a un- »w -..- - l‘ l‘ ‘\ . ‘. 1 - x . 1:3! :1 « a. ‘ , v ‘- -- r‘ r‘ . , , . . . \- . :v.‘ .- 4 A. 1- 1“ NN'511; ' W 4‘ ‘ . ‘ .' ‘ ‘ l.L;lN$'-\l . z ‘. u ' \: : “ .. '. v - . . . a x: . .. Igr‘K‘J 5,. I . ."\‘. . .s..u. |{"i15«.1.,gn‘- ’\ It *2 3 L 121w"- 4“ ‘ "“1". - “ U34“. 7 "ugi ABSTRACT THE PATHOGENICITY AND POPULATION MANAGEMENT OF PRATYLENCHUS PENETRANS ON POTATO BY Ernest C. Bernard Pratylenchus penetrans (Cobb) Filipjev and Schuurmans- Stekhoven significantly decreased the tuber yields of potato cultivars Katahdin, Kennebec, and Superior, had no observed effects on Russet Burbank, and stimulated an in- crease in tuber yield in Onaway. All initial population densities (Pi) of E. penetrans (Pi = 38, 81, 164, and 211/ 100 cc soil) decreased tuber yields of Superior; a moderate Pi (81/100 cc soil) decreased yields of Kennebec; and a moderate Pi increased yields of Katahdin, followed by a marked decline at higher Pi's. In general, yields were related to tolerance of the cultivars to nematode colonization. Highest nematode popu— lations in roots were found in Russet Burbank, followed by Kennebec, Katahdin, and Superior. Symptoms of nematode colonization were confined to reductions in tuber weight and root weight. A \ Ernest C. Bernard An estimate of the economic thresholds for root-lesion nematode colonization was made using Pi, cultivar grown, anticipated yield, and value of yield. Each cultivar exhi- bited a different threshold value. For instance, maximum feasible treatment costs for cultivars at a Pi of 100/100 cc soil are: $38 for Katahdin, $70 for Kennebec, $10 for Russet Burbank, and $160 for Superior. Field studies demonstrated that fumigation may inhibit the emergence of sprouts if climatic conditions are not within acceptable ranges. Heavy rainfall may seal the soil and prevent fumigant dispersal. Greenhouse nematicide trials using 1,3 D-MIC and PHENAMIPHOS reduced nematode populations 90-95% and increased yields of Russet Burbank 20% and 40%, and Superior 35% and 80%, on treated-infested and treated-noninfested soil, respectively. THE PATHOGENICITY AND POPULATION MANAGEMENT OF PRATYLENCHUS PENETRANS ON POTATO BY Ernest C. Bernard A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Entomology 1974 ACKNOWLEDGEMENTS I would like to thank Dr. Charles W. Laughlin for his generous aid and encouragement during the course of these studies. To Drs. Richard W. Chase, Donald C. Cress, Melvyn L. Lacy, and especially George W. Bird, I extend my appre- ciation for serving on my guidance committee. I would especially like to extend my sincere thanks to John F. Davenport for his able and willing assistance throughout these studies. I also wish to thank the Michigan Potato Industry Commission for financial support which made this study possible. ii TABLE OF CONTENTS Page LIST OF TABLES . . . . . . . . . . iV LIST OF FIGURES . . . . . . . . . . V INTRODUCTION . . . . . . . . . . . 1 LITERATURE REVIEW . . . . . . . . . 5 MATERIALS AND METHODS . . . . . . . . 9 1. Pathogenicity Studies . . . . . . . . 9 Greenhouse Cultivar Trials . . . . . 9 Field Microplot Cultivar Trials . . . . 10 2. Population Management Studies . . . . . 12 Field Soil Fumigation . . . . . . . 12 Greenhouse Nematicide Studies . . . . . 13 RESULTS AND DISCUSSION . . . . . . . . 15 l. Pathogenicity Studies . . . . . . . 15 Greenhouse Cultivar Trials . . . . . 15 Field Microplot Cultivar Trials . . . . 15 2. Population Management Studies . . . . . 33 Field Soil Fumigation . . . . . . . 33 Nematicide Greenhouse Studies . . . . . 37 SUMMARY AND CONCLUSIONS . . . . . . . . 40 LITERATURE CITED . . . . . . . . . . 42 iii TABLE OF CONTENTS Page LIST OF TABLES . . . . . . . . . . iv LIST OF FIGURES . . . . . . . . . . V INTRODUCTION . . . . . . . . . . . 1 LITERATURE REVIEW . . . . . . . . . 5 MATERIALS AND METHODS . . . . . . . . 9 l. Pathogenicity Studies . . . . . . . . 9 Greenhouse Cultivar Trials . . . . . 9 Field Microplot Cultivar Trials . . . . lO 2. Population Management Studies . . . . . 12 Field Soil Fumigation . . . . . . . 12 Greenhouse Nematicide Studies . . . . . 13 RESULTS AND DISCUSSION . . . . . . . . 15 1. Pathogenicity Studies . . . . . . . 15 Greenhouse Cultivar Trials . . . . . 15 Field Microplot Cultivar Trials . . . . 15 2. Population Management Studies . . . . . 33 Field Soil Fumigation . . . . . . . 33 Nematicide Greenhouse Studies . . . . . 37 SUMMARY AND CONCLUSIONS . . . . . . . . 40 LITERATURE CITED . . . . . . . . . . 42 iii Table 10. LIST OF TABLES Occurence of plant-parasitic nematode genera in samples submitted from potato fields to the Cooperative Extension Service from 1963-1973 . . . . . . . . Relative abundance of Pratylenchus penetrans in samples submitted from potato fields to the Cooperative Extension Service from 1963-1973 c o o o o o o 0 Effect of initial population density (P1) of Pratylenchus penetrans on tuber yield of two potato cultivars grown under greenhouse conditions . . . . . . . . Effect of selected potato cultivars on the final population density (Pf) of Pratylenchus penetrans in microplots . . Effect of four potato cultivars on the popula- tion dynamics of four initial population densities (P1) of Pratylenchus penetrans at selected intervals during the growing season . . . . . .‘ . . Effect of initial population density (P1) of Pratylenchus penetrans on fresh root weights of selected potato cultivars . . Effect of spring fumigation on the emergence of selected potato cultivars . . . . Effect of spring fumigation on the tuber yield of selected potato cultivars . . . . Effect of spring fumigation on the mean tuber yield per plant of selected potato cultivars Effect of nematicides and initial population density (Pi):of Pratylenchus penetrans on tuber yield Qf two potato cultivars . . iv Page 16 20 21 23 34 35 36 38 Figure 1. LIST OF FIGURES Effect of initial population level (Pi) of Pratylenchus penetrans on the tuber yield of selected potato cultivars . . . Effect of initial population level (Pi) of Pratylenchus penetrans on the appearance of Kennebec potato root systems . . Correlation of tuber yield with number of secondary roots/cm lateral root for Superior potatoes . . . . . . . . Feasibility of treatment based on initial population densities (Pi) of Pratylenchus penetrans for Katahdin potato tuber yield of 225 cwt/a valued at $2.70/cwt . . Feasibility of treatment based on initial population densities (Pi) of Pratylenchus penetrans for Kennebec potato tuber yield of 225 cwt/a valued at $2.70/cwt . . Feasibility of treatment based on initial population densities (Pi) of Pratylenchus penetrans for Russet Burbank potato tuber yield of 225 cwt/a valued at $2.70/cwt. Feasibility of treatment based on initial pOpulation densities (Pi) of Pratylenchus penetrans for Superior potato tuber yield of 225 cwt/a valued at $2.70/cwt . . Page 18 24 26 30 30 32 32 INTRODUCTION Root-lesion nematodes (Pratylenchus spp.) are among the most ubiquitous of phytoparasitic nematodes, occuring in most of the world's agricultural areas (11). Most cul- tivated crops are susceptible to attack from one or more species of root-lesion nematodes. Most Species of this genus are capable of parasitizing a wide range of plant species. The potential economic importance of this genus in Michigan is supported by examination of Cooperative Extension Service samples submitted for nematode analysis from 1963-1973. Pratylenchus is distributed throughout the state (10), and is the most frequently encountered phytopathogenic nematode genus in potato (Solanum tuber- osum L.) fields, often occuring in high numbers (Tables 1 and 2). Pratylenchus penetrans (Cobb 1917) Filipjev and Schuurmans-Stekhoven 1941 is the predominant species of root-lesion nematode recovered from potato roots and asso- ciated soil in Michigan (N. A. Knobloch, personal communi- cation). Nematode-incited losses to potato tuber crops are estimated at ten percent annually (35), giving a potential loss of productivity in Michigan of 45,000 metric tons, or $2.8 million (15). A better understanding of the pathogenic TABLE 1. Occurrence of plant-parasitic nematode genera in samples submitted from potato fields to the Cooperative Extension Service from 1963-1973. Percent of samples containing: U) m m H s m m a s 5 U .c: m m c 0 s m m 5 >( G .C m c a n m w o 5 - >1 0 H H H a H m o m a o m >1 m o E z o w n U u :4 'U m 'U H o o 0 >1 0 c: F; -H >1 : H 0 .u n -H m o u w w -H m o a ‘5 E {3 H 8 3 i3 '3 .5} COUNTY B 2 a. a1 :11 a: n. B x Allegan 14 33% 93% 13% - — - _ _ Bay 15 7 29 21 86% 64% 57% 21% 7% Berrien 2 - 50 - - - - - 50 Calhoun 7 - 100 14 - — — - - Clinton 30 17 77 - - — - - _ Ingham 15 13 60 - - - — _ _ Lapeer 8 50 87 - - - 13 - - Lenawee l - 100 - — - - _ - Luce l - 100 - - — _ _ _ Macomb 5 100 80 100 - - 20 - - Manistee 2 - 100 - - — _ _ _ Missaukee 2 - 100 - - — _ _ _ Montcalm 3 - 100 - - - - - - Newaygo 1 100 100 - - - - - - Tuscola 3 - 67 - — — _ _ _ Washtenaw 4 25 100 - - - - - — TOTAL 113 21 75 13 11 8 15 3 2 .Oh swap once u m NOBIHv n v uO¢IHN n m “ONIHH_u N NHHom oo OOH\mmUoumEmc OHIH u H ”msHumm H mm I I mm . on o w s swampsmmz I I I om om O N N MHoomDB OOH I I I I H O H om>m3mz mm I I mm mm H N m EHmoucoS OOH I I I I O N N mwxsmmmHS I I om om I O N N mmuchmz mm mN I I I O H v Q5008: OOH I I I I O H H mood I I I I OOH O H H mmsmcwq mm I 4H 4H 4H N m a ummmmn so I I I mm o m m amsmcH av v om mH v «H m mN noucHHo Om mH I ON I O n n csosHmo OOH I I I I O H H :mHHHmm I I wom wom I o v w mam wmm wm I I wmm m m 4H cmmmaaa m w m N H HHOm UHCMOHO HHOm HmnmcHE mmHmEmm mucsou Eonm .oz Eonm .oz topmoqu .oz HumcHumu um mmHmfimm mo w .manImomH Eoum 00H>Hom,c0Hmcmuxm w>Humnmmoou may on mpHmHm Opmuom Eoum UmuUHEQsm mmHmEMm qH mcmnumcum manocmeumnm mo mocmpcsnm m>HHMHmm .N mHmNB potential of P. penetrans and an increased awareness of pro- per control alternatives should enable workers to make more accurate control recommendations. The objectives of this study were: 1) to determine the population density of P. penetrans necessary to incite an economic decrease in potato tuber yields; 2) to investi— gate the tolerance of selected potato varieties to g, pegg- Egans; 3) to study the influence of fumigant and non- fumigant nematicides on potato growth and tuber yield in both nematode-infested and non-infested soil. LI TE RATURE REVI EW The first species of root-lesion nematode was described in 1880 by DeMan. The generic names Tylenchus Cobb 1893 and Anguillulina Cobb 1893 were used until Filipjev esta- blished the genus Pratylenchus in 1934. Cobb reported g. penetrans in 1917 from potato and cotton plants (3). E. penetrans is a migratory endoparasite. Sontirat et 31. (36) reported that late larval and adult stages in- vaded roots more readily than 2nd-stage larvae. Ferris (5) reported that E. penetrans could survive for at least six months in moist, fallow soil at room temperature, and even increased its numbers in refrigerated soil (1-4 C), probably due to the hatching of eggs. Dunn (unpublished research) found that P. penetrans could survive for up to two years at 4 C, but only 1—4 days at -4 C. A wide range of soil types are suitable for the main- tenance and reproduction of g. penetrans. Mountain and Boyce (19) found that gravelly sand and sand were most suit- able, while clay loam was least suitable for nematode survival and reproduction; whereas Kleyburg and Oostenbrink (9) found approximately equal numbers in clay, sandy peat soil, and sandy soil. P. penetrans appears to survive well in both mineral and organic soils in Michigan (Table 2). Root and soil population densities of g. penetrans often fluctuate during the growing season. Mountain and Boyce (20) noted that the number of root-lesion nematodes per gram of peach root declined during the summer and rose again in autumn. They postulated that the suberization of roots and decay of tissue suitable for nematode colonization led to this decrease in density. Miller and Edgington (16) found a similar natural decline on potatoes, from a high of 90/100 g soil in spring to 45/100 g soil in late summer. Late summer population densities increased over spring populations in other herbaceous vegetables, according to Olthof and Potter (25), except in potatoes and onions. Hastings (6) estimated that the life cycle of £3 22227 trans required 54-65 days: 25-31 days for egg and larval development, and 29-34 days for maturation of the adult, but he did not give an experimental temperature. Dunn (unpubli- shed data) and Mamiya (12) similarly observed that at 30 C only 30 days were required for complete development. Dickerson gt 31. (4) observed that a soil temperature range of 16-20 C gave the fastest increase in P. penetrans popula- tions. Hastings (6) calculated an average rate of egg-laying for P. penetrans at 0.8 eggs/day for 30-40 days, while Mamiya (12) estimated the mean rate at between 0.8 and 1.1 eggs/day for 35 days. P. penetrans has a wide host range, including members of the families Poaceae, Fabaceae (8), Asteraceae and Brassi- caceae (38). Oostenbrink gt 31. (30) determined that P. penetrans was able to survive and reproduce on 164 plant species and varieties (the total number tested). Root-lesion nematode parasitism can cause the produc- tion of hyperplastic, hypoplastic, or necrotic symptoms. Affected plants may exhibit increased secondary root proli- feration above the invasion site (37), but more commonly, hypOplastic symptoms, such as reduced shoot and root growth, reduced plant yields, chlorosis, and reduced plant vigor, are seen (4, 18, 37). Necrotic symptoms on the roots con- sist of lesions of variable size composed of dead cortical cells (20). Hastings and Bosher (7) reported an average growth reduction of 59.6% for potato seedlings infested with P. penetrans. Oostenbrink confirmed the pathogenicity of this nematode on potato, demonstrating that P. penetrans could reduce tuber yields 20-50% (26) and total plant weight 50% (27). The initial population density of nematodes at planting (Pi) is a consistent parameter for estimating yield reductions, Oostenbrink (28) demonstrating a significant negative linear regression between initial density of P. penetrans and tuber weight of potato. Olthof St El. (23) and Olthof and Potter (25) showed that initial nematode density is related to yield loss in tobacco, potato, and other vegetable crops. Minimum nematode population levels necessary to incite damage on potato have been established by several authors. Seinhorst (32) stated that 1.0 P. penetrans/g soil produced damage on potato plants. Oostenbrink, in a synthesis of earlier work, gave a range of 0.4-l.0/g soil on sandy soil and 0.7-2.0/g soil on loam and organic soils as the minimum number necessary to decrease potato tuber yields (29). Olthof and Potter (25) established the threshold level for economic damage to potatoes at 2.0 nematodes/g soil. MATE RI ALS AND METHODS l. Pathogenicity Studies Greenhouse Cultivar Trials Five initial population densities (Pi) of g. penetrans and potato cvs. 'Onaway' and 'Russet Burbank' were used to investigate the influence of P. penetrans on the growth and development of potatoes. Initial inoculum densities of 0, 51, 84, 137, and 270 P. penetrans/100 cc soil were prepared by mixing a steam-sterilized sandy clay loam greenhouse soil with infested soil grown to Navy beans (Phaseolus vulgaris L.) for six months in greenhouse benches. Soils of different infestation levels were placed in plastic-lined metal cans 25 x 30 cm, and planted with a seed piece of one _of the varieties on November 22, 1972. Each infestation level was replicated ten times in a completely randomized design. Onaway and Russet Burbank were grown for 117 and 134 days, respectively. Plants were maintained under greenhouse conditions at 21 i 4 C. One nitrogen treatment was made 45 days after planting using urea at a rate of 250 kg/ha. The appearance and growth of each variety was monitored throughout the trials. At harvest, the tubers of each plant were weighed. 10 Bioassays to determine Pi's were prepared by planting four 500 ml pots from each infestation level with Navy bean. After 30 days, the roots of each bioassay plant were re- moved, washed free of soil, cut into 1 cm lengths, and placed in a mist chamber (31). Roots were sprayed with a solution of dihydrostreptomycin sulfate (50 ppm) and ethoxy- ethyl mercuric chloride (10 ppm) for 45 seconds every 20 minutes (2) for five days. The extracted nematodes were concentrated on a 400-mesh screen, washed into a grated dish of 100 squares and numbers recorded. Twenty randomly chosen squares were counted and the sum multiplied by five to esti- mate the number of nematodes present. Field Microplot Cultivar Trials Microplots similar to those described by Olthof and Potter (24) were used to study the effects of different Pi's on the growth and yield of five potato cultivars. Cylindrical clay drainage tiles 20 x 30 cm were covered at one end with 1.19 mm mesh nylon screen and placed screen down in a 30-cm deep hole. Excess field soil was packed around the outside of each tile. Microplot sites were cen- tered at 0.9 m intervals. Nematode-infested soil containing 0, 38, 81, 164, or 211 P. penetrans/100 cc soil was prepared as previously described. These Pi's were determined by the bioassay method described earlier. Soils for all five Pi's were put into tiles on April 25, 1973, at the Michigan State University Entomology Experiment Farm and planted with five 11 potato cultivars ('Katahdin', 'Kennebec', 'Norchip', 'Russet Burbank', and 'Superior') in a completely randomized design with each inoculation level replicated ten times per culti- var. Superior and Russet Burbank were planted as whole seed, the others as cut seed. All cultivars were planted at a depth of 6 cm. During the summer, samplings of each cultivar-infestation combination were examined at three intervals to study the nematode population dynamics. One set of plants was removed 83 days after planting, and two sets each were removed 95 and 105 days after planting. The re- maining microplots were removed when each variety had reached its maximum growth. When plants of a given cultivar exhibited signs of dieback, all the plants of that cultivar were removed, washed free of soil, and the tops and root systems and tubers weighed separately. Superior, Kennebec, Katahdin, and Russet Burbank were harvested 116, 122, 133, and 147 days after planting, respectively. The cultivar 'Norchip' was excluded because of unhealthy seed and plants. Final nematode population densities (Pf) were determined by processing 2 9 root samples from each plant using the previously described mist chamber and counting techniques. Additional root samples of cv. 'Superior' were stained with acid fuchsin in a 1:1 solution of 95% ethanol and glacial acetic acid, modified from McBryde (l3), and destained with chloral hydrate to determine the position of nematodes in the roots, These destained samples, and additional root samples of Superior preserved in 4% formalin, were also 12 used to count the number of secondary roots. Five 1-cm sections of each preserved root sample were examined. 2. Population Management Studies Field Soil Fumigation A study of the effects of soil fumigation on five potato cultivars was initiated at the Montcalm Experimental Farm, Entrican, Michigan, in the spring of 1973. The pur- pose of this study was to observe differences between fumigated and non-fumigated plants. On April 30, a mixture of methyl isothiocyanate and chlorinated C3 hydrocarbons (1,3 D—MIC) was applied to one- half of a 31 x 15 m plot arranged as a randomized block design with five replications. Fumigant was injected 20 cm beneath the surface with a broadcast eleven-shank pump-driven applicator at a rate of 142 1/ha. Fumigant was applied to the west 31 x 6.1 m and not applied to the east 31 x 6.1 m, leaving a 31 x 3 m buffer between the two test strips. The entire plot was double-disked 12 and 15 days after soil fumi- gation. Five potato cultivars ('Katahdin', 'Kennebec', 'Russet Burbank', 'Norchip', and 'Superior') were planted 19 days after fumigation. Russet Burbank and Norchip were plan- ted in 0.9 m rows at 30-cm intervals, the others in 0.9 m rows at 25-cm intervals. Guard rows were planted between blocks and rows wereégxtended beyond treatment boundaries to provide guard p1ants.I Nutrient treatments consisted of 672 kg/ha of 20-10-10 NPK fertilizer at planting, and 13 sidedresses of 78 kg/ha nitrogen 26 and 34 days after planting. The number of plants for each cultivar-treatment combi- nation was counted 49 days after planting. All five cultivars were harvested with a one-row mechanical harvester 126 days after planting. Tubers were hand-graded and weighed. The buffer zone was not included in the data. Greenhouse Nematicide Studies A greenhouse experiment was used to observe the effects of a fumigant nematicide, 1,3 D-MIC, and a non-fumigant nematicide, ethyl 3-methyl-4-(methylethy1) phosphoramidate (PHENAMIPHOS) on the tuber yield of Russet Burbank and Superior potatoes. The following treatments were used: 1. P. penetrans-infested soil; 2. 1,3 D-MIC + steam-sterilized soil; 3. PHENAMIPHOS + steam-sterilized soil; 4. 1,3 D-MIC + infested soil; 5. PHENAMIPHOS + infested soil. Soil for infested treatments was obtained from a greenhouse culture of P. penetrans grown on broadbean (Vicia faba L.). All treatments except those involving the fumigant were mixed in a cement mixer and placed in cylindrical 25 x 30 cm metal cans. PHENAMIPHOS was mixed with soil in the cement mixer before being placed in cans. The soil for each can received 0.4 g PHENAMIPHOS 15G, equivalent to a treatment of 4.5 kg ai/ha. Soil to be fumigated was treated by dispensing l4 fumigant with a pipet at a soil depth of 20 cm. The injec- tion hole was covered and the soil surface watered to prevent premature fumigant loss. Each fumigant-treated can received 0.5 ml, the equivalent of 142 1/ha. This soil was mixed five days after treatment to release any remaining fumigant. Bioassays, prepared as described earlier, re- vealed Pi's in treatments 1, 4, and 5 of 45, 4, and 2 nema- todes/100 cc soil, respectively. Cultivars were planted in the treatment cans January 4, 1974. Each cultivar was arranged as a randomized complete block design with ten replicates. All replicates were oriented in an east-west direction in a greenhouse. Russet Burbank was planted as whole seed, Superior as cut seed. Both cultivars were planted at a depth of 6 cm. Cultivars were harvested when the majority of the vines began to die back. Superior and Russet Burbank were har- vested 126 and 143 days after planting, respectively. Tubers were recovered and weighed. Due to the loss of fresh vine weight during dieback, it was not possible to weigh foliage. RESULTS AND DISCUSSION 1. Pathogenicity Studies Greenhouse Cultivar Trials Pratylenchus penetrans had no significant influence on the yield of Russet Burbank, but Onaway showed increased tuber yield at a moderate Pi (Table 3). These results indi— cate that potato cultivars respond to nematode colonization in different ways. Field Microplot Cultivar Trials The initial population density of P. penetrans (Pi) influenced tuber weights of three cultivars (Fig. l). Yields of Superior were reduced 20-30% at all Pi's, while Russet Burbank yields were unaffected by increasing densities. Katahdin demonstrated a yield increase at a Pi of 81/100 cc soil, followed by large decreases. Kennebec was unaffected up to 81/100 cc soil. These results indicate that the degree of yield reduction is directly related to the cultivar as well as the Pi. Olthof and Potter (25) found that P. penetrans reduced tuber yields of cv. Sebago at a Pi of 60/100 g soil. In this study, only Superior exhibited yield decreases at a Pi below 60. 15 16 TABLE 3. Effect of initial population density (Pi) of Pratylenchus penetrans on tuber yield of two potato cultivars grown under greenhouse con- ditions.1 Tuber weight (g) Pi/ 100 cc soil Onaway Russet Burbank 0 330 b 358 a 51 398 ab 392 a 84 499 a 396 a 137 462 a 331 a 270 435 ab 373 a 1 Numbers are means of ten replications. Column means followed by the same letter are not significantly dif- ferent according to Duncan's Multiple Range Test (P = 0.05). 17 FIG. 1. Effect of initial population level (Pi) of Pratylenchus penetrans on the tuber yield of selected potato cultivars. Points are the means of five replicates. Points with the same letter for a given variety are not signifi- cantly different according to Duncan's Multiple Range Test (P = 0.05) ( I = experimental Pi's.) Tuber yield (g) 1600‘ 1400 * 18 ‘ \ ‘ \ \ \ \‘ \ ‘ 4!) I Katahdin 800‘ \\\ ' c superior \\\c 600‘ ‘--------___c J b ”/b h 400'i I I fi j If T I T I I I I I ‘ ‘ 5O . 100 150 ‘ 200 I 0 38 81 164 211 19 No consistent relationship was found between Pi and Pf for the cultivars in this study, similar to the results of Olthof and Potter (25). Pf's in this study fluctuated con- siderably. Microplots with the highest Pi possessed a smaller Pf than those with the lowest Pi. This might be explained by Seinhorst's concept of "equilibrium density" (33): i.e., at a certain nematode density the available food is just sufficient to maintain the population. When population density is too large to be supported by the available root tissue, the nematode density falls to some supportable point. This fall can be brought about by several factors: population increase, decrease of suitable feeding sites (competition), or plant physiological changes ( e.g. suberization of roots). This hypothesis is supported by examination of microplots sampled during the growing season (Table 5). Densities fluctuated widely between sampling dates, indicating cycling around an equilibrium level. Nematode equilibrium densities varied between cultivars. They are estimated at 2000/g root for Kennebec and Katahdin, 3000/g root for Russet Burbank, and 1200/g root for Superior. Olthof and Potter (25) estimated the equilibrium density of P. penetrans on cv. 'Sebago' to be 1200-1300/g root. These data also indicate that Pf's were highest in the highest-yielding cultivar, Russet Burbank (Fig. 1), lowest in the lowest-yielding, Superior, and intermediate in Katahdin and Kennebec. Tolerance to nematode colonization 20 m . Amoo u n: umwa mmcmm mHmHuHDS m.cmocsa OH maHpnooom ucmumMMHU NHHGMUHMHGOHm poo Hm N Mo .m .x prumH meow man an UmBOHHOM momma 30H cam .o no .Q .m HmuumH mfimm map Na OQBOHHom mammE CEDHOO .mGOHHMOHHmmH m>Hm mo momma mum mumnEsz H N o mNo x o ooNN NN o HoHH N o Hooa HAN N N oooN x n oooo x N ooom N N NmNN ooH N o Noo x o Noom N on ooNH N on ooom Ho N a NmoH x N oHom N no oomN N no ommm om HOHHmmsm xcmnusm .m ownmccmx GHU£MD0M HHOm 00 OOH \Hm uoou m \w& H.mu0HQ0HoHE CH mcmuumcmm monocmHmumum mo Ammv NuHmch coHumHsmom HMCHM man no mhm>HuHso oumuom @muomHmm mo pommmm .v mqmfie 21 HGMHQ ommp m mmpmoHocH “ OGHHGMHQ kumm mmmo ” .mmumoHHmmH m>Hm mo mammfi may mum paw v MHmNB mo mmHsmHm mpHmcmp :OHHMHDQOQ Hmch wsu ou Unommmnnoo mmsHm> CEdHooInuv “mpcmHm 03¢ mo momma map mum mwsHm> cEdHOOIUHm paw ch NucMHm moo How mum mdem> cEdHoqumHHm H mNm O¢O mmm omOH mvNN mmm mmv mow HHN OOON Nmm mvm mvm voov NHON OOON mmmH OOH mom OOOH HNO mmm NOON nomm mmm MHH Hm bmmH wHHH MMOH QIIII «Ham mmNm mNOH mmOH mm OHH mOH mm Mmm th mOH mm omm HOHHmmsm Mamnusm ummmsm - HOHH hmw ONm ovNH OOOH OOOH mmm mmv HHN ovum NNNN omNN mom NMNN vav ONOH OOON «OH OONH Nmn NNv Hv vaN Nan omvH mow Hm womN mme mum ONOH mmmN OOON mMMH Hmm mm NNH mOH mm omm MMH mOH mm Mmm HHom 00 OOH \Hm omnmccmm QHosmumx Doom 0 \mcmuumcmm .M H.c0mmmm OCH30HO may OGHHDU mHm>HmucH pmuomem um mcmnumcmm monocmHmumum mo AHmV mmHuHmamo COHDMHDQOQ HmHuHaH HDOM mo moHEMCNU GOHDMHDQOQ may no mHm>HuHDo oumuom HSOM mo Hommmm .m mqmfie 22 appears to be directly related to tuber yield. Above-ground symptom expression to root-lesion nema- tode colonization by these cultivars was not as definitive as reported by other workers. No nematode-induced chlorosis, wilt, or differences in fresh weight of foliar growth were observed. Tuber weights were reduced (Fig. 1) and signifi- cant differences in final root weight among cultivars Kenne- bec and Superior were noted (Table 6). Katahdin was not harvested at the proper time of dieback and results for it may not be accurate. Differences between non-infested and infested root systems for Kennebec are illustrated in Fig. 2. Roots of plants in this experiment rarely demonstrated signs of necrosis or discoloration. Occasionally, a general tanning of the entire root system was observed, possibly attributable to either nematode colonization or suberization of the roots. In this study, P. penetrans was recovered infrequently from stolons and tubers, at densities of 1—4/g fresh weight tissue. Dickerson 32 El. (4) reported that P, penetrans caused lesions on the roots of potato cv. 'Antigo'. Mountain and Patrick (21) discovered that lesions on peach roots infested with P. penetrans were caused by hydrolysis of the glycoside amygdalin. A peach cultivar with low amounts of amygdalin supported a high nematode population without noticeable symptoms, whereas a cultivar high in amygdalin was severely stunted in the presence of a lower P. penetrans population. 23 TABLE 6. Effect of initial population density (Pi) of Pratylenchus penetrans on fresh root weights of selected potato cultivars. Fresh root weight (9)1 Pi/lOO cc soil Katahdin Kennebec R. Burbank Superior 0 11.2 a 29.8 a 8.2 a 12.6 a 38 6.6 a 16.2 b 7.4 a 6.6 b 81 9.8 a 16.4 b 8.0 a 5.8 b 164 8.2 a 12.4 b 5.8 a 5.2 b 211 8.8 a 12.0 b 7.0 a 5.8 b 1 Numbers are the means of five replicates. Column means followed by the same letter are not significantly different according to Duncan's Multiple Range Test (P = 0.05). J‘ *5 4'"?! 24 AHHom oo ooH\mmooumamc HHN .Ho .o u Hm .ugmHN op uquv .meumNm uoou oumuom ownwccwm mo wocmummmmm map so mcmngmnmm mSSUGmHhumum mo “Hwy Hw>mH GOHHMHSQOQ HMHuHcH mo uommmm .N .UHm 25 The absence of root lesions in the cultivars of the present study and their presence in Dickerson's cultivar indicate that a similar process may function in potato. The nematode pOpulation studied in these trials may also be a distinct physiological race from Dickerson's popu- lation. Slootweg (34) suggested the possibility of "differ- ent races or even species at present indistinguishable morphologically from P. penetrans." Olthof (22) differen- tiated two races of P. penetrans in Ontario on the basis of their reproductive potential and pathogenicity on tobacco and celery. In culture pots at Michigan State University, the experimental population of E. penetrans caused moderate lesioning of Navy bean roots, and heavy lesioning, blacken- ing and necrosis of broadbean roots. Examination of stained roots of Superior revealed a varying number of apparently necrotic secondary roots. An analysis of the number of healthy secondary roots versus final tuber yield gave a significant positive correlation (Fig. 3). Generally, more nematodes clustered near these than elsewhere in the root. Possibly this root-lesion nema- tode population reduces yields by feeding partially upon the bases of secondary roots, reducing total plant nutrient up- take. Further work will be required to determine precisely the role P. penetrans plays in secondary root reduction and alteration of other morphological structures and physiolo- gical activities. 26 H- 10- ' 8 9- F-I ES 8 o .‘P. ,3 . 71 o B . Q . £2 6‘ . o o H 5. o >. L4 cc 1,3 4‘ . o D a3 31 2.. r-0.746* (920.05) ‘I- _«r 460 560 600 750 800 960 1060 Tuber yield (3) FIG. 3. Correlation of tuber yield with number of secondary roots/cm lateral poot for Superior potatoes. 27 Using Michigan Department of Agriculture data (14), it is possible to estimate the economic loss threshold for nematode damage ( i.e. the minimum yield loss at which treatment becomes feasible). The approximate economic loss thresholds for four cultivars are shown in Figs. 4—7. When the dollar value of the lost yield becomes greater than the cost of control treatment, a grower should seriously con- sider nematode control. A simple method was used to predict economic loss thresholds. Three parameters are necessary: 1) anticipated loss in yield due to a known nematode Pi (obtained from Fig. 1); 2) anticipated value of the crop in $/cwt; 3) anti- cipated yield/acre. The cost of treatment should be less than the anticipated dollar loss, and a maximum feasible cost for treatment can be calculated by multiplying these three parameters. Several points for a cultivar can be plotted on a graph of cost for treatment versus Pi/lOO cc soil to form an economic threshold value. All points below a line connecting experimental points indicate treatment feasibility, while points above indicate treatment unfeasibi- 1ity. The calculation method for Figs. 4-7 follows: For the years 1969-1971, the average value of the Michigan potato crop was $2.70/cwt, with an average yield of 225 cwt/acre (14). The calculation is: Anticipated yield loss (fraction of yield) x cwt yield/acre x $ value/cwt = maximum feasible cost of treatment. 28 For a Pi of 80/100 cc soil in a Kennebec field, the maximum feasible treatment cost is, for an anticipated loss of 0.10 (obtained from Fig. 1): 0.10 x 225 cwt/a x $2.70/cwt = $60/a. Therefore, a control treatment would be profitable for a grower anticipating a 10% loss if treatment costs were below $60/acre. Furthermore, using figures for his own farm or locale, a farmer can reach a more precise estimation of the need for control. 29 FIG. 4. Feasibility of treatment based on initial population densities (Pi) of Pratylenchus penetrans for Katahdin potato tuber yield of 225 cwt/a valued at $2.70/cwt. FIG. 5. Feasibility of treatment based on initial population densities (Pi) of Pratylenchus penetrans for Kennebec potato tuber yield of 225 cwt/a valued at $2.70/cwt. o==experimental Pi's. Cost of treatment ($/acre) Cost of treatment ($/acre) 30 1 200; 2°° 150: treatment not feasible :1 1001 502 . 4 treatment fea51ble ‘0 O O . T I t .I I f I r. I T I I T I I I I I I 1 I. 50 100 150 200 Pi/lOO cc soil .4 250; I 230 1 200. l 1501 treatment not feasible 100: 503 2 treatment feasible 10 . fifi 1 .I I I F I. I l r l I r I I. I I I I I. 50 100 150 200 Pi/lOO cc soil 31 FIG. 6. Feasibility of treatment based on initial population densities (P-) of Pratylenchus penetrans for Russet Burbank potato tuber yield of 225 cwt/a valued at $2.70/cwt. FIG. 7. Feasibility of treatment based on initial population densities (P~) of Pratylenchus penetrans for Superior potato tuber yield of 225 cwt/a valued at $2.70/cwt. 0: experimental Pi's. Cost of treatment ($/acre) Cost of treatment (S/acre) 32 J .1 200- 150: 1 treatment not feasible 100* .I ' 60 50‘ 50 i *0 0 0 . I I I .I I T I I. I I I l I I I I. I I. 50 100 150 200 Pi/lOO cc soil 1 200: treatment not feasible : I72 I72 100‘ 2 treatment feasible 50‘ 1 T I I I | T I I I l I I I I I I. 1 I. ' ' 50 ° 100 150 200 Pi/lOO cc soil 33 2. Population Management Studies Field Soil Fumigation Spring fumigation showed no significant stimulatory effects on the emergence of potato cultivars and gave re- duced emergence on Kennebec and Norchip (Table 7). These results may have been confounded by at least two separate factors. Rainfall for April and May were much above average (17): April, 3.25 inches (5 year mean, 2.39), and May, 3.91 inches (mean, 2.91). These heavy rainfalls may have sealed the soil surface too tightly to release fumi- gant, even with two diskings. In addition, Kennebec and Norchip seed pieces were rotted and not fit for planting. Other varieties were healthy. Weakened seed may have been affected more severely by fumigation. These results suggest that the length of time between fumigation and planting should be varied with regard to the amount of rain. Fall fumigation is less likely to be phytotoxic than spring, due to the length of time between treatment and planting. Total yields for the field fumigation study are pre- sented in Table 8. As in emergence data, these results indicate that Kennebec and Norchip yields were significantly lower in treated plots. Fumigation had little effect on yields of the other cultivars. 34 TABLE 7. Effect of Spring fumigation on the emergence of selected potato cultivars. Percent emergence1 Cultivar Fumigated Not fumigated Katahdin 79 a x 91 ab x Kennebec 56 b x 75 be y Norchip 45 b x 73 c y R. Burbank 86 a x 96 a x Superior 78 a x 83 abc x 1 Numbers are the means of five replicates. Column means followed by the same letter a, b, or c and row means followed by the same letter x or y are not significantly different according to Duncan's Multiple Range Test (P = 0.05). 35 TABLE 8. Effect of spring fumigation on the tuber yield of selected potato cultivars. Tuber yield (kg)l Cultivar Fumigated Not fumigated Katahdin 27.4 a x 28.6 a Ix Kennebec 18.9 a x 25.2 ab y Norchip 10.1 b x 15.3 c y Russet Burbank 20.6 a x 22.7 b x Superior 22.2 a x 21.1 b x 1 Numbers are the means of five replications. Column means followed by the same letter a, b, or c and row means followed by the same letter x or y are not significantly different according to Duncan's Multiple Range Test (P = 0.05). 36 TABLE 9. Effect of spring fumigation on the mean tuber yield per plant of selected potato cultivars. Tuber yield (kg)1 Cultivar Fumigated Not fumigated Katahdin 1.56 a x 1.34 a y Kennebec 1.45 a x 1.43 a Norchip 1.12 a x 1.05 a R. Burbank 1.19 a x 1.18 a Superior 1.20 a x 1.08 a 1 Numbers are the means of five replicates. Column means followed by the same letter a, b, or c and row means followed by the same letter x or y are not significantly different according to Duncan's Multiple Range Test (P=0.05). 37 Using data in Tables 7 and 8, the average yield/plant was calculated (Table 9). A significant difference was observed between fumigated and non-fumigated Katahdin plants. The other cultivars exhibited no significant dif- ferences, although each had a slightly higher yield in fumigated areas. Fumigation decreased plant emergence, but did not decrease yields except in plants grown from weakened seed. Rainfall, temperature, and seed condition made all of these results equivocal at best and difficult to interpret. Because of uncertain climatic and planting conditions, fall fumigation should be favored over spring fumigation. Nematicide Greenhouse Studies The nematicide greenhouse studies indicated that 1,3 D-MIC and PHENAMIPHOS similarly affected nematode popu- lations and tuber yields (Table 10). Both materials reduced a Pi of 45/100 cc soil to 4 and 2/100 cc soil, respectively. Yields were increased substantially in both cultivars. Both chemicals applied to infested soil raised yields about 35% on Superior and 20% on Russet Burbank. Applied to non- infested soil, both materials increased tuber yields 80% on Superior and 40% on Russet Burbank. Using variety trial data it appeared that Russet Bur- bank was not susceptible to yield loss at these Pi's of P. penetrans, while Superior was highly susceptible. However, both cultivars exhibited the same pattern of yield increase 38 TABLE 10. Effects of nematicides and initial population density (Pi) of Pratylenchus penetrans on tuber yield of two potato cultivars. Tuber yield (9)1 Treatment P1/100cc sl. R. Burbank Superior Nematode-infested soil 45 243 c 159 c 1,3 D—MIC + infested soil 4 288 be 217 b PHENAMIPHOS + infested soil 2 304 ab 213 b 1,3 D-MIC + sterile soil 0 343 a 283 a PHENAMIPHOS + sterile soil 0 331 ab 289 a 1 Numbers are the means of ten replicates. Column means followed by the same letter are not significantly different according to Duncan's Multiple Range Test (P = 0.05). 39 when treated with nematicides. The greatest increases oc- cured with treatment of steam-sterilized soil, while smaller increases occured with treatment of infested soil. Differences in cultivar response to treatments may be par- tially due to effects of nematicides on potential plant nutrients. Altman and Tsue (1) reported an increase in available nitrogen following fumigation with dichlorOpro- pane-dichloropropene. It is possible that both types of nematicides in this experiment mobilized nutrients useful to the plants. The reaction of Russet Burbank to treated-infested soil may be due to the effect of the remaining nematode po- pulation. However, from previous pathogenicity trial data it was determined that P. penetrans did not appear to affect Russet Burbank yields. Thus it is more probable that 1,3 D- MIC and PHENAMIPHOS had a stimulatory effect in addition to the reduction of nematode pOpulations. A control of sterile soil, however, is needed to prove such a point. 1. SUMMARY AND CONCLUSIONS Potato cultivars differed in their response to different initial nematode population densities (Pi). Russet Burbank showed no reduction in tuber yield, while Onaway and Katahdin demonstrated increased tuber yield at a Pi of 80 P. penetrans/100 cc soil. Kennebec had reduced yields at 80/100 cc soil, and Superior suffered 20-30% losses at all infestation levels. Final nematode populations (Pf) could not be correlated with Pi's or other factors studied here. Variations in mid-season populations probably indicated fluctuations about the equilibrium density for a specific cultivar. It appears that Russet Burbank can support a higher nema- tode density than Katahdin or Kennebec, with Superior supporting the lowest density. Symptoms attributable to nematode colonization included decreased tuber yields, and in Kennebec and Superior, decreased root weights. Superior demonstrated a positive correlation between yields and the number of secondary roots. Tubers and stolons did not serve as food sources for P, penetrans. Root lesions were not seen, lending support to the hypothesis that different physiological races exist in this nematode species. 40 41 4. A system for determining economic loss thresholds was established for potatoes using four parameters: 1) cul- tivar grown; 2) initial density of P. penetrans; 3) anticipated yield/acre; 4) anticipated dollar value/ cwt. Using these parameters, a grower can predict a need for treatment before the crop is planted. Field fumigation studies indicated that fumigation may inhibit the emergence of sprouts if other conditions are not within acceptable ranges. Rainfall between treat- ment and planting was greater than normal and this factor may have prevented escape of the fumigant from the soil. The planting of weakened seed may have also contributed to the inconclusive emergence and yield data. Nematicide trials using 1,3 D-MIC and PHENAMIPHOS demon- strated that both nematicides produced about the same results on Russet Burbank and Superior. Application to infested and non-infested soil increased yields 20% and 40%, respectively, on Russet Burbank. On Superior, yields increased 35% and 80% on infested and non-infested soil, respectively. This effect may be partially due to soil nitrogen mobilization by the nematicides and the presence of a residual nematode population in treated- infested soil. However, neither alternative satisfies the yield increase in Russet Burbank from treated-infested to treated non-infested soil, since previous tests indica- ted a high tolerance to P. penetrans. LI TE RATURE CI TED LITERATURE CITED Altman, J. and K. M. Tsue. 1965. Changes in plant growth with chemicals used as soil fumigants. Plant. Dis. Reptr. 49: 600-602. Bird, G. W. 1971. Influence of incubation solution on the rate of recovery of Pratylenchus penetrans from cotton roots. J. Nematol. 3: 378-385. Cobb, N. A. 1917. A new parasitic nema found infesting cotton and potatoes. J. Agric. Res. 11: 27-33. Dickerson, O. J., H. M. Darling, and G. D. Griffin. 1964. Pathogenicity and population trends of Pratylenchus penetrans on potato and corn. Phyto- pathology 54: 317-322. Ferris, J. M. 1960. Effect of storage temperatures on survival of plant-parasitic nematodes in soil. (Abstr.) Phytopathology 50: 635. Hastings, R. J. 1939. The biology of the meadow nema- tode Pratylenchus pratensis (DeMan) Filipjev 1936. Can. J. Res. 17 (Sec. D): 39-44. Hastings, R. J. and J. E. Bosher. 1938. A study of the pathogenicity of the meadow nematode and associated fungus Cylindrocarpon radicicola Wr. Can. J. Res. 16 (Sec. C): 225-229. Jensen, H. J. 1953. Experimental greenhouse host range studies of two root-lesion nematodes, Pratylenchus vulnus and Pratylenchus penetrans. Plant Dis. Reptr. 37: 384-387. Kleyburg, P. and M. Oostenbrink. 1959. Nematodes in relation to plant growth. Neth. J. Agric. Sci. 17: 327-343. 42 10. ll. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 43 Knierim, J. A. 1963. Nematodes associated with crop plants in Michigan. Quart. Bull. Mich. Agr. Expt. Sta. 46: 254-262. Loof, P. A. A. 1960. Taxonomic studies on the genus Pratylenchus (Nematoda). Tijdsch. PlZiekten 66: 29-90. Mamiya, Y. 1971. Effect of temperature on the life cycle of Pratylenchus penetrans on Cryptomeria seedlings and observations on its reproduction. Nematologica 17: 82-92. McBryde, M. C. 1936. A method of demonstrating rust hyphae and haustoria in unsectioned leaf tissue. Am. J. Bot. 23: 686-688. Michigan Department of Agriculture. 1972. Michigan Agricultural Statistics, 1971. 49 pp. Michigan Department of Agriculture. 1973. Michigan Agricultural Statistics, 1972. 49 pp. Miller, P. M. and L. L. Edgington. 1962. Controlling parasitic nematodes and soil-borne diseases of potatoes with soil fumigation. Am. Potato J. 39: 235-240. Michigan State University Agricultural Experiment Station. 1973. Montcalm Experimental Farm Research Report. 48 pp. Mountain, W. B. 1961. Studies on the pathogenicity of Pratylenchus. Recent Advances in Botany 1: 414-417. Mountain, W. B. and H. R. Boyce. 1958. The peach re- plant problem in Ontario. V. The relation of para- sitic nematodes to regional differences in severity of peach replant failure. Can. J. Bot. 36: 125-134. Mountain, W. B. and H. R. Boyce. 1959. The peach re- plant problem in Ontario. VI. The relation of Pratylenchus penetrans to the growth of young peach trees. Can. J. Bot. 37: 135-151. Mountain, W. B. and Z. A. Patrick. 1959. The peach re- plant problem in Ontario. VII. The pathogenicity of Pratylenchus penetrans (Cobb, 1917) Filip. and Stek. 1941. Can. J. Bot. 37: 459-470. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 44 Olthof, T. H. A. 1968. Races of Pratylenchus penetrans, and their effect on Black Root Rot resistance of tobacco. Nematologica 14: 482-488. Olthof, T. H. A., C. F. Marks, and J. M. Elliot. 1973. Relationship between population densities of Pratylenchus penetrans and crop losses in flue- cured tobacco in Ontario. J. Nematol. 5: 158-162. Olthof, T. H. A. and J. W. Potter. 1972. Relationship between population densities of Meloidogyne hapla and crop losses in summer-maturing vegetables in Ontario. Phytopathology 62: 981-986. Olthof, T. H. A. and J. W. Potter. 1973. The relation- ship between population densities of Pratylenchus penetrans and crop losses in summer-maturing vegetables in Ontario. Phytopathology 63: 577-582. Oostenbrink, M. 1954. Over de betekenis van vrijlev- ende wortelaaljtes in land- en tuinbouw. Versl. Pl.ziektenk. Dienst. 124: 196-233. Oostenbrink, M. 1958. An inoculation trial with Praty- lenchus penetrans in potatoes. Nematologica 3: 30-33. Oostenbrink, M. 1966. Major characteristics of the relation between nematodes and plants. Meded. Landb. Hogesch. Wageningen 66-4: 1-46. Oostenbrink, M. 1972. Evaluation and integration of nematode control methods, in Economic Nematology, ed. J. M. Webster. New York: Academic Press, pp. 497-514. Oostenbrink, M., J. J. s'Jacob, and K. Kuiper. 1957. Over de Waardplanten van Pratylenchus penetrans. Tijdsch. Pl.ziekten 63: 345-360. Seinhorst, J. W. 1950. De betekenis van de toestand van de grond voor het optreden van aantasting door het stengelaaltje (Ditylenchus dipsaci (Kflhn) Filipjev). Tijdsch. Pl.ziekten. 61: 188-190. Seinhorst, J. W. 1960. Over het bepalen van door aaltjes veroorzaakte opbrengstvermindering bij cultuurge- wassen. Meded. Landb. Hoogesch. Gent 25: 1025-1040. 33. 34. 35. 36. 37. 38. 45 Seinhorst, J. W. 1966. The relationship between popula- tion increase and population density in plant para- sitic nematodes. I. Introduction and migratory nema- todes. Nematologica 12: 157-169. Slootweg, A. F. G. 1956. Rootrot of bulbs caused by Pratylenchus spp. and Hoplolaimus spp. Nematologica 1: 192-201. Society of Nematologists, Committee on Crop Losses. 1971. Estimated crop losses due to plant-parasitic nema- todes in the United States. Special Publication No. 1, Supplement to the Journal of Nematology. Sontirat, S. and R. A. Chapman. 1970. Penetration of alfalfa roots by different stages of Pratylenchus penetrans (Cobb). J. Nematol. 2: 270-271. Steiner, G. 1945. Meadow nematodes as the cause of root destruction. Phytopatholoqy 35: 935-937. Townshend, J. L. and T. R. Davidson. 1960. Some weed hosts of Pratylenchus penetrans in Premier straw- berry plantations. Can. J. Bot. 38: 267-273. M'CIIHHIIGHWINIHTIITI [Wii‘1”leISIVIITIWWWIWITI'Es 3 1293 03057 8466