132 882 IVER ITY LIBRAR Itiifli’tiil’iliil’u‘uim 1m will 3 | llllzl 31293 01712 89 This is to certify that the thesis entitled Using TOMCAST, a disease forecasting system, for timing fungicide sprays for purple spot of asparagus presented by Monica P. Meyer has been accepted towards fulfillment of the requirements for M.S. Botany and Plant Pathology degree in / 4 Major professor Date /J'/(]’77 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution LIBRARY Michigan State University PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. DATE DUE DATE DUE DATE DUE 1198 chlRC/DdoOuopes-p.“ USING TOMCAST, A DISEASE FORECASTING SYSTEM, FOR TIMING FUNGICIDE SPRAYS FOR PURPLE SPOT OF ASPARAGUS By Monica P. Meyer A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Botany and Plant Pathology 1997 ABSTRACT USING TOMCAST, A DISEASE FORECASTING SYSTEM, FOR TIMING F UNGICIDE SPRAYS FOR PURPLE SPOT OF ASPARAGUS By Monica P. Meyer Purple spot disease (causal agent: Stemphylium vesicarium (Wallr.) Simmons) affects asparagus worldwide causing lesions on spears and fern. This disease is relatively new in Michigan and has developed as a result of the adoption of a no-till cropping system. Various fungicide regimes were compared for their efficacy in controlling this disease. Calendar-based fungicide spray schedules were used as a control. TOMCAST, a disease forecasting system originally developed to predict the occurrence of Alternaria solani on tomato was tested in three commercial asparagus fields in Michigan for its applicability in predicting purple spot disease on asparagus. The disease pressure in two newly- established asparagus fields was unusually severe in 1997 with 15,500 lesions/unsprayed fern, and moderate in 1996 with 4,000 lesions/unsprayed fern. A mature field showed moderate disease pressure in 1996 and 1997 with a maximum of 2,300 lesions/unsprayed fern. Using TOMCAST 15 DSV eliminated a minimum of two sprays compared to calendar-based schedules and provided similar levels of disease control. The 14-day treatment produced higher yield than the 7- and 10-day treatments when the yields were combined for both years. The results suggest that the use of TOMCAST could potentially reduce grower reliance on fungicides to control purple spot disease on asparagus. ACKNOWLEDGMENTS I would like to thank my major advisor, Dr. Mary Hausbeck for her financial support and guidance throughout my program. My committee members, Dr. Jack Kelly and Dr. Larry Olsen, provided me with suggestions that were very helpful and much appreciated. All of the members of the Hausbeck lab participated in the asparagus project in one way or another. The help and patience of all of you especially during the tedious task of data collection is appreciated. I owe a special thanks to Brian Cortright and Bill Quackenbush for the long hours involved in managing my field equipment and spraying my experimental plots. Dr. Robert Podolsky was an invaluable source of statistical help, and I can not thank him enough. I would like to thank Norm Myers, John Bakker, and Mary J o Bakker for their help in Oceana County. I would also like to thank the Oceana County asparagus growers, Tom and Rick Oomen, for use of their commercial field for my experimental plots. Finally, I would like to thank my husband for encouraging me to follow my aspirations. He is truly a blessing in my life. iii TABLE OF CONTENTS LIST OF TABLES ....................................................... v LITERATURE REVIEW .................................................. 1 Introduction ...................................................... l Asparagus Production .............................................. l Asparagus Harvest and Postharvest .................................... 3 Purple Spot Disease ................................................ 4 Pleospora herbarum, the Teliomorph of S. vesicarium ..................... 6 Disease Forecasting and Calendar-based Chemical ........................ 7 Stemphylium on Other Crops ........................................ 8 Literature Cited .................................................. 10 INTRODUCTION ...................................................... 1 3 MATERIALS AND METHODS ........................................... 16 Evaluation of TOMCAST and Conventional Programs Using Chlorothalonil and Mancozeb in a Newly-established Asparagus Field ...................... 16 Yield and Assessment of Disease on Spears in a Newly-established Asparagus Field ....................... 17 Assessment of Disease on F em in a Newly-established Asparagus Field ............................... 18 Evaluation of TOMCAST DSV Thresholds Using Chlorothalonil in a Mature Asparagus Field ....................... 19 Assessment of Disease on Fern in a Mature Asparagus Field ............... 20 RESULTS ............................................................ 22 Yield and Disease Severity on . Spears in a Newly-established Asparagus Field ......................... 22 Assessment of Disease on Fern in a Newly-established Asparagus Field ............................... 22 Disease Assessment on Fern in a Mature Asparagus Field ................. 24 DISCUSSION ......................................................... 4O LITERATURE CITED .................................................. 44 iv LIST OF TABLES Table 1’ng 1. Average weight (g) of spears per harvest during 1996 and 1997 in a newly-established commercial asparagus field with two cultivars when fern was not treated the year prior or treated with Chlorothalonil or mancozeb according to a calendar schedule or according to TOMCAST disease predictor during 21 July to 30 August 1995 and 3 July to 3 September 1996 .............................. 25 2. Summary of analysis of variance for total spear yield for 1996 + 1997 for both cultivars ............................................. 26 3. Summary of contrast results for total spear yield for 1996 + 1997 for ‘Jersey Knight’ ...................................................... 27 4. Summary of analysis of variance from purple spot disease severity index data for spears from ‘Jersey Knight’ for 1996 (8 harvests) and 1997 (13 harvests) ......................................... 28 5. Summary of analysis of variance from purple spot disease severity index data for spears from ‘Jersey Giant’ for 1996 (8 harvests) and 1997 (13 harvests) ......................................... 29 6. Number of purple spot lesions on ‘Jersey Knight’ and ‘Jersey Giant’ fern from a newly-established fields when not treated with Chlorothalonil or mancozeb according to a calendar schedule or according to the TOMCAST disease predictor during 1996 and 1997 .............. 30 7. Summary of analysis of variance for average lesion numbers on ‘Jersey Knight’ fern ..................................................... 31 8. Summary of contrast results for average lesion numbers on ‘Jersey Knight’ fern ........................................................... 32 9. Average fern defoliation rankings for both cultivars for 1997 .................. 33 10. Summary of analysis of variance for fern defoliation rankings for both cultivars in 1997 .................................................... 34 11. Summary of contrast results comparing the efficacy of the treatment regimes in controlling defoliation .......................................... 35 labia Page 12. Summary of analysis of variance for average lesion numbers on Giant’ ................................................................ 36 13. Summary of contrast results for average lesion numbers on ‘Jersey Giant’ fern ............................................................ 37 14. Number of purple spot lesions on ‘Viking KB3' fern from a mature commercial field when not treated or treated with Chlorothalonil every seven days or according to the TOMCAST disease predictor during 1996 and 1997 ................................................... 38 15. Summary of analysis of variance and orthogonal contrasts for number of purple spot lesions on ‘Viking KB3' fern from a mature commercial asparagus field when not treated or treated with Chlorothalonil every seven days or according to the TOMCAST disease predictor during 1996 and 1997 ............................................ 39 vi LITERATURE REVIEW Introduction Ninety-five percent of asparagus grown in the United States is produced by California, Washington, and Michigan (Elmer et al., 1996). In 1995, Michigan ranked third nationally for asparagus production at 10,545 kg which was 11.2% of the total US. production (Michigan Department of Agriculture, 1995). Within Michigan, the west central area (Oceana Co.) produces the most asparagus (8,227 kg) (Michigan Agricultural Statistics Service, 1992) with 175 farms with 9,700 acres. Average yields are 682 kg. per acre in a mature field. The all-male hybrids can yield 1,818 kg. or more per acre (Zandstra etaL,l992) Asparagus Production Commercial asparagus fields have been established prior to the 19805 using open- pollinated cultivars, such as ‘Mary Washington’, ‘Martha Washington’, or ‘Viking KB3’. These cultivars are dioecious and have been replaced by hybrids (Zandstra et al., 1992). Advancements in tissue-culture technology for asparagus have helped with the development of clonal hybrids (Zandstra et al., 1992). Male and female parents are selected and micropropagated. They are placed in seed-production blocks that have one male plant for each four to five female plants (Zandstra et al., 1992). All-male hybrids are preferable because the food and energy that goes into the production of fruit in dioecious varieties is available for transport into the storage roots. All-male hybrids are currently available from New Jersey, Germany and the Netherlands. 2 The New Jersey varieties are typically recommended for Michigan (Zandstra et al., 1992). Asparagus is a perennial crop that grows best in a temperate zone. Spring frosts occurring after spears have emerged destroy these spears and delay subsequent spear development. Since early spears tend to be larger than later spears, so the loss of the early production can significantly reduce total yields (Zandstra et al., 1992). Deep freezes that occur during winter can injure crowns, so they must be planted at least 20.3-cm deep for protection (Zandstra et al., 1992). Asparagus will grow well on any soil that drains well, especially sands and sandy loams with organic matter and moisture-holding capacity (Zandstra et al., 1992). The soil pH is ideal at 6.8, and asparagus will tolerate soil pH down to 6.0. However, a pH below 6.0 favors F usarium (F usarium proliferatum T. Matsushima and F. oxysporum Schlechtend.:F r.) crown rot (Zandstra et al., 1992). Crowns should be established on land that has not had asparagus grown on it before to avoid F usarium crown rot and other diseases. The land should be free of perennial and annual weeds before planting. The field should have good drainage, and manure should be spread (15 to 20 tons per acre) and a cover crop like Sudangrass or clover should be planted (Zandstra et al., 1992). In the fall before planting, the cover crop should be plowed down, and winter wheat or rye should be planted. About 250 lb each of P205 and K20 per acre should be available in the soil when the asparagus is planted. Planting should take place as early in the spring as possible. Large, one-year-old, disease-free crowns are ideal. The crowns need to be spaced 0.15-m apart, center to center, in rows 1.24- to 1.55-m apart. This will require 9,000 to 10,000 crowns per acre. 3 Furrows, made with a middle-buster plow, should be 20- to 25-cm deep. The crowns need to be covered with 2.5- to 5-cm of soil the day of planting (Zandstra et al., 1992). Asparagus is managed with a no-tillage system, because of the damage that tilling causes to the crowns. This damage provides F usarium with avenues to infect the crowns. Herbicides are required to control weeds, so in the spring, the fern is chopped as low as possible and herbicides are applied (Zandstra et al., 1992). Asparagus Harvest and Postharvest Asparagus is not harvested until the third year after planting the crowns. Limited harvest occurs until the plants are five years old (Zandstra et al., 1992). New varieties may be able to tolerate more than 24 harvests per year, but older varieties should not be harvested more than 24 times. Buds need to be left to develop plenty of fem growth. Asparagus harvest in Michigan typically begins around the middle of May and continues through late June. An asparagus field should remain productive for up to 17 years. Asparagus is harvested by hand-snapping or by riding on picking aids (Zandstra et al., 1992). Asparagus that is picked for processing is picked by bending the spear until it breaks, whereas spears picked for fresh market are snapped near the ground to keep the spears more uniform in size (around 20-cm long). Harvesting occurs every day to every other day, depending on the temperature. Asparagus can grow at a rate of 2.5-cm an hour when temperatures reach 26.7 C or warmer, so on warm days harvesting will be on a daily basis (Zandstra et al., 1992). In Michigan, the maximum length of spears for processing should not exceed 19.1-cm and they must be 0.12-cm or more in diameter 12.7-cm below the tip. Fresh 4 market spears should be a minimum of 0.15-cm in diameter, and minimum length is 17.8- cm (Zandstra et al., 1992). Fresh market spears need to be cooled as soon as possible after harvesting at 0 to 2.2 C at 95% relative humidity. The quality of spears will be maintained in these conditions for two to three weeks. Processing spears should be kept in a cold storage while waiting to be shipped (Zandstra et al., 1992). Purple spot disease Purple spot disease affects asparagus worldwide and gets its name from the numerous purplish lesions that are found on the spears as a result of infection (Lacy, 1982). The fungal organism responsible for purple spot disease, Stemphylium vesicarium (Wallr.) Simmons, has been isolated from asparagus spears in California (F alloon, et al., 1984), Washington (Johnson and Lunden, 1984), Michigan (Lacy, 1982), New Zealand (Sing, 1977), France (Blancard et al., 1984), and Switzerland (Gindrat et al., 1984). Lesions are found predominantly on the windward side of the spears, because blowing sand causes wounding which creates an avenue for infection by S. vesicarium (Lacy, 1982). Although wounds allow an avenue for more rapid infection, they are not necessary, since S. vesicarium uses natural epidermal Openings such as stomata to penetrate plants (Johnson and Lunden, 1986). Using scanning electron microscopy, it was observed that germ tubes from ascospores or conidia penetrated the spears exclusively through stomata (Falloon, et al., 1987). Epidermal cells collapsed around the point of penetration to form a slightly sunken lesion (F alloon et al., 1987). Environmental factors play a significant role in disease severity. According to a greenhouse study the most severe fem infection occurs under conditions of low light, 5 100% relative humidity, one week of 26—28 C before inoculation, and two days of leaf wetness at 14 C during the infection period (Menzies et al., 1991). Purple spot was worse in the field during the early part of the harvest season when rain occurred and temperatures ranged between 1.1 and 20 C ( F alloon et al., 1987). Plant age also affects disease severity (Menzies et al., 1991). Small differences in the shoot-emergence date, and therefore, tissue age at inoculation greatly influence tissue susceptibility. As the tissue ages, susceptibility decreases. In California, disease severity was correlated positively with rainfall events (Falloon et al., 1987), if temperatures were between 0 and 20 C, respectively. When atmospheric concentrations of conidia and ascospores of S. vesicarium were monitored with Burkard volumetric spore traps in no-till asparagus fields, peak concentrations of ascospores usually were associated with rainfall, and purple spot disease developed on spears when harvesting followed rainfall-prompted ascospore discharge by 48 hours or more (Hausbeck, 1993). Falloon et a1. (1984) looked at the overwintering structures of S. vesicarium on the field debris from the previous year, and suggested that pseudothecia are the means by which the fungus overwinters and is the main source of wind-bome ascospores causing spear infection early in the spring. Burial or removal of the previous year's fern growth reduces severity of purple spot on spears (Elmer et al., 1996). Burial does not necessarily decompose the pseudothecia before harvest, but it prevents ascospores and conidia from becoming airborne (Elmer et al., 1996). However, the damage caused to crowns during debris burial may provide avenues for infection by F usarium proliferatum f. sp. asparagi, the causal agent of crown and root rot. Cover crop mulches and wind barriers that reduce 6 blowing sand might be effective in disease management (Elmer et al., 1996). Purple spot contributes to the diseases causal to asparagus decline by damaging the fern during the growing season, causing defoliation of the cladophylls (Elmer et al.,l996). The destruction of photosynthetically active fern tissue reduces the potential photosynthate translocated to the crown and storage roots. Thus, smaller and fewer spears emerge the next year, affecting spear quality and marketability (Elmer et al., 1996). A two-year study conducted by Johnson and Lunden (1992) looked at the effects on yield of premature defoliation due to rust (Puccinia asparagi DC. In Lam. & DC.). Rust significantly reduced total weight of spears in susceptible cultivars such as ‘Mary Washington’ and ‘WSU-l'. Cultivars with slow-rusting resistance, such as ‘Jersey Giant’ and ‘UC-157', had no significant yield loss due to rust (Johnson and Lunden, 1992). Conway et al. (1990) noted that reductions in yield are correlated with the amount of Cercospora blight (Cercospora asparagi Sacc.) developing on the fern during the summer (Conway et al., 1990). Pleospora herbarum, the teliomorph of S. vesicarium. Ascostomata are scattered and are immersed to erumpent in the tissue of the host. Ascospores are globose and somewhat flattened (100-500u diameter). Asci are bitunicate, cylindrical to clavate 90-250 x 20-50u, with eight irregularly distichous ascospores. Ascospores are light to dark yellow brown, ellipsoid to clavate, 7—septate, slightly constricted at the three primary transverse septa, finally muriforrn and 26-50 x 10-20u in size (Commonwealth Mycological Institute, 1967). In culture, aerial mycelium is filamentous, sparse, hyaline to brown, branched, 7 5-8u wide. Thicker hyphae develop later on the surface of the agar darker in color. The Stemphylium conidial state has erect flexuous conidiophores 1-7 septate, 20- 72 x 4—6u, pale brown to brown, with a swollen apical sporogenous cell 7-11u diameter, and slightly roughened toward the apex. They possess a single apical pore 5-8u in diameter. Several successive sporogenous cells may form by proliferation through the apical pore (CMI, 1967). Conidia are oblong, olive to brown, ovoid to subdoliiform, occasionally constricted at 1-3 transverse septa and at the 1-3 longitudinal septa if these are complete, 195 x 285}; with a single basal pore 8 u in diameter and a rougher outer wall (CMI, 1967) Disease forecasting and calendar-based chemical control of purple spot disease. Montesinos and Vilardell (1992) evaluated the applicability of a forecaster for Alternaria solani on tomato (FAST) for predicting infection periods and timing fungicide applications for managing S. vesicarium on pear. The FAST Forecasting system incorporates two empirical models based on the following daily environmental parameters: maximum and minimum air temperature, hours of leaf wetness, maximum and minimum temperature during the wetness period, hours of relative hmnidity higher than 90% and rainfall (Madden et al., 1978). The results showed that 25-3 5% fewer fungicide applications were necessary when the FAST forecaster was used than when a 7- day commercial schedule was used (Montesinos and Vilardell, 1992). TOMCAST, a simplified version of FAST, uses hourly temperature and leaf wetness values to calculate Daily Disease Severity Values (DSVs). This disease model was designed for the control 8 of early blight, Septoria leaf spot, and anthracnose fruit rot on tomato (Pitblado, RE, 1992). Many different fungicides have been tested for effectiveness in controlling foliar disease on asparagus. According to a study by Hausbeck and Kusnier (1995), Chlorothalonil applied every seven days effectively controlled purple spot. Stemphylium on other crops Necrotic spotting of pear, caused by S. vesicarium, is a disease of economic importance in Mediterranean production areas (Montesinos and Vilardell, 1992). Montesinos et a1. (1995) conduced a study that tested the optimal temperature range (C) and wetness duration (h) necessary for infection of S. vesicarium on pear fruit and leaves. Disease severity ranged from 0 to 14.6 mean lesions per firm and 0 to 3.5 mean lesions per leaf. Maximum disease severity was found at 20 to 25 C with 18 to 24 h of wetness duration (Montesinos et al., 1995). A new leaf disease of onion (Allium cepa L.) in New York caused by S. vesicarium was discovered by Shishkoff and Lorbeer (1987). Miller et al. (1978) reported that losses in Texas onion crops due to S. vesicarium were as high as 90%, most damage occurring after rains lasting more than 24h. The affected onion leaves had water-soaked lesions that coalesced to girdle the leaves, with pseudothecia forming on dead tissue (Shishkoff and Lorbeer, 1987). Disease developed on wounded and on nonwounded leaves. The number of lesions per leaf increased with the number of hours of exposure to free moisture in the mist chamber at 20 C, appreciable damage occurring only when moisture could be seen on the foliage. When onion plants of the same age were 9 inoculated, there was a slightly greater incidence of disease on rubbed leaves compared with nonrubbed leaves, and a greater incidence of disease on the oldest leaves compared with the youngest leaves (Shishkoff and Lorbeer, 1989). The results indicated that disease can occur after 18-24 h of exposure to moisture after inoculation (Shishkoff and Lorbeer, 1989). Studies have been conducted on Stemphylium leaf spot on alfalfa. One study involved using different relative virulence and inoculurn concentrations of S. botryosum, in which differences in disease severity on alfalfa were demonstrated by changes in the number, but not the size, of lesions on leaves (Cowling and Gilchrist, 1980). From this same study, it was shown that relative virulence of the pathogen appears to be determined before or during penetration, and successful infection resulted in lesions with dimensions that were limited by factors other than relative virulence of the pathogen. A study conducted by Cowling et al. in 1981 looked at symptom differences between Stemphylium leaf spot on alfalfa in Eastern North America and in Califomia. The results indicate that the distinctive symptom differences of Stemphylium leaf spot of alfalfa recognized in the eastern areas of North America and California (Cowling, 1980, Cowling and Gilchrist, 1980 and Graham et al., 1979) are due to inherent differences between the pathogens, and not the environments or cultivars characteristic of the two regions. There are also significant differences in disease severity detected among cultivars (Thal and Campbell, 1987). LITERATURE CITED Blancard, D., Piquemal, J .P., and Gindrat, D. 1984. La Stemphyliose de l’asperge. P.M.H. Revue Hortic. (248):27-30. Commonwealth Mycological Institute, Ferry Lane, Kew, Surrey, England. Eastern Press Ltd., London and Reading. 1967. Conway, K.E., Motes, J .E., and F oor, C.J. 1990. Comparison of chemical and cultural controls for Cercospora blight on asparagus and correlations between disease levels and yield. Phytopathology 80:1 103-1 108. Cowling WA. 1980. Environmental, genetic, and physiological factors influencing disease severity in Stemphylium leafspot of alfalfa in California. Ph.D. dissertation, University of California, Davis. 211p. Cowling WA. and Gilchrist, D.G. 1980. Influence of the pathogen on disease severity in Stemphylium leafspot of alfalfa in California. Phytopathology 70:1148-1153. Cowling, W.A., Gilchrist, D.G., and Graham, J .H. 1981. Biotypes of Stemphylium botryosum on alfalfa in North America. Phytopathology 71 :679-684. Elmer, W.H., Johnson, DA, and Mink, 6.1. 1996. Epidemiology and management of the diseases causal to asparagus decline. Plant Dis. 80:117-125. Falloon, P.G., Falloon, L.M., and Grogan, R.G. 1984. Purple spot and Stemphylium leaf spot of asparagus. California Agriculture, July-August. Falloon, P.G., Falloon, L.M., and Grogan, R.G. 1987. Etiology and epidemiology of Stemphylium leaf spot and purple spot of asparagus in California. Amer. Phytopathol. Soc. 77:407-413. Gindrat, D., Varady, C., and Neury, G. 1984. Asperge: Une nouvelle maladie du feuillage, provoquee par un Stemphylium. Rev. Suisse Vitic., Arboric. Hortic. 16(2):8l- 85. Graham, J .H., Frosheiser, F.I., Stuteville, D.L., and Erwin, DC. 1979. A compendium of alfalfa diseases. Am. Phytopathol. Soc., St. Paul, MN 65 pp. Hausbeck, M.K., 1993. Epidemiology of Stemphylium leaf spot and purple spot in no-till asparagus. Phytopathology 83: 1344. 10 11 Hausbeck, M.K., and Kusnier, J .J . 1995. Control of foliar diseases on asparagus with foliar sprays, 1994. Fungicide Nematicide Tests 50:88. Johnson DA. and Lunden, J .D. 1984. First report of purple spot (Stemphylium vesicarium) of asparagus in Washington. Plant dis. 6821099. Johnson, DA. and Lunden, J .D. 1986. Effects of wounding and wetting duration on infection of asparagus by Stemphylium vesicarium. Plant Dis. 70:419-420. Johnson, D.A., and Lunden, J .D. 1992. Effect of rust on yield of susceptible and resistant asparagus cultivars. Plant Dis. 76:84-86. Lacy, ML. 1982. Purple spot: A new disease of young asparagus spears caused by Stemphylium vesicarium. Plant Dis. 66:1198-1200. Madden, L., Pennypacker, SP, and MacNab, AA. 1978. FAST, a forecast system for Alternaria solam' on tomato. Phytopathology 68: 1354-1358. Menzies, S.A., Bansal, R.K., and Broadhurst, PG. 1991. Effect of environmental factors on severity of Stemphylium leaf spot on asparagus. New Zealand J. Crop Hort. Sci. 19:135-141. Michigan Agricultural Statistics Service. 1992. Michigan Rotational Survey, Vegetables. Michigan Department of Agriculture. Michigan Department of Agriculture, 1995. Michigan Agricultural Statistics. Miller, M.E., Taber, R.A., and Amador, J .A. 1978. Stemphylium blight of onion in south Texas. Plant Dis. 62:851-853. Montesinos, E., Moragrega, C., Llorente, I., Vilardell, P., Bonaterra, A., Ponti, I., Bugiani, R., Cavanni, P., and Brunelli, A. 1995. Development and evaluation of an infection model for Stemphylium vesicarium on pear based on temperature and wetness duration. Phytopathology 85:586-592. Montesinos, E. and Vilardell, P. 1992. Evaluation of FAST as a forecasting system for scheduling fungicide sprays for control of Stemphylium vesicarium on pear. Plant Dis. 76:1221-1226. Pitblado, RE. 1992. The development and implementation of TOM-CAST. Ministry of Agriculture and Food. Ontario. 12 Shishkoff, N. and Lorbeer, J.W. 1987. New leaf disease of onion in New York caused by Stemphylium vesicarium. Phytopathology (abstract) 77:1713. Shishkoff, N. and Lorbeer, J .W. 1989. Etiology of Stemphylium leaf blight of onion. Phytopathology 79:301-304. Sing, G. 1977. Plant pathogenic species of Stemphylium Wallr in New Zealand. Ph.D. thesis. Massey Univ., NZ. 141pp. Thal, W.M. and Campbell, CL. 1987. Assessment of resistance to leaf spot diseases among alfalfa cultivars in North Carolina fields. Phytopathology 77:964-968. Zandstra B.H., Kelly J .F., Hausbeck M.K, Grafius E.J., and Price HQ 1992. Commercial Vegetable Recommendations for Asparagus. M.S.U. Cooperative Extention Bulletin E- 1304. INTRODUCTION Ninety-five percent of asparagus grown in the United States is produced by California, Washington, and Michigan (Elmer et al., 1996). In 1995, Michigan ranked third nationally for asparagus production at 10,545 kg which was 11.2% of the total US. production (Michigan Department of Agriculture, 1995). Purple spot disease affects asparagus worldwide causing purplish lesions on the spears (Lacy, 1982). The causal agent of purple spot disease, Stemphylium vesicarium (Wallr.) Simmons (teliomorph Pleospora herbarum), has been isolated from asparagus spears in California (Falloon et al., 1984), Washington (Johnson and Lunden, 1984; Johnson and Lunden, 1986), Michigan (Evans and Stephens, 1984; Lacy, 1982), New Zealand (Falloon, 1982), France (Blancard, et al., 1984), and Switzerland (Gindrat et al., 1984). Purple spot lesions are often found predominantly on the windward side of the spears, because blowing sand causes wounding, creating an avenue for infection, by S. vesicarium (Lacy, 1982). Wounds, however, are not necessary for infection since S. vesicarium uses natural epidermal openings such as stomata to penetrate asparagus (Johnson and Lunden, 1986). Purple spot disease can affect asparagus fern, causing death and premature defoliation which may result in reduced photosynthesis. Johnson and Lunden (1992) examined the effects of premature defoliation on yield due to rust (Puccinia asparagi DC. In Lam. & D.C.). Rust significantly reduced total weight of spears for susceptible cultivars such as ‘Mary Washington’ and ‘WSU-l .’ Cultivars with slow-rusting 13 14 resistance, such as ‘Jersey Giant’ and ‘UC-157' had no significant yield loss due to rust (Johnson and Lunden, 1992). Reductions in yield are also correlated with the amount of Cercospora blight (Cercospora asparagi, Sacc.) developing on the fern during the summer (Conway et al., 1990). Environmental conditions play a significant role in purple spot disease severity on spears and fern. In a greenhouse study, the most severe fern infection occurred under conditions of low light, 100% relative humidity, one week of 26-28 C before inoculation, and two days of leaf wetness at 14 C during the infection period (Menzies et al., 1991). In the field, purple spot disease is favored during the early part of the harvest season following rainfall events and when temperatures are between 1.1 and 20 C ( Falloon et al., 1987). Menzies et al. (1991) demonstrated that as the tissue ages, susceptibility to S. vesicarium decreases. Pseudothecia of Pleospora herbarum overwinter on the previous summer’s fern debris lying on the soil surface (Evans and Stephens, 1984 and Falloon et al., 1984), providing a source of inoculum for spear infection (Falloon et al., 1984). Atmospheric concentrations of conidia and ascospores of the purple spot pathogen were monitored using Burkard volumetric spore traps in two no-till asparagus fields ( Hausbeck, 1993). Peak concentrations of ascospores in the atmosphere were usually associated with rainfall and purple spot disease developed on spears when harvesting followed rainfall-prompted ascospore discharge by 48 hours or more. Burial or removal of the previous year's fern growth reduced severity of purple spot on spears by preventing ascospores and conidia from becoming airborne (Elmer et al., 1996). 15 Fungicides are used routinely to manage purple spot disease on asparagus. Mancozeb fungicides are registered for use and typically provide control (Hausbeck and Kusnier 1995), however, large processors refuse to buy spears if the previous season’s fern has been treated with mancozeb (personal communication, Campbell Soup Co.). Because of this problem, Michigan obtains a yearly section 18 Specific Exemption for Chlorothalonil. Chlorothalonil applied every seven days consistently and effectively controls purple spot disease (Hausbeck and Kusnier, 1995). Montesinos and Vilardell (1992) evaluated the applicability of a disease forecaster for predicting infection periods and timing fungicide applications for control of S. vesicarium on pear. The FAST forecasting system was developed to manage Alternaria solani on tomato, and it incorporates two empirical models based on the maximum and minimum air temperature, hours of leaf wetness, maximum and minimum temperature during the wetness period, hours of relative humidity higher than 90% and rainfall (Madden et al., 1978). The results from the study conducted by Montesinos and Vilardell (1992) showed that 28-38% fewer fungicide applications were necessary when the FAST forecaster was used compared with a 7-day fungicide application schedule (Montesinos and Vilardell, 1992). TOMCAST is a simplified version of FAST utilizing the duration of leaf wetness and the average temperature during the leaf wetness period to identify environmental conditions favorable for A. solam' development (Pitblado, 1992). The objective of this research was to assess the efficacy and economics of genetic resistance and fungicides applied according to conventional schedules or TOMCAST for purple spot disease control in asparagus. MATERIALS AND METHODS Evaluation of TOMCAST and conventional programs using chlorothalonil and mancozeb in a newly established asparagus field. Two-year-old crowns of ‘Jersey Giant’ and ‘Jersey Knight’ were established in Benona sand fields in Oceana County, Michigan in 1995. The cultivars were spaced 0.4-km apart. Crowns were spaced 30.77- cm apart within the rows and 3.91 -m spacing between rows. The experimental design for each cultivar was a randomized complete block with four 175.85-m blocks containing nine treatments randomly assigned within each block. Each treatment was contained within 19.54-m row sections in the blocks with two buffer rows between treatment rows and 7.82-m buffers between treatments within the rows. The fungicides Chlorothalonil (Bravo Weather Stik at 2.5 kg active ingredient (a.i.)/ha., ISK Bioscience, Mentor, OH) and mancozeb (Penncozeb 75DF at 2.25 kg a.i./ha., Elf Atochem, Philadelphia, PA) were applied with a CO2 backpack sprayer operated at 40 psi through two XR Tee-Jet 8004 flat-fan nozzles calibrated to deliver 373.0 L/ha. Mancozeb or Chlorothalonil treatments were applied at 7-day intervals (1996, 10 sprays; 1997, 8 sprays), 10-day intervals (1996, 7 sprays; 1997, 6 sprays), l4-day intervals (1996, 5 sprays; 1997 4 sprays), or applied according to TOMCAST with a threshold of 15 disease severity values (DSV) (1996, 4 sprays; 1997, 4 sprays). Calendar-based sprays were initiated when the plants produced secondary branching and the cladophylls were beginning to emerge ( 3 July 1996, 30 June 1997). The TOMCAST program uses the duration of leaf wetness and the average air 16 17 temperature during the wetness period for each 24-hr period (11AM to 11AM) to determine a DSV of 0 to 4 corresponding to an environment unfavorable to highly favorable for A. solani conidial formation (Pitblado, 1992). Hourly averages of leaf wetness and temperature data were collected with a digital recorder (Omnidata DP223; Omnidata International, Inc., Logan, Utah). The digital recorder was located approximately 400-m from the ‘Jersey Knight’ experimental plot, and approximately 565-m from the ‘Jersey Giant’ experimental plot. The leaf wetness sensor was placed approximately 30-cm above the ground. Yield and assessment of disease on spears in a newly established asparagus field. In 1996, spears were harvested on 19, 20, 23, 26, and 29 May, and 1, 3, and 7 June. In 1997, spears were harvested on 23, 26, 29 May, and 1, 3, 6, 8, 10, 12, 14, 16, 19, and 21 June. The spears were hand picked and yield was measured by weight for each treatment. The spears were assessed for disease; a ranking system from 1 to 5 corresponding to no disease and severe infection, respectively, was used. According to these ratings, the following criteria were used to rank the spears, or place them in the appropriate disease class: 1=no symptoms; 2=1-20 lesions on each spear; 3=21-50 lesions on each spear; 4=51-90 lesions on each spear; 5= more than 90 lesions on each spear. A disease severity index (DSI) was calculated for each harvest by modifying the formula of Sherwood and Hagadom (195 8): 2(disease class x no. of spears in that class)100 DSI= Total no. of spears x 5 18 Treatment effects on disease severity were tested for 1996 and 1997 using an analysis of variance of a randomized complete block design with DSI as the dependent variable. Treatment effects on total two-year spear yield ( 1996 and 1997) were analyzed using an analysis of a randomized complete block design (ANOVA). The treatments were subsequently compared using the following orthogonal contrasts: (1) untreated control contrasted with any treatment receiving a spray; (2) Chlorothalonil contrasted with mancozeb; (3) 7-day schedule contrasted with 10- and l4-day schedules; (4) 10-day contrasted with 14-day schedules; (5) the difference between the 7-day schedule and the 10- and 14-day schedules compared between the fungicides; (6) the difference between the 10- and 14-day schedules compared between fungicides. Assessment of disease on fern in a newly established asparagus field. On 13 September 1996 and 15 September 1997, four fern were harvested randomly from each treatment within each cultivar and the total number of purple spot lesions were counted. In 1997, because of the severe disease pressure, a fern ranking system was utilized to assess defoliation of the cladophylls. The ranking ranged from 1 to 5 according to the following criteria: 1=healthy plant with no defoliation; 2=1-20% defoliation; 3=21-50% defoliation; 4=51-80% defoliation; 5=more than 80% defoliation and plant death. Treatment effects on lesion numbers were tested for 1996 and 1997 using an analysis of variance of a randomized complete block design. For 1996 and 1997, the treatments were subsequently compared using the following orthogonal contrasts: l9 (1) untreated control compared with any treatment receiving a spray; (2) Chlorothalonil contrasted with mancozeb; (3) TOMCAST contrasted with the 7-day schedule; (4) TOMCAST and 7-day schedules were compared with the 10- and 14-day schedules; (5) 10-day schedule contrasted with the 14-day schedule; (6) the difference between TOMCAST and 7-day schedules, contrasted between fungicides; (7) the difference between TOMCAST/7-day schedule and 10-/14-day schedules, compared between fungicides; (8) the difference between 10- and 14-day schedules compared between fungicides. The data used in the 1996 analysis of treatment effects on lesion number were not normally distributed and were transformed to normality using the following equation: {(lesion number +1). The 1997 data were not normally distributed and were transformed to normality using the following equation: log (lesion number +1). The 1997 data analyzed for rank, or amount of defoliation, were not normally distributed and were transformed to normality as follows: 1f (rank +1 ). Evaluation of TOMCAST DSV thresholds using chlorothalonil in a mature asparagus field. ‘Viking KB3’ asparagus established in Benona sand fields in Oceana County, Michigan in 1984 was used during the 1996 and 1997 study. Crowns were spaced 30.77 cm apart within rows with 3.91-m spacing between rows. The experimental design was an incomplete block with eight blocks containing two treatments randomly assigned within each block and were separated by a buffer plot. The treatments consisted of untreated, or treated with Chlorothalonil (Bravo 2.5 kg a.i./ha) at 7-day intervals, or according to TOMCAST with a threshold of 12 or 15 DSV. Each of the eight blocks consisted of one row containing three 11.72-m plots with one buffer row between each 20 block. Each block was staggered 11.72-m from the previous block. Fungicide applications were made with a C02 backpack sprayer operated at 40 psi through two XR Tee-Jet 8004 flat-fan nozzles calibrated to deliver 373.0 L/ha. Calendar-based sprays were initiated when the plants produced secondary branches and the cladophylls were beginning to emerge (17 July 1996, 30 June 1997). Hourly averages of leaf wetness and temperature data were collected with a digital recorder (Omnidata DP223; Omnidata International, Inc., Logan, Utah) with the leaf wetness and temperature sensors located in a buffer row and placed approximately 30 cm above the ground. Assessment of disease on fern in a mature asparagus field. Four fern were selected randomly from each treatment within each cultivar and were harvested on 10 September 1996 and 8 September 1997. The total number of purple spot lesions was counted on each fern for each treatment. In 1997, because of the severe disease pressure, a fern ranking system was utilized to assess fern defoliation. The ranking ranged from 1 to 5, corresponding to no defoliation and severe defoliation, respectively. The fern were ranked according to the following criteria: 1=healthy plant with no defoliation; 2=l -20% defoliation; 3=21-50% defoliation; 4=51-80% defoliation; 5=more than 80% defoliation and plant death. Treatment effects on fern were tested using an analysis of variance of a randomized complete block design. There were significant differences among treatments, so the following contrasts were used to compare them: (1) untreated control compared with any treatment receiving a spray; (2) TOMCAST compared with the 7-day schedule; 21 (3) TOMCAST 12 DSVs compared with TOMCAST 15 DSVs. RESULTS Yield and disease severity on spears in a newly established asparagus field. When ‘Jersey Knight’ spear yields were combined for 1996 and 1997, treatment yields differed significantly (p=0.0110) (Tables 1 and 2), so orthogonal contrasts were utilized as a multiple comparison test. When fungicide was applied every 14 days, yields were significantly higher than when fungicide was applied every 10-days (p=0.0019, Table 3). When Chlorothalonil was applied, the 10 and 14-day treatments produced significantly more yield than the 7-day treatment (p=0.0356, Table 3). There was only one year of yield data for the TOMCAST treatments, so these data are not included. There were no significant yield differences among the fungicide treatments (p=0.1420) for ‘Jersey Giant’ (Table 2). Harvest date was the only factor that had a significant effect on disease severity for either ‘Jersey Knight’ and ‘Jersey Giant’ in 1996 and 1997 (Tables 4 and 5). Disease severity on spears was not significantly influenced by treatment in either year for either cultivar. Assessment of disease on fern in a newly established asparagus field. The efficacy of the treatment programs differed significantly (p=0.0001) in both years for ‘Jersey Knight’ (Tables 6 and 7), so orthogonal contrasts were utilized as a multiple comparison test. The weekly fimgicide regime resulted in 10 (1996) or 8 (1997) fimgicide applications (Table 6). Using TOMCAST eliminated a minimum of 4 and 2 sprays compared to a weekly or lO-day application regime, respectively. In 1996 and 1997, 22 23 significantly fewer lesions occurred on treatments receiving fungicide sprays compared with the untreated control (p=0.0001, Table 8). In 1997, Chlorothalonil was more effective in controlling purple spot disease than mancozeb (p=0.0002, Table 8), the TOMCAST and 7-day treatments provided better control than the 10- and 14-day treatments in 1997 (0.0001, Table 8), and the 10-day treatment provided better control than the l4-day treatment in 1997 (p=0.0062, Table 8). In 1997, when ranking asparagus fern for disease severity (cladophyll defoliation), the difference due to the spray programs was significant (p=0.0001) for ‘Jersey Knight’ (Tables 9 and 10), so orthogonal contrasts were utilized as a multiple comparison test. The amount of defoliation was significantly greater in the untreated control than for any treatments (p=0.0001, Table 11). Chlorothalonil provided better disease control than mancozeb (p=0.0024, Table 11). Defoliation was significantly reduced in the TOMCAST and the 7-day treatments compared to the 10- and 14-day treatments (p= 0.0001 , Table 11). The lO-day treatment was significantly more effective in limiting defoliation than the 14-day treatment (p=0.0034, Table 11). The efficacy of the treatment programs differed significantly (p=0.0001) in both years for ‘Jersey Giant’ (Table 6 and 12), so orthogonal contrasts were utilized as a multiple comparison test. In both years, any treatment that received fungicide sprays had significantly fewer lesions than the untreated control (p=0.0001, Table 13). The 7-day treatment provided significantly better control than the TOMCAST treatment in both years (p=0.0036 1996; p=0.0001 1997, Table 13). In 1997, Chlorothalonil provided better disease control than mancozeb (p=0.0001, Table 13). The TOMCAST and 7-day 24 treatments provided significantly better disease control than the 10- and 14-day treatments in 1997 (p=0.0001, Table 13). In 1997, when ranking asparagus fern for disease severity (cladophyll defoliation), the difference due to the treatments was significant (p=0.0001) for ‘Jersey Giant’ (Tables 9 and 10), so orthogonal contrasts were utilized as a multiple comparison test. Defoliation was significantly higher in the untreated control compared with all other sprayed treatments (p=0.0001, Table 11). Defoliation was significantly higher in the TOMCAST treatment than the 7-day treatment (p=0.0485, Table 11). Disease assessment on fern in a mature asparagus field. In 1996 and 1997, there were significant differences between the treatments, so orthogonal contrasts were utilized as a multiple comparison test (p=0.0001, Tables 14 and 15). Eight applications of Chlorothalonil were made according to the 7-day schedule (Table 14). Using TOMCAST resulted in eliminating a minimum of 4 sprays (DSV=12) or 5 sprays (DSV=15). In both years, any treatments that received fungicide sprays had significantly fewer lesions than the untreated control (p=0.0001, 1996; p=0.0001, 1997, Table 15). In 1996, TOMCAST treatments provided significantly less disease control than the 7-day spray schedule (p=0.0001, Table 15), although the TOMCAST 12 and 15 DSV disease control did not differ significantly from each other due to treatment ( Table 15). 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