53 Table 3.7 Main effects analyses (probability of difference: p=0.05) of the interaction between planting time and cultivar planted on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2004 and 2005 ............................................................................................................................................ 54 Table 3.8 Interaction effect between planting time and cultivar on select agronomic factors and disease symptoms caused by Rhizoctonia solani during field studies at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2004 ............................ 55 Table 3.9 Interaction effect between planting time and cultivar on agronomic factors during field studies at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2005 ............................................................................................................................................ 56 Table 3.10 Interaction effect between planting time and cultivar on disease symptoms caused by Rhizoctonia solani during field studies at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2005 ............................................................................................................................................ 57 Table 4.1 Experimental layout (for one of four potato cultivars: FL1833, FL1879, Russet Norkotah, and Superior) for the study of the effects of soil temperature at planting, the use of a seed treatment, additional fungicide and the application timing of the additional fungicide on agronomic factors and disease symptoms, caused by Rhizoctonia solani, in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. ............... 63 Table 4.2 Main effects analyses (probability of difference: p=0.05) of the effect of soil temperature at planting on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 .................................................................................................................................... 66 Table 4.3 Main effects analyses (probability of difference: p=0.05) of the effect of cultivar planted on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. ............... 67 ix Table 4.4 Main effects analyses (probability of difference: p=0.05) of the effect of the use of the seed treatment Maxim (fludioxonil) on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. ............................................................................................ 68 Table 4.5 Main effects analyses (probability of difference: p=0.05) of the effect of use of additional fungicide (Amistar: azoxystrobin, or Moncut: pyraclostrobin) on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 ......................................................... 69 Table 4.6 Main effects analyses (probability of difference: p=0.05) of the effect of application timing of additional fungicide on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 ............................................................................................ 70 Table 4.7 Main effects analyses (probability of difference: p=0.05) of the interactive effect of soil temperature at planting, seed treatment, additional fungicide and application time, on the cultivar planted for agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 .......................................................................................................................... 71 Table 4.8 Effect of the interaction of soil temperature at planting, seed treatment, additional fungicide and application time, on the cultivar FL1833 for agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 .................................................................... 72 Table 4.9 Effect of the interaction of soil temperature at planting, seed treatment, additional fungicide and application time, on the cultivar FL1879 for agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 .................................................................... 73 Table 4.10 Effect of the interaction of soil temperature at planting, seed treatment, additional fungicide and application time, on the cultivar Russet Norkotah for agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 .......................................... 74 Table 4.11 Effect of the interaction of soil temperature at planting, seed treatment, additional fungicide and application time, on the cultivar Superior for agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006 .................................................................... 75 xi LIST OF FIGURES Figure 3.1 Historical soil temperatures (10cm) around planting time for potatoes East Lansing, MI, 2000 to 2003 ........................................................................................ 46 Figure 4.1 Five day average of soil temperature (10cm) and precipitation total (mm) Muck Soils Research Farm, Michigan State University; Bath, MI, 2006 ......................... 79 xii CHAPTER 1 INTRODUCTION Host: Potato (Solanum tuberosum) Importance The potato (Solanum tuberosum) is the fourth most important food crop (behind corn, soybeans and wheat) in the United States, in terms of the value of production, with the 2007 growing season estimated at over 5.2 billion dollars (USDA, 2008). In addition, the potato ranks fifth (behind sugarcane, maize, wheat, rice) worldwide for the amount harvested, which was estimated at over 20 million tonnes (F AOSTAT, 2008b). Worldwide, on average, people consume 32 kilograms of potatoes per year, and in the United States people consume 63 kilograms per year (F AOSTAT, 2008a). Potatoes have been cultivated for over 5000 years, dating back to pre-Incan times in South America (Thurston, 1994). As with other important crops, there has been a great deal of research for crop protection due to pest problems associated with growing monocultures over large areas. Basic potato plant anatomy The potato plant is an annual, herbaceous, dicotyledonous plant that is commercially grown from vegetative seed, either whole or cut tubers. Potatoes can also be grown from botanical seed, although this method of propagation is commercially limited and used mainly by breeding programs. Meristematic tissue on the potato seed tuber gives rise to stems, stolons and roots. The stems produce pinnately compound leaves in a phyllotactic spiral pattern (Cutter, 1992). A stolon, which is a below-ground stem, produces a tuber at its apex (Peterson, 1985). A potato tuber is a modified stem possessing leaves and axillary buds that are much reduced, shortened internodes, and radially expanded stem axis (Cutter, 1992). Tubers act as an energy sink and store carbohydrates in the form of starch granules, in varying concentrations, in amyloplasts of the parenchyma cells located throughout the tuber (Peterson, 1985). The tubers of the various cultivars can range widely on a number of attributes including tuber type, shape, taste, skin and flesh color, internal qualities, specific gravity, usage and disease or insect resistance. In addition, the plants can also vary significantly in appearance. Pathogen problems There are nearly 60 important diseases affecting potatoes caused by fungi, fungal- like organisms, bacteria, viruses, nematodes, plasmids, and insect toxins (Stevenson, 2001). In order for any pathogen to cause severe problems in a cropping system, three things must be present: the host, pathogen, and appropriate environmental conditions. Different pathogens affect the potato crop at different times, but all can lead to lowered yield quality and/or quantity. For most diseases, an integrated pest management strategy is used, which incorporates cultural, chemical, and biological methods. Pathogen: Rhizoctonia solani Ki’lhn Introduction The anarnorph of the basidiomycete Thanatephorus cucumeris (Frank) Donk is Rhizoctonia solani Kiihn, a ubiquitous soilbome plant pathogen, saprophyte, and is occasionally found in mycorrhizal association with orchids (Warcup, 1985). As a pathogen, R. solani has been reported on over 500 different genera of plants (Farr, 1989). Some of the more important anamorphic features of R. solani are: the production of sclerotia without differentiated rind and medulla, the lack of conidia, hyphal branching at right angles prior to septa, hyphal constrictions at the dolipore septa, multiple nuclei, no clamp connections, and the production of monilioid cells (Parmeter and Whitney, 1970). Monilioid cells and hyphae, that give rise to monilioid cells, form the loosely woven sclerotia (Tu and Kirnbrough, 1975). The species R. solani is part of the Rhizoctonia complex, which is a large grouping based upon vegetative (anamorphic) features, and has teleomorphs in multiple genera: Helicobasidium, Thanatephorus, Ceratobasidium, Waitea, Tulasnella and Sebacina (Ogoshi, 1996). The taxonomy of the teleomorphs is based primarily upon the basidiospores produced (Stalpers and Andersen, 1996). Importance There are a number of important disease signs and symptoms resulting from an infection by R. solani on potatoes. Cankers on stem and stolons (starting as sprouts), can significantly delay emergence (Frank and Leach, 1980), reduce plant stand and yields (Banville, 1989), as well as lead to increased production of misshapen, variable tuber sizes (Otrysko and Banville, 1992), and (inedible) aerial tubers. Further, sclerotia produced on the skin of daughter tubers reduce the value of fresh market tubers due to the appearance, and value of seed tubers. It is recommended that seed tubers be free of visible sclerotia, but 2 — 5% coverage can be tolerated (Secor and Johnson, 2008). If seed has more than 5% coverage, it is recommended that the seed lot be rejected due to the increased potential for disease (Powelson and Rowe, 2008). Separation by Anastomosis Groups Rhizoctonia solani is separated into anastomosis groups (AG), which are based upon vegetative incompatibility (Anderson 1982, Ogoshi 1987, Carling 1996). There are thirteen groups, some of which have subgroups, known as vegetative compatible populations (VCP) (Carling 1996). The more-homogenous VCPs within an AG differ for pathogenic, biochemical, or genetic characteristics (Sneh et. al. 1991, Carling 1996, Campion 2003). When two isolates are grown on the same media, they will naturally grow in a radial fashion outward as they colonize the media. The area where the two initially come into contact can be studied under a light microscope for one of four reactions to take place. The reaction that follows dictates if the isolates are closely related, and of the same AG and VCP (most likely the same isolate); related and part of the same AG, but not the same VCP; distantly related, possibly either part of the same AG or different AGs; or not related, due to different AGs (Carling, 1996). When isolates are of the same AG and VCP, the hyphal tips will be attracted to each other. This attraction can be detected from as far as 100 pm, as the tips will turn towards each other and meet tip to tip (Ogoshi, 1987). Once the tips come into contact with each other, growth stops, branchlike appendages are formed, the cell walls of both tips dissolve, and the protoplasts join (Ogoshi, 1987). In addition, the anastomosis point is frequently not obvious, the diameter of the anastomosis point is equal or nearly equal to the rest of the hyphae, and the anastomosing and adjacent cells may occasionally die (Carling, 1996). This interaction is now referred to as the C3 reaction (Carling, 1988). If the two isolates are of the same AG but not the same VCP, the tips are attracted to each other, but this time the anastomosis location is obvious (Carling, 1996). The diameter of this anastomosis point is smaller than the surrounding hyphae, and the reaction always results in the death of the fused and adjacent cells (Carling, 1996). This reaction is called the C2 reaction or killing reaction (Carling 1988). The C1 reaction occurs when two isolates, whose hyphae are distantly related, come into contact and have an apparent connection of the cell walls but no penetration of the membrane or membrane to membrane contact, and occasionally one or both of the anastomosing and adjacent cells die (Carling, 1996). Finally, with the C0 reaction, when the isolates are not related, there is simply no interaction between hyphae as they grow towards and past one another (Carling, 1996). Host range In addition to potatoes, from which it was first described by Kuhn in 1858, the diseases caused by R. solani on many other important plants have been extensively reported (Parameter, 1970) and recently reviewed by Sheh et al. (1996). Most major crops are potential hosts, including rice, wheat, sugar beet, cotton, peanut, many vegetables, canola, turf grass, flower bulbs, forage and oil seed legumes, omamentals, and forest trees (Farr et al., 1989; Sheh et al., 1996). Some of the anastomosis groups (AGs) are rather host specific, while others can be found on a wide host range (Anderson, 1982; Gonzalez Garcia et al., 2006). Rhizoctonia solani AG 3 is considered host specific to potatoes and the causal agent of Rhizoctonia canker on stems, stolons and roots, as well as black scurf on the progeny tubers of potato plants (Bandy, 1988; Carling, 1989; Hill, 1989; Ogoshi, 1996; Campion 2003). In addition to AG 3, groups 1, 2, 4, 5, 8, and 9 have also been found on potatoes as sclerotia, isolated from stem and stolon cankers or from the soil around the plants (Bains and Bisht, 1995; Balali et al., 1995; Virgen—Calleros et al., 2000; Campion, 2003; Woodhall et al., 2007). Disease cycle and infection process The disease cycle of R. solani on potatoes starts in the spring with one of three scenarios: tubers are planted that have black scurf (sclerotia) on their skin; black scurf— free tubers are planted into soil containing R. solani, or both seed-borne and soil-borne inoculum are present. Soil-borne inoculum is found as sclerotia or mycelium that can be on nearby debris from crop residue or free in the soil matrix. The presence of R. solani inoculum can reduce both yield quality and quantity (Carling, 1989; Platt, 1989; Nolte, 2003; Tsror, 2005). Both sources of inoculum are important, but there have been conflicting reports as to which is more important. Some studies have put emphasis on seed-bome inoculum sources (Banville, 1989; Carling, 1989), while others have suggested that soil-bome inoculum is more important (James and McKenzie, 1972). Tsror (2005) recently reported that both sources of inoculum are equally important, and that together cause a significantly higher level of disease severity of black scurf, than either inoculum source separately. Similarly, Frank and Leach (1980) reported that both inoculum sources are important to the overall impact of the disease, but that tuber borne inoculum may be more important for early stem pruning, while soil-borne inoculum may be more important for later season stolon pruning and production of sclerotia on tubers. After germination of sclerotia, the mycelia grow towards plant tissues in response to exudates (Kerr, 1956; Jeger, 1996; Keijer, 1996). Once in contact with plant tissue, the mycelia grow across the surface. It is at this point that the infection process begins. The mycelia can attempt to penetrate the host in a number of ways, depending upon the location on the plant: through natural wounds or openings (stomata or lenticels); by creating an infection cushion [and eventually penetration peg(s), or appressoria (Dodman and F lentje, 1970)]. On contact with intact, healthy plant surface, the mycelium continues to grow along the surface, usually along the junction lines of the underlying epidermal cells, becomes flattened and adheres to the surface (Dodman and F lentje, 1970; Hofinan and Jongbloed, 1988; Keijer, 1996). Hyphae branch in a T-shaped pattern and can either form appressoria, or with repetitive branching form infection cushions (Dodman and F lentje, 1970, Keijer, 1996). One or more infection pegs push into the host tissue from the appressoria or infection cushions via hydrostatic pressure (Keijer, 1996) and possible enzyme activity (Hofrnan and Jongbloed, 1988). In contrast, when the hyphae directly penetrates through a stomatal or lenticel opening, the diameter of the hyphae reduces in size to facilitate entry (Dodman and Flentje, 1970). Following penetration, enzymes are released that lead to cell wall and cytoplasm degradation, and cell death (Bateman, 1970; Hofrnan and Jongbloed, 1988; Weinhold and Sinclair, 1996). Hofinan and Jongbloed (1988) found that AG 3, on potato stems, caused cell death beyond the depth of colonization of the mycelia, up to 12 cells deep. As a result of the infections, light brown to black lesions form which can girdle and kill the stem, stolon, or root (Banville et al., 1996). Organs that are pruned due to severe lesions may produce secondary growth, and the cycle of growth and apical pruning has been noted to occur up to 11 times (Baker, 1970). Infection occurs throughout the growing season (Hofrnan and Jongbloed, 198 8), but plant shoots are most susceptible in the spring, especially between planting and emergence (Baker, 1970). Van Emden (1965) observed that sprouts that were exposed to sunlight became resistant to infection. The phenomenon of early susceptibility has been demonstrated to be caused by plant exudates, produced by young shoots and roots, which decline as the shoots age and the waxy cuticle layer builds up (Flentje et al., 1963). Older shoots can again become susceptible if the cuticle is damaged, as described in the experiments of Flentje et a1. (1963). Once the potato plants reach maturity and begin to senesce, either naturally or after herbicide application, there is an increase in the production of sclerotia on progeny tubers (Van Emden, 1965; Dijst, 1990; Banville, 1996). The production of sclerotia increases with time while the tubers remain in the soil, up to three weeks afier vine desiccation (Gudmestad et al., 1979). Allington (1935) demonstrated that changes in fungal nutrition, specifically a decrease in carbohydrates, played a role in the formation of sclerotia. Later studies by Dijst (1986, 1988, 1990) have pointed to exudates of the tuber and roots as a cause of increase in sclerotia formation at the time of harvest. Potato plants exude both inhibitory and stimulatory products throughout the growing season, and after vine-kill the production of inhibitory substances decreases (Dij st, 1990). The production of stimulatory exudates continues because the roots continue to absorb water and nutrients from the soil. Dij st (1986) demonstrated that by removing the vines, or cutting the roots and stolons from the tubers, the formation of sclerotia is greatly reduced. Rhizoctonia solani over-winters primarily as sclerotia, which appear as small, dark growths that form on the skin of the tuber, or on crop residue left in the soil. The sclerotia are commonly referred to as “black scurf’ or “the dirt that won’t wash off’ when found on potatoes. Although the sclerotia are superficial, it is unwanted for both table stock due to the unpleasant appearance, and seed potatoes since it would not be certified as disease-free (Banville et al., 1996). Sclerotia are of little importance when found on potatoes that are bound for chipping or French fiies, since the skin is pealed, thus removing the blemishes (Banville et al., 1996). Environmental Factors The stems and stolons of a potato plant, especially prior to emergence, are susceptible to infection by R. solani (Baker, 1970; Banville et al., 1996). In addition, the progeny tubers, following plant senescence are susceptible to colonization (Spencer and Fox, 1979; Dij st, 1990; Jeger, 1996). A number of environmental factors have been linked to the incidence and severity of Rhizoctonia stem and stolon canker and black scurf, with perhaps the most important being soil temperature. Soil moisture (Frank and Leach, 1980; Hide et al., 1985), pH, and soil type have also been linked to the disease, but indirectly in studies of fungicide efficacy (Hans et al., 1981; Rushdi and Jeffers, 1956) Early studies by Richards (1921) demonstrated that R. solani attacks potato stems over a wide range of temperatures, between 9 and 27°C, with 18°C being the optimal temperature for stem canker incidence and severity. Further, he found that the destruction of the growing points of sprouts was highest at 12°C. Bolkan et al. (1974) furthered the study by Richards (1921) by testing different seed-bome inoculum levels over the range of temperatures. Although they did not formally quantify the macroscopic sclerotia, they did group the seed pieces as clean (no sclerotia), low, moderate, and high levels, (based upon a representative picture estimated as approximately: clean = 0 sclerotia; low = 0 - 5% surface coverage; moderate = 5 — 10% surface coverage; and high = >10% surface coverage). They found that if inoculum levels were low, stem canker severity decreased as temperature increased, from 15 to 24°C. However, when inoculum levels were higher, they found that temperature increases did not control stem canker severity. One in vitro study of radial growth of sclerotia from potatoes (LeClerg, 1942) showed that the optimal temperature was 25°C. This finding was supported by a study in which AG 3 was isolated (from sugar beet), and the optimal temperature for growth was found to also be 25°C (Windels et al., 1997). Carling and Leiner (1990) demonstrated that isolates of AG 3 infected potato sprouts at a temperature range from 10 to 21 .1°C, and was most destructive at 10°C. However, other AGs (4, 5 and 8) tended to be more virulent to potato sprouts between 15.5 and 21 .1°C (Carling and Leiner, 1990). There is a range of temperatures at which R. solani can infect its various hosts, depending upon the AG. Yitbarek et al. (1988) found that while both AG 2-1 and 4 had optimal growth temperatures near 25°C (24 and 26°C, respectively), AG 2-1 was significantly more virulent on canola at lower air temperature schedules (7-8°C, night minimum-day maximum temperatures = 4-8°C night minimum-day maximum for soil temperature) than AG 4, regardless of inoculum level. Conversely, AG 4 was significantly more virulent on canola at higher air temperature schedules (26-35 °C, night minimum-day maximum; 25-36 °C for soil) regardless of inoculum level (Yitbarek et al., 1988). Chang et al.. (2004) found variation in chickpea susceptibility to the same AG 4 10 isolate was linked to soil temperature. In a gradient plate study, both cultivars tested were susceptible to root rot at all temperatures tested (from mean temperatures of 7.5 to 275°C) and susceptible to shoot infection above 10°C (Chang, 2004). Grosch and Kofoet (2003) found that the optimal growth temperature for AG l-IB (isolated from lettuce fields) was 25°C, while virtually no growth occurred at 35°C. Dorrance et al. (2003) found that AG 2-2 IIIB was able to cause hypocotyl lesions on soybeans at all temperatures tested (20 to 32°C), with significantly more lesions at 32°C than at 20°C. In addition, all AG 2-2 IIIB isolates collected grew at 35°C (Dorrance et al., 2003). In addition to R. solani, other pathogens, such as F usarium spp. causing potato seed piece decay, are also important around the time of planting. Like R. solani, F usarium seed piece rot is found wherever potatoes are grown. Some fungicide seed treatments, such as fludioxonil and flutolanil alone or in combination with mancozeb, are recommended for control of both R. solani and F usarium spp. (Wharton et al. 2007b, 2007c). Cut seed pieces are particularly vulnerable to decay, since the wound provides an entry point for the pathogen. Escande and Echandi (1988) found that cut tubers that were allowed to heal for five days, provided significant protection from infection of F usarium spp. and Erwim'a spp. or their combination, compared to cut tubers that were not allowed to heal at soil temperatures of 10 and 15°C. Temperatures between 15 and 28°C are considered optimal for growth (Arora et al., 2004). Although soil temperature was not taken into account, Nolte et al. (2003) found that whole seed had significantly less seed decay caused by F usarium spp. compared to cut seed pieces, but not cut and fungicide- treated pieces, over the five year study. Further, Wharton et al. (2007a) found that cut 11 and inoculated seed pieces with a fungicide treatment up to 10 days prior to planting was effective against decay caused by F. sambucinum. Management Strategies There are three classifications for the control methods of Rhizoctonia solani on potatoes: biological, chemical, and cultural. Biological control agents Biological control agents are either organisms or the products of organisms that control the pathogen through antagonism, parasitism, or competition. The amendment of certain fungi into growing media, Trichoderma harzianum, Verticillium biguttatum, or T richoderma virens, or bacteria, Burkholderia cepacia or Bacillus subtilis, have had varying levels of effectiveness for controlling R. solani (Tronsmo, 1996; Brewer and Larkin 2005; Larkin, 2001, 2002, 2007 and 2008). Another control method is inoculation with a hypovirulent strain of R. solani (Bandy and Tavantzis, 1990). Hypovirulent strains exhibit certain traits, like reduced growth rates, production of sclerotia and virulence that can be transferred, via anastomosis, to more virulent strains. Further, the hypovirulent strains also may induce protection or directly compete with pathogenic strains, which could be especially effective with complete coverage of the seed tuber (Bandy and Tavantzis, 1990). Fungicides Many plant pathogens are at least in part managed with fungicides, which is a major component of integrated pest management. The first reported use of a chemical for the control of Rhizoctonia was by Winston (1913), in which soil was fumigated with a mixture of steam and formalin. Since then, a number of fungicides have been tested for 12 efficacy against R solani. Multiple mercury-based products have been tested (Maine Ag. Exp. St. Bull, 1937) and used for a number of years, but all are now banned in most countries (Rich, 1983). Today, there are a variety of chemical groups that can be used against R. solani including: aromatic hydrocarbons, carboxamides, benzamidazoles, dicarboximides, triazoles, morpholine, phenylurea, validamycin, phenylpyrroles, and strobilurins (Kataria and Gisi, 1996). Recent fungicide efficacy tests on potatoes have focused on strobilurins (azoxystrobin, pyraclostrobin), caboxamides (flutolanil), and phenylpyrroles (fludioxonil), (Stevenson et al., 2000, 2001, 2003, 2004; Kirk et al., 2001 , 2002, 2003, 2004; Bains et al., 2002; Inglis et al., 2002, 2003; Nolte et al., 2003; van den Boogert and Lutikholt, 2004). There are four possible (registered) application timings for these fungicides for effective protection of potato plants against R. solani: seed treatment; soil (in firrrow); foliar (around the time of emergence); and tuber pre-harvest treatment (via green crop lifting). Liquid or dust formulations of seed treatments can be applied on cut or whole seed pieces. Nolte et al. (2003) found that, over a five year period, cut and treated seed performed at least as well as whole seed that had not been treated with a fungicide. However, the study did not compare whole seed with a seed treatment compared to cut seed with the same treatment. Bains et al. (2002) examined fungicide efficacy, applied as a seed treatment, over a range of cultivars, and found that fludioxonil consistently provided control of R. solani black scurf and stem and stolon canker. In addition, he found that cultivar susceptibility varied, but none were resistant to either symptom. Efficacy of in-fiirrow and foliar applications (around emergence), have been tested by Kirk et al. (2002, 2004, 2007, 2008) for agronomic and disease parameters comparing l3 treatments at different application timings with the non-treated control and the industry standard of fludioxonil but few differences in efficacy were reported. Seed and soil-borne inoculum are important for stem and stolon canker, respectively (Frank and Leach, 1980). Seed treatments would therefore be expected to be most effective managing seed stem cankers, while later fungicide applications, in furrow and foliar, would be effective at managing stolon cankers. This separation is difficult, since environmental factors like soil temperature, depth of planting, soil type, and soil moisture content, play a role in persistence and efficacy of the fungicide as well as the growth of the plant and fungus. Green crop lifiing involves destroying the stems, lifting the progeny tubers so as to break up the roots and stolons, and placing the tubers back into the soil (Turkensteen et al., 1994). Just prior to the tubers being covered with soil, an application of a fungicide can be made as was tested by van den Boogert and Luttikholt (2004). However, this practice has only been experimented with in Europe, and no implementation of this practice has taken place in the United States (Kirk, personal communication). Cultural practices Cultural control measures are farming practices aimed at avoiding pathogen contact, creating environmental conditions that are unfavorable to the pathogen or avoiding conditions that are favorable, eradicating or reducing inoculum levels, and improving resistance (Agrios, 1997). There are a number of strategies that are commonly recommended for the control of Rhizoctonia stem canker (R solani) that can help to relieve disease pressure; use of black-scurf free certified seed (Jeger et al., 1996), planting once the temperature of the soil has risen sufficiently in the spring (Sirnons and Gilligan, 1997; Secor and Gudmestad, 1999), plant green-sprout seed (Rich, 1983), use 14 cover crops, rotate crops (Honeycutt et al., 1996), plant seed tubers close to the surface or after planting drag a chain or board to level off the hill and thus have the potatoes closer to the surface (Jeger et al., 1996), and proper management of water (Simons and Gilligan, 1997) and nutrients. Black scurf can be partially controlled by early harvesting, with the peak of sclerotia formation at about three weeks after vine destruction (Gudmestad, 1979). Dij st (1986, 1990) however, pointed out that plant exudates, in conjunction with the method of vine destruction, played the key role in the formation of black scurf on progeny tubers. Research Goals Integrated pest management is the key to successful and sustainable agriculture. Common recommendations for the management of R. solani include shallow planting in warm soil (Jeger, et al., 1996) around 15°C (Banville, 1996), planting with disease free or certified seed (Banville, 1996; Secor and Gudmestad, 1999), rotation of crops (Honeycutt et al., 1996), and the use of seed and post emergence fungicide treatments (Secor and Gudmestad, 1999). However, there are very few studies that have incorporated both cultural and chemical management strategies in potatoes. Recent studies have tested out and fungicide treated seed against whole seed Nolte et al., (2003), and Bains et al. (2002), tested fungicide efficacy of seed treatments and cultivar susceptibility. However, no study has examined fiingicide efficacy at different application times using multiple cultivars. A series of field experiments were established that attempted to validate the prior claim of control with planting based upon soil temperature, and to test the efficacy of fungicides available and recommended for R. solani control. The first set of experiments 15 (Chapter 2), in planta, examined the efficacy of early-season fungicide applications alone or in combination with a seed treatment. The commonly recommended (and tested) seed treatment fludioxonil (Maxim) was tested with the early-season fungicides azoxystrobin (Amistar), flutolanil (Moncut), and pyraclostrobin (Headline). Each early-season fungicide was tested at three different application times: in-furrow, at emergence, and 14 days post emergence. The main questions this experiment attempts to address were: for effective management of the various disease symptoms, are both seed treatments and . early-season treatments needed or is just one sufficient? Is there one particular fimgicide that effectively manages disease symptoms better than the others, based upon measured agronomic and pathological variables? Does one particular application timing (of the early-season fungicides) provide more effective control of disease symptoms, based upon measured agronomic and pathologic variables? The second set of experiments (Chapter 3) examined the effect of planting time based upon soil temperature among multiple varieties using three soil temperatures that can occur during the early part of the growing season; namely when soil (at 15cm depth) reached a five-day average of 8, 14, or 20°C. The main questions addressed by this approach were: does planting later result in a reduction in stem and/or stolon canker and/or black scurf, compared to early planting? Due to the shorter growing season, does planting later result in a reduction of yield? Are there differences among varieties for the effect of planting date on the incidence and severity of stem and stolon canker and black scurf? The final experiment (Chapter 4) tested the combined effect of planting time based upon soil temperature and fungicide efficacy in four commonly grown potato l6 varieties. The seed treatment fludioxonil, and two early-season fungicides (azoxystrobin and flutolanil) were evaluated at two possible application times. All combinations of treatments were tested in two separate plantings that occurred once the average soil temperature at 15 cm depth averaged 14 and 20°C (as described above). The questions this experiment attempted to answer were: does a particular combination of planting date and early season fungicide application result in a reduction of disease incidence and severity (stem and/or stolon canker, and/or black scurf)? Does a late planting and non- treated seed result in lower yield? Also, is one type of control, cultural (planting based upon soil temperature) or chemical (using seed/early-season fungicide(s), more effective for disease management? 17 CHAPTER 2 EFFECT OF FUNGICIDE AND APPLICATION TIMING ON THE INCIDENCE AND SEVERITY OF RHIZOCTONIA CAN KER AND BLACK SCURF ON POTATOES Introduction Fungicides are the primary component of integrated management for Rhizoctonia diseases of potato. There are a wide variety of fungicides for the management of Rhizoctonia solani that differ in the primary target site of action as well as fungicide type and class (systemic or protective). Recent efficacy studies have tested some of these fungicides, including azoxystrobin, fludioxonil, flutolanil, iprodione, mancozeb, pencycuron, thiophanate-methyl, and thiabendazole (Bains et al., 2002; Nolte et al., 2003; van den Boogert et al., 2004). In order to have effective chemical management of Rhizoctonia stern and stolon canker and black scurf, the correct rate of the chosen fungicide as well as the best application timing must be established. The most common timing for fungicide application is either as seed treatments on cut or whole seed, or in-firrrow during planting. However, some fungicides are also labeled for applications around emergence or during cultivation. Some fungicides like flutolanil may have multiple formulations, such that it can be applied as a dust to seed pieces or sprayed in-furrow. Other fturgicides, like azoxystrobin, can be sprayed in-furrow, at the time of emergence, or at hilling/cultivation. Efficacy testing of in-furrow and foliar applications around emergence showed few significant differences among treatments for agronomic and disease parameters Kirk et al. (2001, 2002, 2003, 2004, 2007a,b, 2008). Generally, all treatments tested had significantly lower disease incidence and severity when compared to the untreated control but rarely were there differences among treatments. 18 Fungicides should be applied at timings that maximize efficacy. Seed and soil- bome inoculum of R. solani are important sources of stem and stolon canker, respectively (Frank and Leach, 1980; Powelson and Rowe, 2008). Seed treatments should therefore be more effective for managing stem cankers, while later frmgicide applications, in furrow and foliar, should be effective at managing stolon cankers and tuber black scurf. This separation, however, is difficult since environmental factors like soil temperature, depth of planting, soil type, and soil moisture content, play a role in persistence and efficacy of the fungicide as well as the growth of the plant and pathogen. Field experiments were established in 2004 and 2005 to examine the effects of seed and early season fungicide applications. These early season applications were evaluated in an attempt to manage the difierent sources of potential inoculum. If these inoculum sources can be managed effectively, through timely fungicide applications, there could be a significant reduction in economic losses that are linked to R. solani. Materials and Methods The effects of application timing of three different ftmgicides (azoxystrobin, flutolanil, and pyraclostrobin) were tested over two growing seasons, 2004 and 2005 in the absence or presence of a seed treatment (fludioxonil). In addition, three application times were evaluated: in-fin'row at-planting, at emergence, and 14 days post-emergence (Table 2.1). In 2004, cvs. FL1879 and Russet Norkotah were grown on Houghton muck at the Muck Soils Research Farm near Bath, MI and cv. Superior was grown on sandy loam in Antrim County, MI by a cooperating farmer. In 2005, Russet Norkotah was again grown at the Muck Soils Research Farm, and Superior was again grown, in Antrim l9 County, MI. These sites were chosen due to their known history of Rhizoctonia canker infection and black scurf. Table 2.1 Treatment matrices for seed and early-season fungicides and application timings for field experiments that occurred during the 2004 and 2005 growing seasons. In-furrow at- 14 days post Seed Treatment planting Emergence emergence 1 No seed treatment + Azoxystrobin 2 + Azoxystrobin 3 + Azoxystrobin 4 + Flutolanil 5 + Flutolanil 6 + Flutolanil 7 + Pyraclostrobin 8 + Pyraclostrobin 9 + Pyraclostrobin 10 1 1 + Fludioxonil + Azoxystrobin 12 + Azoxystrobin 13 + Azoxystrobin 14 + F lutolanil 15 + Flutolanil 16 + Flutolanil 17 + Pyraclostrobin 18 + Pyraclostrobin 19 + Pyraclostrobin 2O Single-cut potato seed tubers cv. ‘Superior’ were planted in sandy loam soil near Elmira, M1 on 2 June, 2004 and near Mancelona, M1 on 26 May, 2005 with and without fludioxinil (see below). Cut ‘F L1 879’ seed and whole ‘Russet Norkotah’ seed pieces were planted in Houghton muck soil at the MSU Muck Soils Research Farm near Bath, M1 on 23 June, 2004, and 1 June, 2005. The grower-cooperator at the Elmira and Mancelona sites applied all fertilizer, irrigation and pesticides for control of weeds, insects, and other pathogenic fungi as part of their farm’s proprietary program. 20 At the Bath, MI location, fertilizer was formulated according to results of soil tests was and drilled into plots before planting in both 2004 and 2005. A permanent irrigation system was established prior to the commencement of protective fungicide sprays (for other plant pathogens) and the fields were maintained at soil moisture capacity throughout the season by frequent (minimum 5-day) irrigations. In 2004, chlorothalonil (Bravo WS 6SC) was applied at 1.75 L/ha on a ten day interval, for a total of six applications, starting two weeks after the last application of experiment treatments. Weeds were controlled with metolachlor (Dual 8E) at 2.34 L/ha 15 DAP (days after planting), bentazon (Basagran) at 2.34 L/ha 25 and 45 DAP and sethoxydim (Poast) at 1.75 L/ha 63 DAP. Insects were controlled with irnidacloprid (Admire 2F) at 1.46 L/ha at-planting, carbaryl (Sevin 80S) at 1.46 kg/ha 36 and 60 DAP, endosulfan (Thiodan 3 EC) at 2.73 L/ha 70 DAP and perrnethrin (Pounce 3.2EC) at 0.561kg/ha 53 DAP. In 2005, porpamocarb hydrochloride (Previcur Flex) was applied at 1.4 L/ha on a ten day interval, for a total of four applications, starting two weeks after the last application of experiment treatments. Weeds were controlled with metolachlor (Dual 8E) at 1.2 L/ha and metribuzin (Sencor 75DF) at .67 kg/ha at planting, bentazon (Basagran) at 2.34 L/ha 27 and 45 DAP (days after planting) and sethoxydim (Poast) at 1.75 L/ha 63 DAP. Insects were controlled with thiamethoxam (Platinum) at 0.59 L/ha at-planting, cyfluthrin (Baythroid 2) at 0.146 L/ha 36 and 60 DAP. 8 Environmental conditions were monitored (2004, 2005) at the Muck Soils Research Farm with the onsite Michigan Automated Weather Network (MAWN, http://www.agweather.geo.msu.edu/) Campbell Scientific weather station. Of particular 21 interest were the soil temperatures available at about 5 and 10 cm depth, air temperature, and precipitation. Although the data were not used to make experimental decisions, such as with the experiment on the effect of soil temperature at planting on varietal susceptibility (described below), the information did provide insight into experimental results. At both the Elmira (2004) and Mancelona (2005) locations a Campbell Scientific weather station was placed in the grower—cooperator field. The weather station used a CRIOX data logger, and had probes for temperature (surface, 10, and 20 cm deep), and precipitation (the same as the MAWN stations described above). At both locations, the seed pieces were planted in plots that were four rows wide (86 cm spacing between rows) and 6.1 m long, with approximately 33 cm spacing between plants give a target population of 72 plants for each plot. The experiment was replicated four times in randomized strip plot design. The fludioxonil seed treatment was applied, in a water suspension of at a rate of 0.5mL/kg, onto the entire seed surface. Early season applications were made over the seed in—furrow, or emerged shoots/plants and applied with a single nozzle R&D spray boom delivering 46.8L/ha (551.6 kPa) and using one XR11003VS nozzle per row. Nine parameters were measured at various stages of the growing season. Emergence was counted at all locations, from which the relative area under the emergence progress curve, RAUEPC, was calculated using the following formula: RAUEPC = 2(ti + 1 - ti) *((Ei + 1 + Ei)/2) T total * 100 where t was the time in days after planting and E was the percentage of plant emergence (Wharton et al., 2007). Each variety had one midseason harvest (four plants per treatment), per growing season, during which time the tuber, stem and stolon numbers, 22 and percentage of stems and stolons with greater than five percent girdling due to R. solani were counted and calculated. At the end of the season, plots were machine- harvested and a sample weight was taken from one row of the four row plots. In addition, 20 tubers from each plot were stored for 60 days at 10°C then evaluated for incidence and severity of black scurf on the mature tubers. The number of tubers with visible sclerotia were counted and a percentage of the sample was calculated for the incidence value. The severity index of black scurf was calculated by summing the class value assigned to each tuber (n = 20 per replicate), which was based upon the surface area covered by visible sclerotia. The severity classes used were 0 = 0%; l = 1 - 5%; 2 = 6 - 10%; 3 = 11 - 15%; 4 = >16% surface area. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia. Data Analyses All data were analde by AN OVA and tested with Tukey’s Honestly Significant Differences (HSD), P = 0.05 (JMP: SAS Institute, Cary, NC, USA). Response analyses were determined by the main effects analyses using the probability of difference between fungicide treatments (seed and/or early-season), application timing, and/or interaction of the treatments. Three types of response were determined; Type A= significant effect of seed treatment (P < 0.05) but no effect of any other factors (P > 0.05), (only the mean effect of seed treatment presented); Type B: significant effect of more than one factor, or factor other than seed treatment (responses to seed treatment and application type presented); Type C= no effect of any factor, no further analysis. 23 Results Year There were significant differences between the 2004 and 2005 growing seasons in both cultivars Russet Norkotah and Superior with respect to several measured parameters, (Table 2.2). The mid-season sample timings for cv. Russet Norkotah were 50 and 69 days after planting (DAP) in 2004 and 2005, respectively. Despite being plantedl9 days earlier, the 2004 planting of cv. Russet Norkotah yielded a significantly higher RAUEPC, number of tubers, stems and stolons. However, the 2004 planting also yielded significantly higher percentages of diseased stems and stolons. Harvest was completed 90 and 113 DAP in 2004 and 2005, respectively. Even with 23 fewer days in the growing season 2004 still had a significantly higher sample weight, but also significantly higher black scurf incidence and severity on the mature tubers. Cv. Superior was grown in Antrim County by a grower-cooperator in both 2004 and 2005. Due to rotation of crops, the plots were located in different fields, but with similar sandy-soil type. The 2004 and 2005 mid-season harvests were 50 and 67 DAP respectively, while the fall harvests were 84 and 112 DAP. The 2005 growing season was only significantly different from the 2004 growing season with respect to three parameters; 2005 yielded significantly lower RAUEPC and higher numbers of tubers and stems. 24 Table 2.2 Main effects analyses (probability of difference between years: p=0.05) of growing season for cultivars 'Russet Norkotah' and 'Superior' on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted at two sites in MI between 2004 and 2005. Russet Norkotah Superior Variable Measured p-value 2004 2005 p—value 2004 2005 Emergence (RAUEPC)Z < 0.0001 0.197 0.058 < 0.0001 0.434 0.061 Number of tubers/plant < 0.0001 30.3 22.4 < 0.0001 19.2 27.5 Number of stems/plant 0.0002 4.6 3.8 0.4810 3.6 3.7 Percent of stems with girdling 0.0803 65.0 70.3 0.2177 17.5 20.3 Number of stolons/plant < 0.0001 13.4 9.0 < 0.0001 16.3 22.3 Percent of stolons with girdling < 0.0001 33.2 19.9 0.0638 7.2 9.5 Yield: metric tons/hectare < 0.0001 26.8 20.2 0.2373 33.0 31.8 Black scurf incidence)I < 0.0001 48.9 23.7 0.5033 8.3 6.7 Black scurf severityx < 0.0001 21.9 9.1 0.3669 4.0 2.9 z: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to the last emergence count y: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken during fall harvest x: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%; 1 = 1 - 5%; 2 = 6 - 10%; 3 = 11 - 15%; 4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia Seed and Early Season Treatments The application of the seed treatment fludioxonil (Maxim 4FS) had some significant effects on the parameters for all varieties in both 2004 and 2005 growing seasons (Tables 2.3 — 2.6). The use of fludioxonil on seed potatoes, compared to treatments without fludioxonil (-ST), significantly increased RAUEPC (2004: cvs. FL1879 and Superior); tuber number (2004: cv. Superior); stem number (2004: all varieties); stolon number (2004: cv. Superior); sample weight (2005: cv. Russet 25 Norkotah); percent stems with >5% girdling (2004: cvs F L1 879 and Superior); percent stolons with (>5%) girdling (2004: cvs Russet Norkotah and Superior; 2005: cv. Superior); and incidence and severity of black scurf (2004: cv. Superior). F ludioxonil use resulted in a significant reduction in the stem number (2005: cv. Superior) and sample weight (2004: cv. Superior). In addition, cv. Russet Norkotah (2005) had a significant increase in the percent of stems with >5% girdling and both incidence and severity of black scurf on the mature tubers. In 2004, no significant differences were found among treatments for all measured parameters of cvs F L1 879 and Russet Norkotah (Tables 2.3 - 2.4). However, the application of fludioxonil and pyraclostrobin at emergence or 14 days post emergence (14 DPE), on cv. Superior (Table 2.6), resulted in significantly higher numbers of tubers and stolons when compared to the non-treated control (no ST or early-season ftmgicide) and a portion of treatments without fludioxonil (-ST). However, pyraclostrobin (+ST) at emergence or 14 DPE was not significantly different from the treatments with fludioxonil. Similarly, pyraclostrobin (+ST) at emergence also yielded a significantly higher stem number compared to the non-treated control, and a portion of the treatments without ST, but was not significantly different from any treatment including fludioxonil seed treatment. In addition, fludioxonil and pyraclostrobin applied at emergence resulted in a higher RAUEPC compared to azoxystrobin (-ST) applied 14 DPE and pyraclostrobin (-ST) applied in-furrow but not to the non-treated control. In 2005 on cv. Russet Norkotah, fludioxonil (ST) plus azoxystrobin applied in furrow or with flutolanil applied at emergence, (Tables 2.7 and 2.8) resulted in a significantly higher RAUEPC when compared with azoxystrobin (- ST) applied at 26 emergence, flutolanil (- ST) applied at emergence, and pyraclostrobin (- ST) applied either in furrow or 14 DPE, but was not significantly different from the untreated control. Similarly, fludioxonil with either azoxystrobin applied at emergence or pyraclostrobin applied in-furrow resulted in a higher yield than azoxystrobin (-ST) applied 14 DPE, but was not significantly different from the untreated control. Also in 2005, pyraclostrobin (-ST) applied 14 DPE to cv. Superior had a significantly higher percent of stolons with (>5%) girdling compared with all other treatments (Tables 2.9 and 2.10). 27 Table 2.3 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted near Bath, Ml during the 2004 growing season on cultivar ‘F L1 879’. Effect of interaction Effect of of seed, early- Effect of early- Efiect of seed treatment (ST) 53350“ application 59350“ and on variable fungicide timing of application treatment fungicide timing on . Mean on variable on variable variable Vanable Measured p-value - ST + ST p—value p-value p-value Analysisz Emergence (RAUEPC)y 0.0006 0.125 0.151 0.0915 0.2775 0.1527 A Number of tubers/plant 0.1421 35.5 40.1 0.7218 0.7179 0.9472 C Number of stems/plant 0.0056 4.6 5.8 0.4940 0. 1 829 0.0446 C Percent of stems with girdling 0.0262 44.4 32.0 0.4782 0.4725 0.1444 A Number of stolons/plant 0.0689 44.4 51.9 0.4357 0.0919 0.6827 C Percent of stolons with girdling 0.0888 33.8 26.1 0.2422 0.3483 0.3171 C Yield: metric tons/hectare 0.1354 35.5 38.9 0.5608 0.9591 0.5805 C Black scurf incidencex 0.4563 62.9 58.7 0.8322 0.1805 0.7542 C Black scurf severityw 0.4565 32.5 29.8 0.9001 0.0621 0.7671 C 2: Response analysis determined by the probability of difference among fungicide treatments (seed and/or early-season), application timing, and/or interaction of the treatments. Type A= significant effect of seed treatment (p < 0.05) but no effect of other factors (p > 0.05), mean of effect of seed treatment shown only in Table 2.3. Type B= significant effect of more than one factor, or factor other than seed treatment, treatment means shown in Table 2.4; Type C= no effect of any factor, no firrther analysis. y: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated fi'om the day of planting to the last emergence count x: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken during fall harvest w: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%;1=l- 5%; 2 = 6 - 10%; 3 =11-15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia 28 Table 2.4 Main effects analyses (probability of difference among treatments: p =0.05) of the effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted near Bath, MI during the 2004 growing season on cultivar 'Russet Norkotah'. Effect of Effect of interaction early- Effect of of seed, season application early- Effect of seed treatment (ST) fungrcrde timing-of season a.“ . treatment fungrcrde application on vanable . . on on timing on Variable Mean variable variable variable Measured p-value - ST + ST p-value p—value p-value AnalysisZ Emergence (RAUEPC)y 0.121 I 0.195 0.200 0.8197 0.4329 0.0707 C Number of tubers/plant 0.3109 29.4 3 I .3 0.1941 0.8901 0.7889 C Number of stems/plant 0.0312 4.2 4.9 0.3376 0.4134 0.5140 A Percent of stems with girdling 0.1006 69.1 60.9 0.8612 0.6991 0.1981 C Number of stolons/plant 0.1 198 12.5 14.3 0.0731 0.3518 0.5275 C Percent of stolons with girdling 0.0390 37.8 28.6 0.7292 0.9935 0.2622 A Yield: metric tons/hectare 0.6870 26.4 27.1 0.5186 0.9655 0.9721 C Black scurf incidence" 0.3134 51.1 46.8 0.7324 0.7960 0.7071 C Black scurf severityw 0.2780 23 .2 20.5 0.7151 0.7475 0.4792 C 2: Response analysis detemrined by the probability of difference among fungicide treatments (seed and/or early-season), application timing, and/or interaction of the treatments. Type A= significant effect of seed treatment (p < 0.05) but no effect of other factors (p > 0.05), mean of effect of seed treatment shown only in Table 2.5. Type C= no effect of any factor, no further analysis. y: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to the last emergence count x: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken during fall harvest w: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling intoeachclassO=0%; l =1-5%;2=6-10%;3 =11-15%;4=>16%surfaceareacoveredwith sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia 29 Table 2.5 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatments and application timing agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted in Antrim Co., Ml during the 2004 flowingiason on cultivar 'Superior'. Effect of Effect of interaction early- Effect of of seed, season application early- Effect of seed treatment (ST) fungicide timing of season and on variable treatment fungicide application on on timing on Variable Mean variable variable variable Measured p-value - ST + ST p—value p-value p—value AnalysisZ Emergence (RAUEPC)y 0.0032 0.422 0.446 0.4142 0.0081 0.0119 B Number of tubers/plant < 0.0001 16.2 22.1 0.0427 0.0597 < 0.0001 B Number of stems/plant < 0.0001 3.1 4.1 0.2335 0.4710 < 0.000] B Percent of stems with girdling < 0.0001 24.3 10.7 0.1727 0.8217 0.0235 B Number of stolons/plant < 0.0001 13.4 19.2 0.0697 0.0809 < 0.000] B Percent of stolons with girdling 0.0001 10.7 3.6 0.5136 0.8791 0.0067 B Yield: metric tons/hectare < 0.0001 35.7 30.2 0.9131 0.3166 0.0090 B Black scurf incidencex 0.0071 13 .5 3.1 0.3459 0.2159 0.1089 A Black scurf severityw 0.0081 6.5 1.4 0.3038 0.23 82 0.1 147 A 2: Response analysis determined by the probability of difference among fungicide treatments (seed and/or early-season), application timing, and/or interaction of the treatments. Type A= significant effect of seed treatment (p < 0.05) but no effect of other factors (p > 0.05), mean of effect of seed treatment shown only in Table 2.6. Type B= significant effect of more than one factor, or factor other than seed treatment, treatment means shown in Table 2.7; Type C= no effect of any factor, no further analysis. y: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to the last emergence count x: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken during fall harvest w: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%;1=1- 5%; 2 = 6 - 10%; 3 =11-15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. 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Co 82:52 no 2.8.8“. 93 8:253: Ammv 533. 3.80 use 2.9 Bow .Lotomsm. 332:... so 833. Ezaomvoom 05 mezzo =2 :00 EEE< E 383:8 23: Soc 5 .533. .eEoBonEx ,3 333 mEoEExm 38mm 98 Boaofl o_Eo=e._wm co mafia: sanguine use Econ—“mob oEEwEE :83?on new team he 89mm ed 038. 31 Table 2.7 Main effects analyses (probability of difference among treatments: p=0.05) of the effect of seed and early-season fungicide treatment and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted near Bath, MI during the 2005 growing season on cultivar 'Russet Norkotah'. Effect of Efiect of interaction early- of seed, Effect of seed treatment (ST) on “3.50." Effect 9f early ' d variable fungicide application season an treatment timing of appllcat1on on fungicide timing on Variable Mean variable on variable variable Measured p-value - ST + ST p-value p-value p-value AnalysisZ Emergence (RAUEPC)y < 0.0001 0.050 0.065 0.7198 0.9778 < 0.0001 B Number of tubers/plant 0.6361 22.7 22.1 0.1337 0.1351 0.1459 C Number of stems/plant 0.6528 3.8 3.9 0.4147 0.3470 0.4749 C Percent of stems with girdling 0.0363 66.7 73.9 0.6948 0.0702 0.0777 C Number of stolons/plant 0. 1062 9.6 8.5 0.4947 0.2757 0.404] C Percent of stolons with girdling 0.5356 19.0 20.7 0.1568 0.4777 0.032] C Yield: metric tons/hectare < 0.0001 16.7 23.8 0.7652 0.8781 0.0010 B "Black scurf incidencex 0.0293 20.1 27.3 0.3188 0.0752 0.1652 A Black scurf severityw 0.0045 7.0 11.1 0.1429 0.1 198 0.0384 C 2: Response analysis determined by the probability of difference among fungicide treatments (seed and/or early-season), application timing, and/or interaction of the treatments. Type A= significant effect of seed treatment (p < 0.05) but no effect of other factors (p > 0.05), mean of effect of seed treatment shown only in Table 2.8. Type B= significant effect of more than one factor, or factor other than seed treatment, treatment means shown in Table 2.9; Type C= no effect of any factor, no further analysis. y: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to the last emergence count x: Percent incidence of tubers with sclerotia of R. solani fi'om a sample of 20 tubers per replicate taken during fall harvest w: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%; 1 = 1 - 5%; 2 = 6 - 10%; 3 = 11 - 15%; 4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 -10%;51- 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia 32 Table 2.8 Effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted near Bath, Ml during the 2005 growing season on cultivar 'Russet Norkotah'. Seed (ST) and early season (ES) treatments and application timings QTY Emergence Yield: metric ST ES AT ‘ (RAUEng tons/hectare -sr azo IF 0.054 abcdx 18.8 ab Em 0.046 d 16.625 ab PE 0.052 abcd 12.425 b flu IF 0.05 bcd 16.825 ab Em 0.046 d 17.05 ab PE 0.054 abcd 14.8 ab pyr lF 0.048 cd 16.425 ab Em 0.053 abcd 20.1 ab PE 0.046 d 17.6 ab none 0.053 abcd 16. l ab + ST azo lF 0.07 ab 23.625 ab Em 0.069 abc 26.775 a PE 0.064 abcd 24.725 ab flu [P 0.059 abcd 23.625 ab Em 0.073 a 19.875 ab PE 0.064 abcd 23.2 ab pyr IF 0.067 abcd 25.15 a Em 0.06 abcd 22.9 ab PE 0.062 abcd 23.875 ab none 0.064 abcd 24.3 ab 2: Seed treatment: fludioxonil (Maxim 4F S) 2.37mU45.4 kg seed. Early season treatment per 305 m: azo = azoxystrobin (Amistar 80WD) 7.09 g; flu = flutolanil (Moncut 70DF) 33.45 g; pyr = pyraclostrobin (Headline) 6.21 mL. Application timings: [F = ln-furrow at planting; Em = emergence; PE = 14 days post-emergence y: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to the last emergence count x: Values followed by the same letter are not significantly different at p = 0.05 (Tukey’s HSD) 33 Table 2.9 Main effects analyses (probability of difference among treatments: (1 =0.05) of the effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted in Antrim Co., Ml during the 2005 _growing season on cultivar 'Superior'. Effect of Effect of early- interaction of season Effect of seed, early- Effect of seed treatment fungicide application season and (ST) on variable treatment timing of application on fungicide on timing on Mean variable variable variable Variable p- Measured value - ST + ST p-value p—value p—value AnalysisZ Emergence (RAUEPC)y 0.6391 0.060 0.061 0.2926 0.9595 0.9708 C Number of tubers/plant 0.6929 27.3 27.8 0.3925 0.2711 0.5373 C Number of stems/plant 0.0003 4.0 3 .4 0.7662 0.5337 0.2185 A Percent of stems with girdling 0.3317 21.7 18.8 0.0673 0.0418 0.6042 C Number of stolons/plant 0.1068 23.1 21.6 0.6380 0.5346 0.7758 C Percent of stolons with girdling 0.0019 12.1 6.9 0.2067 0.3382 0.0014 B Yield: metric tons/hectare 0.1029 30.5 33.1 0.5020 0.7968 0.7482 C Black scurf incidence" 0.1792 8.5 4.9 0.5532 0.7081 0.8898 C Black scurf severityW 0.0545 4.1 1.6 0.4252 0.8832 0.6287 C 2: Response analysis determined by the probability of difference among fungicide treatments (seed and/or early-season), application timing, and/or interaction of the treatments. Type A= significant effect of seed treatment (p < 0.05) but no effect of other factors (p > 0.05), mean of effect of seed treatment shown only in Table 2.10. Type B= significant effect of more than one factor, or factor other than seed treatment, treatment means shown in Table 2.11; Type C= no effect of any factor, no further analysis. y: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to the last emergence count x: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken during fall harvest w: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%; l = l - 5%; 2 = 6 - 10%; 3 =11 ~15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia 34 Table 2.10 Effect of seed and early-season fungicide treatments and application timing on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials conducted in Antrim Co., Ml during the 2005 growing season on cultivar 'Superior'. Seed (ST) and early season (ES) treatments and application timings (ATY‘ ST ES AT Percent of stolons witLgirdling - ST azo 1F 10.7 by Em 9.2 b PE 10.1 b flu [F 8.7 b Em 8.3 b PE 10.6 b pyr IE 7.3 b Em 1 1.4 b PE 30.5 3 none 14.2 ab + ST azo IF 5.9 b Em 10.9 b PE 5.8 b flu [F 6.6 b Em 4.8 b PE 7.1 b pyr [F 10.4 b Em 6.3 b PE 6.9 b none 4.1 b 2: Seed treatment: fludioxonil (Maxim 4F S) 2.3 7mL/45.4 kg seed. Early season treatment per 305 m: azo = azoxystrobin (Amistar 80WD) 7.09 g; flu = flutolanil (Moncut 70DF) 33.45 g; pyr = pyraclostrobin (Headline) 6.21 mL. Application timings: [F = ln-furrow at planting; Em = emergence; PE = 14 days post-emergence y: Values followed by the same letter are not significantly different at p = 0.05 (Tukey’s HSD) 35 Results of similar studies Previous studies of the diseases caused by Rhizoctonia solani at the MSU Muck Soils Research Farm (on cvs Pike, Snowden, or FL 1879) have included the use of the seed treatments including fludioxonil, and early season treatments of azoxystrobin, flutolanil and pyraclostrobin tested at multiple concentrations and formulations (Kirk et al., 2001, 2002, 2003, 2004, 2007a, 2007b, 2008). In most cases, the use of seed treatment fludioxonil or early-season treatments resulted in healthier plants, when compared to the non-treated control, in at least one measured variable (increased RAUEPC, canopy development, stem number, and yield; or decreased stem and stolon girdling and incidence and severity of black scurt). However, when compared to other seed or early-season fungicides tested, these treatments generally resulted in statistically similar ratings of the tested variables. In other fungicide testing programs throughout the country, the use of fludioxonil, azoxystrobin, flutolanil and pyraclostrobin has had mixed results. During the 2003 growing season, May et al. (2004) found that flutolanil (at two rates) applied alone or in combination with fludioxonil (ST) resulted in similar ratings of R. solani on stems and stolons, when compared to the inoculated non-treated control. On the other hand, azoxystrobin (two formulations) applied alone or in combination with fludioxonil (ST) resulted in significantly less disease. During the 2004 growing season, Stevenson et al. (2005) found no significant differences between treatments of fludioxonil (ST) alone or in combination with early-season applications azoxystrobin or treatments with azoxystrobin or pyraclostrobin and the non-treated control for early emergence, severity of infection on stems, stolons or tubers, or yield. In addition, there were no significant 36 differences among relevant treatments for average stem and stolon numbers. Finally, during the 2007 growing season, Zitter and Drennan (2008), found that fludioxonil (ST) or in combination with one of two concentrations of an in-furrow application of azoxystrobin resulted in a significantly lower black scurf rating compared to the non- treated control, but there were no significant differences among relevant treatments. Discussion and Conclusions The 2004 planting was delayed approximately three weeks due to significant rainfall (233.7 mm) in the pervious month, which led to flooding. The location of the Muck Soils Research Farm is geographically a low point for the surrounding area, and as such a backup/ failure of the drainage system resulted in the entire farm being under nearly 50 cm of water for over a week. Even with the delay in planting, the agronomic differences found between the 2004 and 2005 growing seasons, for cv. Russet Norkotah, cannot be explained by meteorological data. The soil temperature at planting depth of 10 cm for a five-day average was 18.7°C on 23 June, 2004 and 15.8°C on 1 June, 2005. In spite of being planted just over three weeks later in 2004, the temperature was only 3°C higher. The two and four week average soil temperatures following planting were 19.6°C and 205°C in 2004 and 208°C and 207°C in 2005, respectively. Sale (1979) noted in growth chamber studies on cvs Sebago and Sequoia that “emergence was quickest at a mean temperature of about 21°C and was progressively longer as mean temperature either decreased or increased from this value.” In both 2004 and 2005 growing seasons, the mean temperatures at and following planting were very close to the range of optimal temperature for emergence. This may have resulted in a faster and more uniform 37 emergence than if the seed pieces were planted at cooler soil temperatures, thus decreasing time for initial interaction between R. solani and the emerging sprouts. A possible reason for the agronomic differences in growing seasons may have been more the result of seed physiological age, something that was not accounted for. Allen and O’Brien (1986) found that an increase of time between seed breaking dormancy and planting date resulted in a quicker emergence. This faster emergence could lead to more stolons and tubers formation. All seed used had broken dormancy, but the number of days between dormancy break and planting was not recorded. Increased disease symptoms for cv. Russet Norkotah (percent stolons with greater than five percent girdling due to R. solani, incidence and severity of black scurf) in 2004 may have been the result of optimal growing conditions for R. solani the year prior to the experiment, leading to an increased level of soil-bome inoculum. Soil inoculum levels were not measured but previous experiments at the same location have had disease signs and symptoms of R. solani. In addition, the selection of certified seed for the experiments ensured seed-borne inoculum less than 2% on the majority of tubers. With planting near the optimal temperature range for emergence, the stems may have been able to emerge prior to significant stem infection. However, as stolons grew, they encountered soil-borne inoculum leading to stolon girdling, and as tubers formed, black scurf occurred. Simons and Gilligan (1997) reasserted that soil-bome inoculum is more important for the development of stolon canker and black scurf, while seed-borne inoculum is more important for stem canker (Van Emden 1965; Frank & Leach 1980). In 2004, cv. Russet Norkotah vines were desiccated on September 23 and harvested October 13 (20 days) while in 2005 vines were desiccated and harvested 38 August 25 and September 21 (27 days), respectively. The span of time between desiccation and harvest, especially three to seven weeks, has been shown to significantly increase the number of sclerotia per tuber (Gudmestad, 1979). In both years the final harvests were in the time span which could have resulted in an increase in tuber black scurf. Similar to prior growing seasons at the Muck Soils Research Farm, treatments in 2004 that included the fludioxonil seed treatment had significantly less disease (percent of stems and stolons with greater than five percent girdling and black scurf incidence and severity). Treatments with fludioxonil (ST), especime on cv. Superior, produced more robust plants early in the growing season (significantly greater RAUEPC and tuber, stem and stolon numbers). However, at the end of the season the yield was significantly less for treatments with fludioxonil, which was also found by Stevenson et al. (2000, 2004). In 2005 treatments with the fludioxonil seed treatment, on cv. Russet Norkotah, had significantly higher percent of stems with greater than five percent girdling as well as higher black scurf incidence and severity. Only in 2002 did a treatment with fludioxonil have a parameter that was significantly worse than other treatments tested including the untreated control: RAUEPC and RAUCPC (Kirk, 2003). However, other efficacy studies have also noted either significantly less healthy plants or worse disease symptoms. The current chemical management strategy for R. solani is for a seed treatment (fludioxonil, flutolanil, thiophanate-methyl) or in—furrow (PCNB, azoxystrobin or flutolanil) application or both (Bird, 2008). The rationale is that the seed treatment will control the seed-bome and some soil-borne inoculum, while the in-furrow application 39 would control mostly soil-borne inoculum. Of the early—season treatments tested, alone or in combination with the seed treatment fludioxonil, there was not one that consistently improved plant vigor (faster emergence resulting in a higher RAUEPC, increased stem, stolon, tuber number, or yield) or decreased disease incidence and severity on stems, stolons or tubers. Further, the choices of the early-season fungicide tested (azoxystrobin, flutolanil or pyraclostrobin) or the application timing (in-furrow, at emergence, or 14 days post-emergence) were not consistently significantly different from the non-treated controls. Some of the lack of differences among treatments may have been due to interactions with the soil. The Houghton muck, at the MSU Muck Soils Research Farm, is 91.9% organic matter which is significantly higher than the sandy loam in Antrim County. Organic matter tends to adsorb pesticides, slowing the rate of leaching and volatilization, but also reduces the amount that is available for absorption by plants or target organisms. This may have lead to the limited number of significant differences found among treatments grown on muck (cvs FL1879 and Russet Norkotah). However, a comparative study of the efficacy and availability of these fimgicides on different soil types has not been completed, which could provide insight. Another possible source for the lack of visible significant differences among treatments may have been in part due to the reliance on a naturally occurring inoculum. The addition of inoculum, and a non-inoculated control, could have led to some insight on existing inoculum levels at the Muck Soils Research Farm. The application of inoculum was not an option for the sites in Antrim Co, grown by a cooperating farmer. 40 In a summary of nearly 50 trials, Wale (2008) found that even Rhizoctonia stem canker specific (seed and soil) treatments had variable results. For example, of the trials with azoxystrobin that examined black scurf incidence (15) and severity (11), or stem canker incidence (9) and severity (10), there was never more than 50% of the trials that resulted in significantly less disease when compared to the non-treated control (Wale, 2008). Based upon the results of this summary, these two field experiments and given consideration for the concerns stated, only use of the one treatment, whether seed or early season was necessary. However, the use of fungicides for management should not be the sole source of control, but rather part of an integrated pest management strategy. Although the presence of R. solani has been noted via previous efficacy studies by Kirk et al. (2000 through 2008), the quantity, anastomosis group(s) and virulence of the soil-borne inoculum were not examined. These parameters should be evaluated for future studies at the Muck Soils Research Farm. 41 CHAPTER 3 EFFECT OF SOIL TEMPERATURE ON VARIETY SUSCEPTIBILITY TO THE INCIDENCE AND SEVERITY OF RHIZOCTONIA CANKER AND BLACK SCURF ON POTATOES Introduction The time potatoes need to reach maturity can range from 90 to around 130 days, and potatoes can be lefi in the ground up to one month, or more, after vine desiccation. In addition to a wide window of opportunity for harvest, a similarly large window exists for planting the potato seed pieces. Whole or cut seed pieces are generally planted once the temperature of the soil has risen sufficiently after thawing and unlikely to refreeze after seed pieces are in the ground. Many of the cultural practices involving how and when to plant seed pieces have been developed for the management of early-season diseases especially stem and stolon canker caused by Rhizoctonia solani. Although current recommendations suggest not planting until the soil is 15°C, as this promotes quick emergence (Secor, 2008; Banville et al., 1996), there are few recent evaluations of the effect of soil temperature on disease incidence and severity of potato varieties. Simons and Gilligan (1997) examined three different planting times, and found that as the soil temperature increased (with later planting) the incidence and severity of stem canker decreased. However, the planting times were apparently based upon time (2 to 3 weeks apart) instead of the measured soil temperature. In this set of experiments, the effect of soil temperature on stem and stolon canker, and black scurf incidence and severity, was examined on several potato varieties. Ten potato cultivars commonly grown in Michigan in 2004 and eight in 2005 were evaluated at three different planting times based upon the soil temperature surpassing a threshold. Planting occurred once the temperature at 15 cm (approximate planting depth) 42 as a five day average surpassed thresholds of 8, 14 and 20°C. These temperatures were chosen to provide a testable range, within which planting was typically done in Michigan, either commercially or at the home garden level. In addition, local historical soil temperature (10cm) data (Michigan Automated Weather Network, Michigan State University: http://www.agweather.geo.msu.edu/mawn/) were examined to determine possible temperature thresholds (Figure 3.1). A five—day average was used to reduce fluctuations that could result in targeted plantings within a short (and insignificant) time period. The temperature thresholds resulted in an average of 21 .5 days between plantings in 2004, and 35 days in 2005. The main questions this experiment attempted to answer were: 1) Does planting later result in a reduction in stem and/or stolon canker and/or black scurf, compared to early planting? 2) Due to the shorter growing season, does planting later result in a reduction of yield? 3) Do varieties differ for the effect of planting date on the incidence and severity of stem and stolon canker and black scurf? Materials and Methods The experiment was located at the Plant Pathology Farm (Michigan State University) in East Lansing, MI. The seed pieces were hand-planted into pre-hilled sandy soil in a split-plot randomized complete block design with temperature-determined planting timing as the major split. All plantings had three replications, and within each replicated plot were four rows, three meters long and spaced one meter apart. Nine seed pieces per row were spaced 30 cm apart, for a total of 36 seed pieces total per replication. Table 3.1 lists the cultivars, and their common usage, that were planted in both years. 43 The temperature of the soil at 15 cm (approximate planting depth) was measured hourly by a Cole-Parmer 12 channel scanning thermometer (one at each planting) and a five-day moving average of three temperature probes was calculated. This moving average was monitored, and planting occurred once the threshold of 8, 14, or 20°C was surpassed. In 2004, plantings occurred on April 19th, May 17th, and June 28‘“, while in 2005 they occurred on April 215‘, May 9th, and June 3”. Plot Maintenance was similar for both growing seasons. Drip irrigation lines were setup, after planting, for each plot and used regularly to maintain adequate moisture for plant growth. Weeds were controlled by hand as needed. The fungicide propamocarb (Previcur Flex) was applied biweekly (at 1.05L/hectare) to prevent foliar late blight (Phytophthora infestans). Insects were controlled with one in-furrow application of imidacloprid (Admire 2F ) at 1.46 L/ha at-planting. The field was inoculated in 2004 with an AG-3 isolate of R. solani, MI-3, originally isolated from potato tuber sclerotia from Michigan. The isolate was grown on white millet that was twice autoclaved, for one hour on two consecutive days. 10 covered trays (23 x 40 cm) were each inoculated with one Petii plate (100 x 10mm) of the actively growing isolate. The isolate was allowed to colonize the millet seed for 21 days, after which time the inoculated millet was spread out on large (40 x 60cm) trays and dried for 3 days. One batch of 10 trays was created for each planting time and was broadcast over the section of the field to be used, just prior to cultivation and hilling for the plots. The field was not inoculated in 2005 due to contamination of the inoculum intended for the first planting. 44 In 2005, emergence counts were taken for each plot of each planting time, 8, 14 and 20°C, for 62, 53, and 38 days afier planting, respectively. This was then converted to the relative area under the emergence progress curve (RAUEPC) for each plot. Higher values of RAUEPC indicate earlier emergence relative to the length of time in which evaluations are made. In both 2004 and 2005, a mid-season sample from each plot was made (Table 3.2), with the exception of the third (20°C) planting in 2004. In both years samples were taken from one of the two center rows from each plot, with two plants per plot evaluated in 2004 and four plants in 2005. During this evaluation, the number of stems, stolons and tubers were counted. In addition, the incidence of cankers/girdling, caused by R. solani, on stems and stolons was counted and converted to a percentage of the total. Also, in both 2004 and 2005, samples of four plants were taken at the end of the season (Table 3.2), to evaluate yield, and a subset of 20 tubers per replicate were evaluated for the incidence and severity of black scurf. Black scurf incidence was a count of the number of tubers with visible sclerotia on the skin. Black scurf severity was evaluated as the number of tubers with a specific amount of surface area covered by sclerotia. A score of 0 to 4 was possible for each tuber, with 0 = 0%; l = 1 - 5%; 2 = 6 - 10%; 3 = 11 — 15%; 4 = > 15% surface area of tuber covered with sclerotia. The number in each class was multiplied by the class number and summed. The sum was multiplied by a constant to be expressed as a percentage. Indices of 0 - 25 represent 0 - 5%; 26 - 50 represent 6 - 10%; 51 - 75 represent 11 - 15% and 75 - 100 >15% of the average surface area covered with sclerotia. 45 All data were analyzed by AN OVA and tested with Tukey’s Honestly Significant Differences (HSD), p=0.05 (JMP: SAS Institute, Cary, NC, USA). Figure 3.1 Historical soil temperatures (10cm) around planting time for potatoes East Lansing, IVfl, 2000 to 2003. :5 7.5 :3: Q) Q. 2 <1) [-1 O 1 I I I I I T f e o «x 8 «\ 8 if 0‘” 0“” 0'” 5° 0‘ 0‘ a” a”? 6 0‘ 0"} Date I ----- 2000 ----2001 ———2002 2003 —4 year Average J 46 Table 3.1 Cultivars planted during the 2004 and 2005 experiments evaluating the effect of planting time, based upon soil temperature, on disease incidence and severity caused by Rhizoctonia solani; Plant Pathology Farm, Michigan State University, East Lansing MI Variety 2004 2005 Main Use Atlantic X chip FL1833 X chip FL 1 867 X X chip FL 1 879 X X chip Jacqueline Lee X X table Michigan Purple X X table M81 1 52-A X table (advanced breeding selection)Z M 81201-2PY X table (advanced breeding selection) Pike X X chip Russet Norkotah X X table Snowden X X chip 2: Advanced breeding selection provided by the Potato Breeding and Genetics Laboratory of Michigan State University, East Lansing, MI Table 3.2 Important dates and relative number of days afier planting (DAP) for each planting time of the 2004 and 2005 experiments evaluating the effect of planting time, based upon soil temperature, on disease incidence and severity caused by Rhizoctonia solani. Plant Pathology farm, Michigan State Universig, East LansinfiMl. Planting Mid-season Vine Temperature Temperature sampling desiccation Fall Harvest Threshold Threshold Planting Year (0C) Surpassed date date DAP date DAP date DAP 2004 8 4/18 4/19 7/8 80 9/17 151 10/4 168 14 5/11 5/17 7/28 72 9/17 123 10/4 140 20 6/10 6/28 N/A N/A 9/17 81 10/4 98 2005 - 8 4/18 4/21 7/27 97 9/8 140 9/27 159 14 5/11 5/11 7/27 77 9/8 120 9/27 139 20 5/31 6/3 7/27 54 9/8 97 9/27 1 16 47 Results In both 2004 and 2005 differences in several variables were found between planting times. In 2004, planting early (8°C, compared to 14°C) resulted in fewer tubers at the mid-season harvest, fewer stolons and more disease symptoms (percent of stems and stolons with girdling, and black scurf incidence and severity; Table 3.3). Planting early resulted in higher yield than at 14°C, although not significantly different than at 20°C. No significant differences were found between planting at 14°C and 20°C for black scurf incidence and severity, and yield. In 2005, planting early resulted in slower emergence (RAUEPC) and lower number of stems per plant compared to planting at 20°C and higher percent of stems and stolons with girdling and yield (Table 3.4). However, there were no differences among planting times for the number of stolons per plant or black scurf incidence or severity. In contrast to 2004, early planting in 2005 resulted in a higher number of tubers per plant. In 2004 differences were found among cultivars for most of the agronomic variables: numbers of tubers, stems and stolons per plant (Tables 3.3 and 3.5). However, using Tukey’s Honestly Significant Differences (HSD) test, the number of tubers per plant was found to be not significantly different. Also, no differences were found for any disease variables. In 2005 significant differences were found among cultivars planted for all variables tested, except the percent of stems with girdling and yield (Table 3.4 and 3.6). The interaction of planting time and cultivar also resulted in differences in both years. In 2004 Michigan Purple had the highest number of stolons per plant and the highest yield (Tables 3.7 and 3.8). Russet Norkotah had no stems with girdling, 48 significantly less than Atlantic’s 67.6% (however neither was significantly different than any other cultivar). In 2005 Michigan Purple again had the highest number of stolons per plant (at 8°C and 14°C) and highest yield at 20°C (Tables 3.7, 3.9, and 3.10). Snowden had the highest number of stems per plant at all three plantings. 49 €83an0 Om: $325.5 mod M Q 8 382.23 32805an 3: 08 8:2 2:3 05 .3 3323.2 3285 u .m 3 3.2 3% 3 38 .N 3 3.3 33 03 ._ 3 3m 93 mm: 328 £38232 9 08232283 v .3 22» 05 Sea 38:23 3830539 2.33V 22> n: .m 3 3.2 93 no 38 .N 3 3.2 32V cv. ._ 3 3.3 33 wS 328 23.83 :23 893 05 .3 38 828m fem—A 028.. 2: 330 2: - mu ”fem. - Z 033 05 330 mu - E ”foo— - c owafi 05 830 on - mN me\em - o 038 05 330 N 9 o .3 3035 63333 28 .o=_8> 820 33032 35 .3 32322: 35 083 32? 035 83328 :23 383m 38 3.33 2.3—A u v ”fem— - 2 u m ”e32 - o u N gm - _ u _ me\eo n o 3.20 33 85 358.2 38232 .83 ON .1. 5 39:5: 393 05 wasp—=3 .3 38323 335 $58 329 .3 333m S .m 3 3m 3% ma 28 .N 3 3.“ 33 oz ._ 3 3.“ 93 mg 328 08230.. .83 233 ON .3 0389. 8 Bot 328 .M .3 2323. 53 £33 .3 3:232 3883 as N 3 3.2 .83 N» 98 ._ 3 3% 93 cm 323 308232 8 08232 83 383 N .3 03.88 8 Bob 08 .332. 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Number of Number of Number of Cultivar planted tubers/plantz stems/plantz stolons/plantZ Atlantic 8.2 ay 3.6 b l 1.8 c FL 1 833 14.9 a 4.7 ab 20.6 abc FL1867 l 1.3 a 3.8 b 14.4 bc FL 1 879 17.4 a 4.8 ab 25.8 ab Jacqueline Lee 14.7 a 3.4 b 14.3 bc Michigan Purple 21.5 a 5.0 ab 26.9 a MS1201-2PY 22.3 a 7.3 a 23.1 abc Pike 10.8 a 2.8 b 12.8 c Russet Norkotah 14.2 a 4.0 b l 1.3 c Snowden 23.8 a 5.5 ab 19.8 abc 2: Numbers of tubers, stems, and stolons are the average of 2 plants per replicate (3 replicates) taken 80 days after planting (dap) for pt 1; and 72 dap for pt 2 y: Values followed by the same letter are not significantly different at p = 0.05 (Tukey's HSD Comparison) 52 Table 3.6 Efi'ect of cultivar planted on agronomic factors and disease symptoms caused by Rhizoctonia solani in field studies at the Plant Pathology farm, MichiggState University; East LansinhMl in 2005. Emergence Number of Number of Number of Cultivar planted (RAUEPC)z tubers/planty stems/planty stolons/planty FL1867 0.0381 bx 14.1 bcd 4.5 bc 15.3 cd FL1879 0.0492 a 16.9 bc 5.4 ab 25.8 ab Jacqueline Lee 0.0278 cd 14.3 bcd 3.7 bc 15.1 cd Michigan Purple 0.0478 a 23.8 a 4.1 bc 29.9 a MS1152-A 0.0202 (1 12.1 cd 3.5 c 14.6 cd Pike 0.0344 bc 9.5 d 3.3 c 11.6 d Russet Norkotah 0.0480 a 16.3 be 4.2 bc 12.2 d Snowden 0.0516 a 19.5 ab 6.3 a 20.9 bc Percent of stolons Black scurf Black scurf with girdlingW incidencev severityu FL1867 36.1 ab 9.4 ab 3.5 ab FL1879 32.4 abc 10.6 ab 3.5 ab Jacqueline Lee 30.9 abcd 7.5 ab 2.3 b Michigan Purple 12.2 d 2.8 b 1.0 b MS1152-A 15.0 cd 4.2 b 1.0 b Pike 19.9 bcd 7.2 b 3.5 ab Russet Norkotah 39.5 a 17.8 ab 5.3 ab Snowden 33.0 abc 24.4 a 10.8 a z: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to 62 days after planting (dap) for pt 1, 53 dap for pt 2, and 38 dap for pt 3 y: Numbers of tubers, stems, and stolons are the average of 4 plants per replicate (3 replicates) taken 97 days after planting (dap) for pt 1, 77 dap for pt 2, and 54 dap for pt 3 x: Values followed by the same letter are not significantly different at p = 0.05 (T ukey's HSD Comparison) w: Percent of stems and stolons with girdling caused by R solani are from an average of 4 plants per replicate (3 replicates) taken 97 dap for pt 1, 77 dap for pt 2, and 54 dap for pt 3 v: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken 168 dap for pt 1, 140 dap for pt 2, and 98 dap for pt 3. u: Severity of black scutf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%;1=1— 5%; 2 = 6 - 10%; 3 =11 -15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia taken 159 dap for pt 1, 139 dap for pt 2, and 116 dap for pt 3. 53 8 80 :00 0: 0:0 .m 8 .80 800 on: ._ 8 :00 800 an: :00—0: 0000:0380: m0 0:00:80:\m::08 0 00 208 05 80.: 0000—30—00 8:080:58: 050:: 0.0; u: m a :0: 000 0: : 000 .0 :0 :0: 000 0...: .: :0 :0: 000 02 000:0: 0000 a :0 :0: 000 00 000 .0 :0 :0: 000 0: ._ :0 :0: 000 00: 000:0: 0020.00 5:3 .000: 0500 00:0 00005.0. 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"Soon: 0 09C. .04.. 0300. :_ 85: >0 0:0 8 00 8:00:08: 0o :02: .8800 058830 :0 00 >0 0:0 8 00 8:00:08: 0:000:90 "A308 0 09C. .w.m 030... 5 :305: 00808 >000 8 . 8:00:08: 00 :00E .God A 8 >0 0:0 8 00 8:000:08: 0:005:30 "€83 m 0&0. 03305 85.50 0: .33: v 3 >0 0:0 8 00 8:00:08: 0:005:30 0: "$83 < 09C. .A>0v :0>:_:0 0:0 33 0:5 w:::08 00 8:00:08: 05 5:3 000:0:0005 00 5:00:08 05 .3 0058:0000 0:00:05 00:08.00: ”N 0 0000.0 0000.0 0000.0 0 090.0 0000.0 0000.0 008000000: 0:005 H0:0; : 050.0 E :0 82.0 < 0:000 0000.0 0300 0003000300 0:008 0 0000.0 002 .0 0000.0 < 000:0 00:0 3000 0.800200: :38 0:008 0 0:000 0:000 :0000 v < ._ 0000.0 0:000 2000000 5:3 000.00 :0 008:0: 0 02 _ .0 :0000 v :0000 v 0 _. «000.0 0000.0 0000000200 :0 00.002 0 0000.0 «000.0 800.0 m .. 0000.0 0: :0 3050000 .08 0:000 :0 0.00:0: 0 200.0 :0000 v 0000.0 0 .. 200.0 0000.0 000200.020 :0 00502 0 00:00 :0000 v 0000.0 < .. 000: .0 _ E0 000200.003 :0 00052 U 38.: V 2:56 V 208.: V 0 0. .0 .0 fiUmmDév oocow:08m 0:305: 0.. 00 :0 0.. z a 0.. 0 2 00:00:00... 0.. 00 A... 0.. 3 a 0.. 0 9 02000 0.00::0> 0_D0_00> :0 003230 6:0 ogulwflflcflm 030000> :0 0020—30 van QESUWEHEE 00 8:00:08: 0500 00000”: ”moom 00 8:00:08: 05 0: 800m ”33 .0000 0 coon :_ :2 0:00:04 :00: 58:03:: 008m :0wE0:>_ .:::00 308508 0:03 05 :0 0.0:: 0.20 :_ .3080. 0::8800Et .3 000000 0:88:50 000005 0:0 8:000 0_Eo:o:w0 :8 00808 03:80 0:0 0E: w:::08 :003000 8:00:08: 05 00 Rodi: ”00:0:0005 0o £30880 0000.0:0 080000 502 Em 030.0 54 Table 3.8 Interaction effect between planting time and cultivar on select agronomic factors and disease symptoms caused by Rhizoctonia solani during field studies at the Plant Pathology farm, Michigan State University; East Lansing, M1 in 2004. Timing] Timing3 (8°C) Timing 2 (14°C) (20°C) Percent of stems Number of Number of Number of Yield: metric with Yield: metric Cultivar stolongplantZ stems/plantz stolons/plantz tons/hectarey girdlingZ tons/hectarey a Atlantic 9.5 b" 3.8 b 14.0 b 15.0 c 67.6 .. 24.6 a FL1833 15.0 b 5.3 ab 26.2 ab 22.0 abc 26.9 ab 20.1 a FL1867 11.5 b 4.5 b 17.3 ab 15.8 be 58.1 ab 17.6 a FL1879 17.5 ab 6.3 ab 34.2 a 26.0 ab 41.9 ab 18.2 a Jacqueline Lee 14.2 b 3.2 b 14.3 b 17.6 be 52.7 ab 24.4 a Michigan Purple 31.0 a 4.5 b 22.8 ab 29.] a 64.7 ab 29.3 a MS1201-2PY 19.5 ab 9.5 a 26.7 ab 21.1 abc 49.6 ab 20.4 3 Pike 13.8 b 2.5 b 11.8 b 12.7 c 31.3 ab 19.1 a Russet Norkotah 13.2 b 4.0 b 9.5 b 14.2 c 0.0 b 17.0 a Snowden 16.0 ab 5.7 ab 23.7 ab 18.1 be 56.7 ab 18.5 a 2: Numbers of tubers, stems, and stolons are the average of 2 plants per replicate (3 replicates) taken 80 days afier planting (dap) for pt 1; and 72 dap for pt 2 y: Yield (metric tons/hectare) calculated fi'om the yield of 4 p1ants/rep1icate(3 replicates); taken 168 dap for pt 1, 140 dap for pt 2, and 98 dap for pt 3. x: Values followed by the same letter are not significantly difi'erent at p = 0.05 (Tukey's HSD Comparison) 55 Eoflaqfioo Om: mane—PC mod n m E 38ch £23556 8: 2a .532 2:8 05 an 332.8 32m> ”3 .m a .8 a8 2. can .m a 8.2 96 a2 ._ a 8m mac a2 :83 ”$8823.. a 88:90:35..“ v 20 203 05 Set 35:28 338.32: 0.525 22> ”x m a he 93 E 25 .m a he 9% E. ._ a 8.2 Ema wags—a 8cm 3% 3 :83 $08232 3 882%.. 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QM can @6— na 335 ohms“. a odm on 5.: on m.m can v.2 3% «35.0 new _ ur— a w.mm a mi a iv an mdm on 236 covaocm a h. _ m n m6 an m.m n 02 at 236 538.82 833— “ mém n h: n _.m n «.2 («ow memod 8?— a ~.m~ a c6. an ad an 0.3 .“ 2 86 «ran _ 52 a n? a 2;. an 3 a ”.8 a 82; 2&3 53:22 mom: a 93 n ”.2 on N6 an v.3 no 386 84 05338.. a «fin a Sm a ed a QB pm :86 033m a ”.2 n «.2 a 9v 3 Q: 33 :23 $24.... $380522 baa—Emco—om dam—EmEBm £53233 Ngmaév 832:0 2383809 252: 63; mo 69:52 go 69:52 ho 52:32 oocowhoEm was—BE .88 5 :2 £583 Emfliozcs 28m 59:22 .55 322:9. 28E 2: E 8633. 20c wctzu 282a 2838? :0 52:3 can 2:: macaw—m c853 “onto 53825 Qm 2an Table 3.10 Interaction effect between planting time and cultivar on disease symptoms caused by Rhizoctonia solani during field studies at the Plant Pathology farm, Michigan State University; East Lansing, MI in 2005. Percent of Percent of Planting stems with stolons with Black scurf Black scurf Temgrature Cultivar fldlfiz girdlingZ incidencey severityx 3°C FL1867 49.9 abw 54.0 a 16.7 a 7.5 a FL1879 23.8 b 49.1 a 18.3 a 5.8 a Jacqueline Lee 41.2 ab 30.8 abc 13.3 a 4.6 a Michigan Purple 61.7 a 22.9 be 3.3 a 0.8 a MS1152-A 43.8 ab 9.6 c 5.0 a 1.3 a Pike 63.9 a 14.0 c 1.7 a 0.4 a Russet Norkotah 62.9 a 39.6 ab 13.3 a 4.2 a Snowden 66.9 a 49.8 a 15.0 a 4.6 a 14°C FL1867 41.5 ab 43.7 ab 8.3 a 2.1 a FL1879 41.3 ab 33.7 abc 8.3 a 2.1 a Jacqueline Lee 19.0 b 23.2 abc 1.7 a 0.4 a Michigan Purple 23.1 ab 9.0 c 5.0 a 2.1 a M81 1 52-A 38.8 ab 18.9 bc 3.3 a 0.8 a Pike 30.7 ab 32.8 abc 20.0 a 10.0 a Russet Norkotah 59.2 a 57.1 a 20.0 a 5.8 a Snowden 13.3 b 40.4 abc 31.7 a 13.3 a 20°C FL1867 23 .2 a 15.3 b 3.3 b 0.8 b FL1879 1.7 a 9.5 b 5.0 ab 2.5 ab Jacqueline Lee 1.8 a 42.5 a 7.5 ab 1.9 ab Michigan Purple 10.8 a 0.8 b 0.0 b 0.0 b MSIISZ-A 31.7 a 17.2 ab * * Pike 13.2 a 12.8 b 0.0 b 0.0 b Russet Norkotah 9.0 a 21.9 ab 20.0 ab 5.8 ab Snowden 2.8 a 8.9 b 26.7 a 14.6 a z: Percent of stems and stolons with girdling caused by R solani are from an average of 4 (2005) plants per replicate (3 replicates) taken 97 days afier planting (dap) for pt 1, 77 dap for pt 2, and 54 dap for pt 3. y: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate (3 replicates) taken 159 dap for pt 1, 139 dap for pt 2, and 116 dap for pt 3 x: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 =0%;1=1- 5%; 2 =6 - 10%; 3 =11 - 15%;4=>16% surfaceareacovered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia taken 159 dap for pt 1, 139 dap for pt 2, and 116 dap for pt 3 w: Values followed by the same letter are not significantly different at p = 0.05 (Tukey's HSD Comparison) 57 Discussion and Conclusions Planting early in the spring, when soil temperatures are relatively low (around 8°C) resulted in more disease symptoms and less healthy plants. This general result was found among planting times both in the presence (2004) and absence (2005) of additional inoculum. The differences among planting times for percent of stems and stolons with girdling is most likely the result of two factors, soil temperature and inoculum. Bolkan et al. (1974) found that infection of potato shoots decreased with increasing temperature (15, 18, 21, 24°C) when (tuber-borne) inoculum levels were “low”, but did not change with “moderate” and “high” levels of inoculum. Similarly, Simons and Gilligan (1997) found that later dates of planting resulted in a decrease of stem canker (girdling), especially with a “high” level of inoculum (AG-3). Also, Carling and Leiner (1990) found that isolates of AG-3 were able to attack potato sprouts at all three temperatures tested (10, 15.5, and 21.1°C), but caused significantly more damage at 10°C. The results of the 2004 and 2005 growing seasons agree with previous findings, that as the soil temperature at planting increases, the incidence of stem and stolon canker decreases. In 2005, there may have been sufficient inoculum either already in the soil or on the seed to cause Rhizoctonia diseases. This was especially evident in 2005, when no inoculum was incorporated prior to planting, and significant differences were still found for percent of stems and stolons with girdling. Potatoes had been planted in the field used many times prior to 2004, and although not measured in this field, it would be expected that the level of soil-borne inoculum would have built up over time. While the presence or absence of additional inoculum may not have played a major role in the percent of stem and stolon canker, it may have made the difference for 58 the significant differences among planting times for black scurf incidence and severity. In 2005, when supplemental inoculum was not added, there were no differences among planting times for black scurf. The significant differences of black scurf incidence and severity found among planting times in 2004 may have two possible sources. First, there was likely an increase in inoculum level, both naturally over the course of the growing season, and as a result of the addition of the inoculum prior to planting. Tsror and Peretz- Alon (2005) found that black scurf incidence and severity were higher when both seed and soil-bome inoculum were present than either inoculum source separately. In addition, the level of inoculum present was also positively correlated with the incidence and severity of black scurf on progeny tubers (Tsror and Peretz-Alon, 2005). Second, the plots of the early planting were more physically mature at vine desiccation. Although the 17 days between vine desiccation and harvest falls within the recommended time frame to harvest of within three weeks (Gudmestad et al., 1979), the early planting of 2004 was already senescing at vine desiccation. This fact may have increased the likelihood that the increased inoculum would form sclerotia, triggered by the reduction of plant exudates (Spencer and Fox, 1979). Growing seasons vary considerably in Michigan, depending upon location, and waiting to plant until the soil warms to 14 or 20°C may result in a significantly shorter growing season, which could negatively affect the yield. In both years, the early planting (around 8°C) resulted in higher yield. While this is expected when considering the length of the growing season, it is in contrast to the early planting having significantly higher percent of stems and stolons with girdling in both 2004 and 2005. It is expected that if there is significantly high level of disease pressure that the yield would be decreased, as a 59 number of researchers have reported significant yield losses (Banville et al., 1996). All planting times were vine desiccated and harvested on the same dates. Had the later plantings been allowed to continue growing until natural senescence, differences in yield may not have been found. Differences among varieties were expected, with the majority being agronomic factors, such as tuber, stem and stolon number and yield. No variety was noted as resistant to R solani prior to the experiments and all had stem and stolon canker as well as black scurf to varying degrees. As to sorting out the varieties in terms of recommendations for planting, it would require a number of factors to be assessed. Some factors needing evaluation include: the end-product that the potatoes will become (chip processing or table stock; the level of known disease pressures present in the field (R. solani, Phytophthora infestans, Streptomyces scabies etc.); the maturity of the particular variety (early, moderate, or late); the harvest window (prior to, at or after senescence); and the level of management (cost/energy). These experiments did confirm previous findings of reduced disease incidence and severity with later planting. A point of improvement in this experiment may be the testing of the method for establishing planting thresholds. The five day average was chosen to minimize large fluctuations in temperature, but other timeframes could be tested. A larger number of days averaged would continue to smooth the trend of increasing temperature, while a shorter one could lead to premature planting. Another point for improvement would be to use a vine desiccation and harvest date based upon the maturity of the plot, instead of the date of the first planting. While this would not affect stem and stolon canker, it could positively impact the yield of later plantings. 60 CHAPTER 4 EFFECT OF FUNGICIDE APPLICATION TIMING AND SOIL TEMPERATURE AT PLANTING ON VARIETY SUSCEPTIBILITY TO THE INCIDENCE AND SEVERITY OF RHIZOCTONIA CANKER AND BLACK SCURF ON POTATOES Introduction The combination of chemical and cultural control strategies, as part of an integrated pest management strategy, is generally the most effective and sustainable method of disease management. The final experiment, in 2006, tested the combined effect of fungicide efficacy and planting time based upon soil temperature, with multiple varieties, for the management of diseases caused by Rhizoctonia solani. As this experiment incorporated both cultural and chemical control strategies, it more closely resembled actual practices that could be utilized by Michigan potato growers. Treatments from the previous experiments (see Chapters 2 and 3) were combined and tested for their efficacy. Two temperature thresholds were used, 14 and 20°C to trigger the planting of four cultivars (commonly grown in Michigan). In addition to the seed treatment (fludioxonil), two early-season applications (in-furrow and 14 days post- emergence) were evaluated for two of the previously tested fimgicides (azoxystrobin and flutolanil). The main questions this experiment attempted to answer were: 1) Does a particular combination of planting date and early season fimgicide application result in a reduction of disease incidence and severity (stem and/or stolon canker, and/or black scurf)? 2) Does a late planting and non-treated seed result in lower yield? 3) Is one type of control, cultural (planting based upon soil temperature) or chemical (using seed/early- season fungicide(s), more effective at disease management? 61 Materials and methods The potato cultivars F L1 833, F L1 879, Russet Norkotah, and Superior were planted at the Michigan State University Muck Soils Research Farm, Bath, MI. All cultivars were tested for the control of Rhizoctonia disease symptoms under identical chemical regimes at two planting times. Within each regime, the efficacy of the seed treatment fludioxonil (Maxim) alone or in combination with one of two additional fungicides, azoxystrobin (Amistar) and flutolanil (Moncut) were examined. The additional fungicides were tested at two application times, at-planting in—firrrow and 14 days after emergence. Seed pieces were planted once the soil temperature at a 10 cm depth surpassed a threshold of a five-day average of 14°C (threshold surpassed 4 May 2006, planted 9 May 2006) or 20°C (threshold surpassed 30 May 2006, planted 1 June 2006). The soil temperature was monitored with the onsite weather station (Michigan Automated Weather Network; Michigan State University, East Lansing, MI; http://www.agweatherggxmsuedu/mawn/l Data was regularly downloaded and daily and five-day averages were calculated. Except for the cultivar Superior, which was out prior to seed treatment, whole seed was treated with the Maxim seed treatment one day prior to planting. Seed pieces were planted in Houghton muck soil at the Michigan State University Muck Soils Research Farm, Bath, M1 on 9 May (timing 1) and 1 June (timing 2), into two rows by 4.6 meter plots (approximately 30.5 cm between plants give a target population of 30 plants per plot at 1 m row spacing) replicated three times in a randomized strip block design. 62 The fludioxonil-seed treatment was applied, in a water suspension at a rate of 0.5mL/kg, and applied onto the entire seed surface. In-firrrow applications were made over the seed at-planting, applied with a single nozzle R&D spray boom delivering 46.8L/ha (551.6 kPa) and using one XR11003VS nozzle per row. Table 4.1 Experimental layout (for one of four potato cultivars: FL 1 833, FL1879, Russet Norkotah, and Superior) for the study of the effects of soil temperature at planting, the use of a seed treatment, additional fungicide and the application timing of the additional fungicide on agronomic factors and disease symptoms, caused by Rhizoctonia solani, in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. Soil temperature Application timing for additional fungicide threshold at Treatment plantirg Seed treatment ln-fiirrow 14 day post—emergence 1 1 4° C Yes Amistar 2 Amistar 3 Moncut 4 Moncut 5 6 N0 Amistar 7 Amistar 8 Moncut 9 Moncut 10 1 1 2 0°C Yes Amistar 12 Amistar 13 Moncut 1 4 MOHCUI 15 l 6 NO Amistar 18 Moncut '9 Moncut 20 Fertilizer was drilled into plots before planting, formulated according to results of soil tests at the Bath, MI location. Porpamocarb hydrochloride (Previcur Flex) was 63 applied at 1.4 L/ha on a ten day interval, total of four applications, starting two weeks after the last application of experiment treatments. A permanent irrigation system was established prior to the commencement of fungicide sprays and the fields were maintained at soil moisture capacity throughout the season by frequent (minimum 5-day) irrigations. Weeds were controlled with metolachlor (Dual 8E) at 2.34 L/ha and metribuzin (Sencor 75DF) at .67 kg/ha at planting, bentazon (Basagran) at 2.34 L/ha 27 and 45 DAP (days after planting) and sethoxydim (Poast) at 1.75 L/ha 63 DAP. Insects were controlled with thiamethoxarn (Platinum) at 0.59 L/ha at-planting, cyfluthrin (Baythroid 2) at .146 L/ha 36 and 60 DAP. Emergence was rated as the number of plants breaking the soil surface or fully emerged after planting. The rate of emergence was estimated as the area under the plant emergence curve (max=100) from the day of planting until 45 DAP for planting l (14°C) and 28 DAP for planting 2 (20°C). Tuber, stem and stolon numbers, and percentages of stems and stolons with girdling caused by R. solani were measured via a destructive mid- season harvest (4 plants per replication) at 69 DAP (timing 1) and 61 DAP (timing 2). Vines were killed with Reglone 2EC (1 pt/A) on 25 August and 20 progeny tubers were harvested from each plot on 15 September and the individual treatment replications were washed and assessed for black scurf (R. solani) incidence (%) and severity. Severity of black scurf was measured as an index calculated by counting the number of tubers (n = 20) falling inclass 0 = 0%; 1 =1 - 5%; 2 = 6 -10%; 3 =11 —15%;4 = >15% surface area of tuber covered with sclerotia. The number in each class was multiplied by the class number and summed. Indices of 0 - 25 represent 0 - 5%; 26 - 50 represent 6 - 10%; 51 - 75 represent 11 - 15% and 75 - 100 >15% surface area covered with sclerotia. 64 Results Tubers planted once the soil at 10 cm surpassed a five day average of 20°C resulted in significantly faster emergence, higher average number of stems, higher percentage of stems and stolons with girdling, and black scurf incidence and severity (Table 4.2). Of the four cultivars planted, plots of Russet Norkotah resulted in significantly faster emergence, higher average number of stems per plant and percent of stems with girdling (Table 4.3). In addition, plots of Superior had a significantly slower emergence and lower average number of stolons per plant. Plots of FL] 833 had significantly lower average number of stems per plant, and plots of F L1 879 had significantly lower percent of stolons with girdling when compared to the other cultivars planted. The use of the seed treatment fludioxonil, compared to no seed treatment used, resulted in significantly higher average number of stems and stolons per plant, and significantly lower percent of stolons with girdling (Table 4.4). The use of azoxystrobin resulted in a significantly lower average number of stolons per plant, when compared to no additional fungicide being used (Table 4.5). However, in terms of average number of stolons per plant, there was no significant difference between plots with azoxystrobin and flutolanil. Also, plots with flutolanil resulted in significantly lower percent of stolons with girdling when compared to plots with no additional fungicide, but there was no significant difference between plots with flutolanil and plots with azoxystrobin. Fungicides that were applied in-furrow resulted in significantly lower average number of stolons per plant, compared to no additional fungicide applied, but were not significantly different from fungicides applied 14 days after emergence (Table 4.6). Also, fungicides 65 that were applied 14 days after emergence resulted in significantly lower percent of stolons with girdling, compared to no additional fungicide applied, but were not significantly different than fungicides applied in firrrow. The interaction of these treatments (planting based upon soil temperature, seed treatment, additional fungicide, and the timing of application of the fungicide) resulted in a number of significant differences within each cultivar (Tables 4.7 — 4.1 1). However, there was not a single (combined) treatment that proved more effective for R. solani management or improved agronomic variables on all cultivars. Table 4.2 Main effects analyses (probability of difference: p=0.05) of the effect of soil temperature at planting on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. Effect of planting date on variable Variable pvalue 14°C Q/9/2006L 20°g6/1/2006) RAUEPC (emergence)Z 0.0496 0.0626 bu 0.0653 a Number of stemsy 0.0007 2.9 b 3.3 a Percent of stems with girdlingx <0.0001 30.0 b 61.3 a Number of stolonsy 0.1433 19.4 a 18.0 a Percent of stolons with girdlingx <0.0001 16.8 b 24.0 a Black scurf incidenceW 0.0009 13.4 b 21.7 a Black scurf severityv <0.0001 4.0 b 8.5 a z: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to 45 days after planting (dap) for planting time (pt) 1, and 28 dap for pt 2 y: Numbers of stems and stolons are the average of 4 plants per replicate (3 replicates) taken 69dap for pt 1; and 61 dap forpt2 x: Percent of stems and stolons with girdling caused by R solani are from an average of 4 plants per replicate (3 replicates) taken 69 dap for pt 1, and 61 dap for pt 2 w: Percent incidence of tubers with sclerotia of R solani from a sample of 20 tubers per replicate taken 129 dap for pt 1, and 106 dap for pt 2 v: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%; l = l - 5%; 2 = 6 - 10%; 3 =11 -15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia taken 129 dap for pt 1, and 106 dap for pt 2 u: Values followed by the same letter are not significantly different at p = 0.05 (Tukey's HSD Comparison) 66 Table 4.3 Main effects analyses (probability of difference: p=0.05) of the effect of cultivar planted on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. Effect of cultivar planted on variable Russet Variable p-value F L1 833 FM 879 Norkotah Superior RAUEPC(emergence)Z <0.0001 0.0661 15“ 0.0656 b 0.0715 a 0.0527 c Number ofstemsy <0.0001 1.9 d 3.3 b 4.7 a 2.6 c Percent of stems with girdlingx <0.0001 44.9 b 40.3 b 60.1 a 37.2 b Numberofstolonsy <0.0001 17.4 b 32.3 a 15.0 b 10.0 c Percent of stolons with girdlingx <0.0001 20.8 b 16.0 c 26.3 a 18.6 bc Black scurfincidenceW 0.2462 18.5 a 21.3 a 15.3 a 14.9 a Black scurfseverityV 0.0488 7.4 a 7.8 a 5.6 a 4.0 a z: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to 45 days after planting (dap) for planting time (pt) 1, and 28 dap for pt 2 y: Numbers of stems and stolons are the average of 4 plants per replicate (3 replicates) taken 69dap for pt 1; and 61 dap forpt2 x: Percent of stems and stolons with girdling caused by R. solani are from an average of 4 plants per replicate (3 replicates) taken 69 dap for pt 1, and 61 dap for pt 2. w: Percent incidence of tubers with sclerotia of R solani fi'om a sample of 20 tubers per replicate taken 129 dap for pt I, and 106 dap for pt2 v: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%;1=1- 5%; 2 = 6 - 10%; 3 =11-15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia taken 129 dap for pt 1, and 106 dap for pt 2 u: Values followed by the same letter are not significantly different at p = 0.05 (Tukey's HSD Comparison) 67 Table 4.4 Main effects analyses (probability of difference: p=0.05) of the effect of the use of the seed treatment Maxim (fludioxonil) on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. Effect of seed treatment applied on variable Variable p—value Maxim None RAUEPC (emergence)z 0.3581 0.0646 a" 0.0633 a Number of stemsy 0.0001 3.4 a 2.9 b Percent of stems with girdlingx 0.1244 43.6 a 47.6 a Number of stolonsy 0.0001 20.6 a 16.7 b Percent of stolons with girdlingx <0.0001 16.7 b 24.1 a Black scurf incidencew 0.2347 16.1 a 19.1 a Black scurf severityV 0.6087 6.0 a 6.5 a z: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to 45 days after planting (dap) for planting time (pt) 1, and 28 dap for pt 2. y: Numbers of stems and stolons are the average of 4 plants per replicate (3 replicates) taken 69dap for pt 1; and 61 dap for pt 2. x: Percent of stems and stolons with girdling caused by R solani are from an average of 4 plants per replicate (3 replicates) taken 69 dap for pt 1, and 61 dap for pt 2. w: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken 129 dap for pt 1, and 106 dap for pt 2 v: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%;1=1- 5%; 2 = 6 - 10%; 3 =11 - 15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia taken 129 dap for pt 1, and 106 dap for pt 2 11: Values followed by the same letter are not significantly different at p = 0.05 (1‘ ukey's HSD Comparison) 68 Table 4.5 Main effects analyses (probability of difference: p=0.05) of the effect of use of additional fungicide (Amistar. azoxystrobin, or Moncut: pyraclostrobin) on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, MI in 2006. Effect of additional fungicide applied on variable Variable p-value Amistar Moncut None RAUEPC (emergence)z 0.6591 0.0642 au 0.0633 a 0.0648 a Number of stemsy 0.0740 3.1 a 3.0 a 3.4 a Percent of stems with girdlingx 0.8446 44.8 a 46.4 a 45.8 a Number of stolonsy 0.0257 17.4 b 18.9 ab 20.9 a Percent of stolons with girdlingx 0.0253 20.0 ab 19.1 b 23.9 a Black scurf incidencew 0.7698 20.2 a 15.3 a 16.7 a Black scurf severitLv 0.8535 7.2 a 5.2 a 6.4 a z: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to 45 days after planting (dap) for planting time (pt) 1, and 28 dap for pt 2 y: Numbers of stems and stolons are the average of 4 plants per replicate (3 replicates) taken 69dap for pt 1; and 61 dap for pt2 x: Percent of stems and stolons with girdling caused by R solani are fiom an average of 4 plants per replicate (3 replicates) taken 69 dap for pt 1, and 61 dap for pt 2. w: Percent incidence of tubers with sclerotia of R. solani from a sample of 20 tubers per replicate taken 129 dap for pt 1, and 106 dap for pt 2 v: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%;1=1- 5%; 2 = 6 - 10%; 3 =11 - 15%;4 = >16% surface areacovered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 - 10%; 51 - 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia taken 129 dap for pt 1, and 106 dap for pt 2 11: Values followed by the same letter are not significantly different at p = 0.05 (Tukey's HSD Comparison) 69 Table 4.6 Main effects analyses (probability of difference: p=0.05) of the effect of application timing of additional fungicide on agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, M1 in 2006. Effect of application timing of additional fungicide on variable 14 days post No Variable p-value In fiirrow emergence application RAUEPC(emergence)z 0.8237 0.0631 a“ 0-0644 8 0.0648 a Numberofstemsy 0.1013 3.1 a 3-0 a 3.4 a Percent of stems with girdlingx 0.6167 47.0 a 44-2 a 45.8 a Number of stolonsy 0.0179 17.2 b 19-0 ab 20.9 a Percent ofstolons with girdlingx 0.0175 20.3 ab 18-8 b 23.9 a Black scurfincidencew 0.9618 16.9 a 18-6 a 16.7 a Black scurfseveriyv 0.9809 5.8 a 6-7 a 6.4 a z: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated fi'om the day of planting to 45 days after planting (dap) for planting time (pt) 1, and 28 dap for pt 2 y: Numbers of stems and stolons are the average of 4 plants per replicate (3 replicates) taken 69dap for pt 1; and 61 dap forpt2 x: Percent of stems and stolons with girdling caused by R. solani are from an average of 4 plants per replicate (3 replicates) taken 69 dap for pt 1, and 61 dap for pt 2 w: Percent incidence of tubers with sclerotia of R solani from a sample of 20 tubers per replicate taken 129 dap for pt 1, and 106 dap for pt 2 v: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%;1=1- 5%; 2 = 6 - 10%; 3 =11 - 15%;4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. Indices of 0 to 25 cover the range 0 - 5%; 25 - 50 cover the range 6 -10%;51- 75 cover the range 11 - 15%; 75 - 100 cover the range >15% surface area of the tuber with sclerotia taken 129 dap for pt 1, and 106 dap for pt 2 11: Values followed by the same letter are not significantly different at p = 0.05 (T ukey's HSD Comparison) 70 Table 4.7 Main effects analyses (probability of difference: p=0.05) of the interactive effect of soil temperature at planting, seed treatment, additional fimgicide and application time, on the cultivar planted for agronomic factors and disease symptoms caused by Rhizoctonia solani in field trials at the Muck Soils Research Farm, Michigan State University; Bath, M1 in 2006. Effect of interaction of planting date, seed treatment, additional fungicide and application time on variable FL 1 833 FL1879 Russet Norkotah Superior Variable p—value analysisZ p—value analysis p—value analysis p-value analysis RAUEPC (emergence)y 0.0007 A <0.0001 B 0.0005 C 0.0012 D Number of stemsx 0.1048 13 0.0802 E 0.8496 13 <0.0001 D Percent of stems with girdlingw <0.0001 A <0.0001 B <0.0001 C 0.0062 D Number of stolonsx 0.0196 A 0.0004 B <0.0001 C 0.1355 E Percent of stolons with girdlingw 0.0013 A <0.0001 B <0.0001 C 0.0033 D Black scurf incidencev 0.4240 E 0.4669 E 0.8951 E 0.7557 E Black scurf severityu 0.2209 E 0.2356 E 0.7700 E 0.7085 E 2: Response analysis determined by the probability of difference among treatments (interaction of soil temperature at planting, seed treatment, additional fungicide, and application timing of additional fungicide) for each cultivar planted. Type A= significant effect of treatment (p < 0.05) with the cultivar FL1833, mean of treatment effect on variable shown Table 4.8. Type B= significant effect of treatment (p < 0.05) with the cultivar F L1 879, mean of treatment effect on variable shown Table 4.9. Type C= significant effect of treatment (p < 0.05) with the cultivar Russet Norkotah, mean of treatment effect on variable shown Table 4.10. Type D= significant effect of treatment (p < 0.05) with the cultivar Superior, mean of treatment effect on variable shown Table 4.1 1. Type E= no significant effect of treatment (p > 0.05), no further analysis. y: RAUEPC: Relative area under the emergence progress curve (max = 100), calculated from the day of planting to 45 days after planting (dap) for planting date (pd) 1, and 28 dap for pt 2. x: Numbers of stems and stolons are the average of 4 plants per replicate (3 replicates) taken 69dap for pt I; and 61 dap forpt2 w: Percent of stems and stolons with girdling caused by R. solani are from an average of 4 plants per replicate (3 replicates) taken 69 dap for pt 1, and 61 dap for pt 2 v: Percent incidence of tubers with sclerotia of R solani from a sample of 20 tubers per replicate taken 129 dap for pt 1, and 106 dap for pt 2 u: Severity of black scurf (index calculated by counting the tuber number (n = 20 per replicate) falling into each class 0 = 0%; I = 1 - 5%; 2 = 6 - 10%; 3 = 1 1 - 15%; 4 = >16% surface area covered with sclerotia; these values were then multiplied by their respective class value, and combined. 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Plantings of potatoes, fi'om large commercial growers down to home gardeners, tend to take place (in Michigan) between late April and mid June, depending upon location and environmental conditions. soil temperature at planting depth can vary significantly among locations, with northern regions of Michigan ranging between 5 and 15°C, and southern regions ranging between 10 and 20°C. In the case of the Muck Soils Research Farm, the moisture content of the soil is also an important factor in the decision to plant. The farm regularly has saturated soil for a significant portion of the planting time, due to its geographical location and history as a eutrophied lake. As a result, planting potatoes at that location usually occurs in late May through June, and thus only the planting thresholds of 14 and 20°C were applicable in 2006 (see Figure 4.1). Also, a more shallow (10cm instead of 15cm) planting was used, which corresponded to the soil temperature data (100m) collected from the on-site MAWN weather station. Contrary to previous findings (see Chapter 3), the later planting at 20°C resulted in significantly more early-season disease symptoms. This finding was also in contrast to other research that found later plantings to have reduced levels of stem and/or stolon canker (Simons and Gilligan, 1997). Bolkan et al. (1974) found that infection of potato shoots decreased with increasing temperature (15, 18, 21 , 24°C tested) when (tuberbome) inoculum levels were “low”, but did not change with “moderate” and “high” levels of inoculum. Although AG-3 is considered the most important anastomosis group for 76 potatoes, others including AG—4, -5, and -8 have also been shown to readily cause infection to sprouts and stolons (Carling and Leiner, 1990). One possible reason for the increase in disease symptoms at later planting, instead of the predicted decrease, could be the presence in reasonable quantity of an AG other than AG-3. Cultures of AG-3 have been shown to cause more disease symptoms at lower soil temperatures. Carling and Leiner (1990) found that while AG-3 damaged shoots and roots at the three temperatures tested (10, 15.5 and 21.1°C), it caused significantly more damage at 10 °C. In addition, AG-S significantly damaged sprouts at 15.5 and 21.1°C, and AG-8 caused damage to roots at all three temperatures (Carling and Leiner, 1990). Following the previous fimgicide efficacy studies of 2004 and 2005 (see Chapter 2), the use of the seed treatment fludioxonil (Maxim) was again found to be effective against R. solani. In 2006, the use of the fungicide did significantly reduce the incidence of stolon girdling, but was not effective against stem girdling. In addition, no single treatment or combination of treatments proved to be effective against disease symptoms for all cultivars planted or soil temperatures at planting. In a summary of nearly 50 trials, Wale (2008) found that even Rhizoctonia stem canker specific (seed and soil) treatments had variable results. Based-upon the results of this experiment and the experiments of 2004 and 2005, only use of the one treatment, whether seed or early season was necessary. However, the use of fungicides for management should not be the sole source of control, but rather part of an integrated pest management strategy. The lack of differences among treatments for black scurf incidence and severity, as well as no yield data reported was the result of significant rainfall prior to harvest. The potato crop was subjected to fully saturated soil for a significant length of time, which 77 resulted in many tubers succumbing to the disease pressure of bacterial soft rot (Erwinia carotovora subsp. carotovora). Future Investigations The management strategies for R solani should continue to be evaluated for possible improvement over current methods. The integration of a pest management plan with chemical and cultural components is most likely the best strategy. In similar studies in the future, the combination of planting based upon soil temperature and chemical control should be reexamined, with multiple seed treatments. Also, some of the available biocontrol agents should be tested for potential efficacy compared to the current standards. The populations of soilbome R. solani should be ascertained prior to and following the study at each location. Also, the use of inoculum (multiple AGs, either alone or in combination) should be incorporated into the soil prior to planting in future studies. 78 uopmrdtoard AEEV “out 385 u u u w>m that m I Ben I I M M M M o/o M M MI. E M N N M M Z I Z I E I M I M Z 0/1 Z I Z I Z I M M H M M M N M M M H N. N M N M M M N Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z Z 0 0 0 0 0 O 0 0 O 0 0 O O 0 0 O 0 0 0 0 0 0 O 0 O 0 O 0 0 O O 0 O O 0 O 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 b P _ F _ b _ — w L r h F F h pl» _ _ _ h L P h L _ _ _ _ — _ F p o o I ‘ I I I ll N oo~ 11 Q I I I I \ I 11 V ‘ . LI 0 ‘ CON Q ‘ I 00” J1 m S . 2 m. h I I II I COM 11 . «arm a t .. u .. -- Nfi m ~ \ M oov -fi ~ 3 00: J E m «8352 . I 3 m n 9 com II ~\ wH m . . . . e 8 -, on m I I 0 ll ( cow -1 .. s NN I \ ll .. u s mated—m vm o2. it .. .. 05> 258m -r em % mm cow om doom d2 .fiam ”bigab 88m 53522 .83m consumed 26m x032 3:5 33 :ouSEBoE 98 A83: BBSQQEB :8 mo owecozw tame 3E 3‘ oBmE 79 BIBLIOGRAPHY Agrios, G. N. (1997). Plant Pathology (Fourth ed.). New York: Academic Press. Allen, B. J ., & O'Brien, P. J. (1986). The practical significance of accumulated day- degrees as a measure of physiological age of seed potato tubers. Field Crops Research, 14,141-151. Allington, W. B. (1936). Sclerotial formation in Rhizoctonia solani as affected by nutritional and other factors. Phytopathology, 26, 831-844. Anderson, N. A. (1982). The genetics and pathology of Rhizoctonia solani. Annual Review of Phytopathology, 20(1), 329-347. Arora, R., & Khurana. (2004). Major fungal and bacterial diseases of potato and their management. In K. G. Mukerji (Ed.), Fruits and Vegetable Diseases (V 01. 1, pp. 562). New York: Springer. Bains, P. S., and Bisht, V. S. (1995). Anastomosis group identity and virulence of Rhizoctonia solani isolates collected from potato plants in Alberta, Canada. Plant Disease, 79, 241-242. Bains, P. S., Bennypaul, H. S., Lynch, D. R., Kawchuk, L. M., and Schaupmeyer, C. A. (2002). 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