Factors Affecting the Biology and Pathogenicity of Heterodera schachtii on Sugarbeet

A direct relationship exists between soil temperature and Heterodera schachtii development. The average developmental period of two nematode populations from Lewiston, Utah, and Rupert, Idaho, from J2 to J3, J4, adult, and the next generation J2 at soil temperatures of 18-28 C were 100, 140,225, and 399 degree-days (base 8 C), respectively. There was a positive relationship (P < 0.05) between nematode Pi, nematode generations, and sugarbeet yields. The greatest sugarbeet growth inhibition (87%) occurred when sugarbeets were exposed to a Pi of 12 eggs/cm³ soil for five generations (1,995 degree-days), compared with a 47% inhibition when plants were exposed to the same Pi for two generations. There was a negative correlation (P < 0.05) between the Pi, Pf, and sugarbeet yield for each population threshold. The smaller the Pi, the greater the sugarbeet yields and the greater the Pf. Root yields were 80 and 29 t /ha and Pf were 8.4 and 3.6 eggs/cm³ soil when sugarbeet seeds were planted at Pi of 0.4 and 7.9 eggs/cm³. respectively, at a soil temperature of 8 C. The number of years rotation with a nonhost crop required to reduce the nematode population density below a damage threshold level of 2 eggs/cm³ depends on the Pi. A Pi of 33.8 eggs/cm³ soil required a 5-year crop rotation, whereas a Pi of 8.4 eggs/cm³ soil required a 2-year crop rotation.     

Comparative Response of Alfalfa to Pratylenchus penetrans Populations

Four populations of Pratylenchus penetrans did not differ (P > 0.05) in their virulence or reproductive capability on Lahontan alfalfa. There was a negative relationship (r = -0 .7 9 ) between plant survival and nematode inocula densities at 26 ± 3 C in the greenhouse. All plants survived at an inoculum level (Pi) of 1 nematode/cm³ soil, whereas survival rates were 50 to 55% at 20 nematodes/cm³ soil. Alfalfa shoot and root weights were negatively correlated (r = - 0.87; P < 0.05) with nematode inoculum densities. Plant shoot weight reductions ranged from 13 % at Pi 1 nematode/cm³ soil to 69% for Pi 20 nematodes/cm³ soil, whereas root weight reductions ranged from 17% for Pi 1 nematode/cm³ soil to 75% for Pi 20 nematodes/cm³ soil. Maximum and minimum nematode reproduction (Pf/Pi) for the P. penetrans populations were 26.7 and 6.2 for Pi 1 and 20 nematodes/cm³ soil, respectively. There were negative correlations between nematode inoculum densities and plant survival (r = 0.84), and soil temperature and plant survival (r = -0 .7 8 ). Nematode reproduction was positively correlated to root weight (r = 0.89).     

Effect of Initial Nematode Density on Managing Globodera rostochiensis with Resistant Cultivars and Nonhosts

Cropping systems in which resistant potato cultivars were grown at different frequencies in rotation with susceptible cultivars and a nonhost (oats) were evaluated at four initial nematode population densities (Pi) for their ability to maintain Globodera rostochiensis at a target level of <0.2 egg/cm³ of soil. At a Pi of 0.1 to 1 egg/cm³ of soil, cropping systems with 2 successive years of a resistant cultivar every 3 years of potato production reduced and maintained G. rostochiensis at <0.2 egg/cm³ of soil. At a Pi of 1 to 4 eggs/cm³ of soil, 2 successive years of a resistant cultivar followed by 1 year of oats for every 4 years of production were necessary to reduce and maintain G. rostochiensis populations at <0.2 egg/cm³ of soil. At a Pi greater than 4 eggs/cm³ of soil, 2 successive years of a resistant cultivar plus 1 year of oats reduced G. rostochiensis densities to <0.2 egg/cm³ of soil, but the population increased above that density after cropping 1 year to a susceptible cultivar. The numbers of cysts and eggs per cyst in the final population (Pf) of G. rostochiensis were influenced by initial density and the frequency of growing a susceptible cultivar in a cropping system. The lowest number of cysts and eggs per cyst in the final G. rostochiensis population occurred with a cropping system consisting of 2 successive years of a resistant cultivar followed by oats with a susceptible cultivar grown the fourth year of production.     

Effects of Initial Nematode Density on Population Dynamics of Globodera rostochiensis on Resistant and Susceptible Potatoes

The influence of resistant and susceptible potato cultivars on Globodera rostochiensis population density changes was studied at different nematode inoculum levels (Pi) in the greenhouse and field. Soil in which one susceptible and two resistant cultivars were grown and fallow soil in pots was infested with cysts to result in densities of 0.04-75 eggs/cm³ soil. A resistant cultivar was grown in an infested field with Pi of 0.7-16.7 eggs/cm³ soil. Pi was positively correlated with decline of soil population densities due to hatch where resistant potatoes were grown in the greenhouse and in the field but not in fallow soil. However, Pi was not correlated with in vitro hatch of G. rostochiensis cysts in water or potato root diffusate. Under continuous culture o f a resistant cultivar, viable eggs per cyst declined 60-90% per plant growth cycle (4 weeks) and the number of cysts containing viable eggs had decreased by 77% after five cycles. The rate of G. rostochiensis reproduction on both resistant and susceptible cultivars was negatively correlated with Pi. These data were used to predict the effect of resistant and susceptible potato cultivars on G. rostochiensis soil population dynamics.     

Effect of Initial Nematode Population Density on the Interaction of Pratylenchus penetrans and Verticillium dahliae on 'Russet Burbank' Potato

Four similar growth chamber experiments were conducted to test the hypothesis that the initial population density (Pi) of Pratylenchus penetrans influences the severity of interactive effects of P. penetrans and Verticillium dahliae on shoot growth, photosynthesis, and tuber yield of Russet Burbank potato. In each experiment, three population densities of P. penetrans with and without concomitant inoculation with V. dahliae were compared with nematode-free controls. The three specific Pi of JR penetrans tested varied from experiment to experiment but fell in the ranges 0.8-2.5, 1.8-3.9, 2.1-8.8, and 7.5-32.4 nematodes/cm³ soil. Inoculum of V. dahliaewas mixed into soil, and the assayed density was 5.4 propagules/gram dry soil. Plants were grown 60 to 80 days in a controlled environment. Plant growth parameters in two experiments indicated significant interactions between P. penetrans and V. dahliae. In the absence of V. dahliae, P. penetrans did not reduce plant growth and tuber yield below that of the nematode-free control or did so only at the highest one or two population densities tested. In the presence of K dahliae, the lowest population density significantly reduced shoot weight and photosynthesis in three and four experiments, respectively. Higher densities had no additional effect on shoot weight and caused additional reductions in photosynthesis in only one experiment. Population densities of 0.8 and 7.5 nematodes/cm³ soil reduced tuber yield by 51% and 45%, whereas higher densities had no effect or a 15% additional effect, respectively. These data indicate that interactive effects between P. penetrans and V. dahliae on Russet Burbank potato are manifested at P. penetrans population densities less than 1 nematode/cm³ soil and that the nematode population density must be substantially higher before additional effects are apparent.     

Damage Functions and Population Changes of Hoplolaimus columbus on Cotton and Soybean

Damage functions and reproductive curves were determined for Hoplolaimus columbus on cotton cv. Deltapine 90 and soybean cv. Gordon over 2 years in field plots in Georgia. Maximum potential yield suppressions of 18% on cotton and 48% on soybean were predicted with respect to increasing Pi. Similar functions indicated yield suppressions of 38% on cotton and 30% on soybean with respect to increasing midseason nematode densities (Pm). Maximum Pf predicted by reproductive curves were 123 and 474/100 cm³ soil on cotton and soybean, respectively. Thresholds at which 10% yield suppression would occur were lower on soybean (Pi of 4) than on cotton (Pi of 70/100 cm³ soil). The economic threshold for a control measure costing $72/ha was a Pi of 60/100 cm³ soil on cotton, assuming a price for cotton lint of $1.44/kg ($0.60/lb), whereas a similar treatment would not be economically feasible on soybean at any Pi with an assumed price of $0.04/kg ($5.50/bu) soybean seed. Damage functions and reproductive curves as determined in this study offer potentially useful tools for analyzing cropping systems and providing decision tools for nematode management.     

Effect of Meloidogyne incognita and Importance of the Inoculum on the Yield of Eggplant

The relationship between population densities of race 1 of Meloidogyne incognita and yield of eggplant was studied. Microplots were infested with finely chopped nematode-infected pepper roots to give population densities of 0, 0.062, 0.125, 0.25, 0.50, 1, 2, 4, 8, 16, 32, 64, and 128 eggs and juveniles/cm³ soil. Both plant growth and yield were suppressed by the nematode. A tolerance limit of 0.054 eggs and juveniles/cm³ soil and a minimum relative yield of 0.05 at four or more eggs and juveniles/cm³ soil were derived by fitting the data with the equation y = m + (1 - m)z^P⁻T. Maximum nematode reproduction rate was 12,300. Hatch of eggs from egg masses in water or from sodium hypochlorite dissolved egg masses was similar (41% and 39%), but egg viability was significantly greater from egg masses in water (58%) than from sodium hypochlorite dissolved egg masses (12%) after 4 weeks. Greater numbers of nematodes were collected from roots of tomatoes from soil infested with entire egg masses than from tomato roots from soil infested with egg masses dissolved by sodium hypochlorite.     

Broadleaf Tobacco Yield Loss in Relation to Initial Globodera tabacum tabacum Population Density

Field microplot experiments were conducted from 1995 to 1998 to determine the relationship between fresh shoot weight of stalk-cut broadleaf and shade-grown cigar wrapper tobacco types (Nicotiana tabacum L.) and initial density of Globodera tabacum tabacum second stage juveniles (J2) per cm³ soil. Total shoot weight was negatively correlated with initial nematode densities of 12.3 to 747.3 J2/cm³ soil (r = -0.53 and -0.70 for broadleaf and shade-grown tobacco, respectively). Nonlinear damage functions were used to relate initial G. t. tabacum densities to shoot weight. The models described shoot weight losses of less than 14% or 39% for broadleaf and shade tobacco, respectively, at G. t. tabacum densities below 50 J2/cm³ soil. Total shoot weights were reduced by 40% and 60% of uninfested plots as preplant nematode densities approached maximum levels (>600 J2/cm³ soil) for broadleaf and shade tobacco, respectively. Globodera t. tabacum population increase over a growing season was described by a linear relation on a log/log plot (R² = 0.07 and 0.61 for broadleaf and shade, respectively). These experiments demonstrate that G. t. tabacum can directly reduce shoot weight of stalk-cut broadleaf tobacco. Broadleaf is more tolerant to nematode infection than shade tobacco, as shade tobacco shoot weight reductions were greater at the same initial nematode densities in the same years.     

Influence of Meloidogyne chitwoodi and M. hapla on Wheat Growth

Meloidogyne chitwoodi reduced the growth of winter wheat ''Nugaines'' directly in relation to nematode density in the greenhouse, The relationship between top dry weight and initial nematode density suggests a tolerance limit of Nugaines wheat to M. chitwoodi of between 0.03 and 0.18 eggs/cm³ of soil; the value for relative minimum plant top weight was 0.45 g and 0.75 g, respectively. Growth of wheat in field microplots containing four population densities (0.003, 0.05, 0.75 and 9 eggs/cm³ soil) was not affected significantly at any inoculum level compared to controls during September to July, However, suppression of head weights of ''Fielder'' spring wheat grown May-July occurred in microplots initially infested with 0.75 and 9 eggs/cm³ soil. Reproduction (Pf/Pi) was poorer at these two inoculum levels as compared to the lower densities. In another greenhouse experiment, roots of wheat cultivars Fielder, ''Fieldwin,'' ''Gaines,'' ''Hyslop,'' and Nugaines became infected by M. chitwoodi, but not by M. hapla. Reproduction of M. chitwoodi was less on Gaines and Nugaines than on Fielder, Fieldwin, or Hyslop.     

Damage Functions for Meloidogyne arenaria on Peanut

Microplot experiments were conducted in 1989 and 1990 to determine the relationship between yield of peanut (Arachis hypogaea) and inoculum density ofMeloidogyne arenaria race 1. Nine inoculum densities were used, ranging from 0-200 eggs/100 cm³ soil (1989) or from 0-100 eggs/100 cm³ (1990), and each density was replicated 10 times. In 1989, higher final densities (mean of 1,171 juveniles [J2]/100 cm³ soil) were obtained in plots inoculated with 0.5 to 50 eggs/100 cm³ soil than in plots inoculated with 100 to 200 eggs/100 cm³ (313 J2/100 cm³ soil). In 1990, final densities of M. arenaria reached high levels (≥ 1,111 J2/100 cm³ soil) in all inoculated plots. Pod yield and dry weight of foliage at harvest were negatively correlated (P ≤ 0.05) with inoculum density in both seasons. In 1989, the relationship between pod weight (y) and initial density (x) was described by Seinhorst''s equation, with y = 0.088 + 0.91(0.90)⁽x⁻¹⁾ and r² = 0.826. In 1990, the relationship was y = 0.22 + 0.78(0.97)⁽x⁻¹⁾ and r² = 0.794. These equations suggest tolerance limits of approximately 1 egg/100 cm³ soil, which may require specialized methods, such as bioassay, for detection.     

Yield Reduction and Root Damage to Cotton Induced by Belonolaimus longicaudatus

Sting nematode (Belonolaimus longicaudatus) is recognized as a pathogen of cotton (Gossypium hirsutum), but the expected damage from a given population density of this nematode has not been determined. The objective of this study was to quantify the effects of increasing initial population densities (Pi) of B. longicaudatus on cotton yield and root mass. In a field plot study, nematicide application and cropping history were used to obtain a wide range of Pi values. Cotton yields were regressed on Pi density of B. longicaudatus to quantify yield losses in the field. In controlled environmental chambers, cotton was grown in soil infested with increasing Pi''s of B. longicaudatus. After 40 days, root systems were collected, scanned on a desktop scanner, and root lengths were measured. Root lengths were regressed on inoculation density of B. longicaudatus to quantify reductions in the root systems. In the field, high Pi''s (>100 nematodes/130 cm³ of soil) reduced yields to near zero. In controlled environmental chamber studies, as few as 10 B. longicaudatus/130 cm³ of soil caused a 39% reduction in fine cotton roots, and 60 B. longicaudatus/130 cm³ of soil caused a 70% reduction. These results suggest that B. longicaudatus can cause significant damage to cotton at low population densities, whereas at higher densities crop failure can result.     

Pathogenicity of Pratylenchus penetrans to Navy Bean (Phaseolus vulgaris L.)

The pathogenicity of Pratylenchus penetrans (root-lesion nematode) to Phaseolus vulgaris (navy bean) was evaluated in greenhouse experiments. Shoot and root fresh weight of cv. Sanilac plants were increased 4 and 21%, respectively, by an initial population density (Pi) of 25 P. penetrans per 100 cm³ soil. Leaf area and shoot fresh and dry weights were decreased by a Pi of 50 or more P. penetrans per 100 cm³ soil. A significant positive linear relationship existed between initial soil population densities of P. penetrans and final soil and root population densities of this nematode. Three dry bean cultivars, Sanilac, Seafarer, and Tuscola, were susceptible to P. penetrans, and yields were reduced by 43-76% when plants were exposed to a Pi of 150 P. penetrans per 100 cm³ soil. P. penetrans also reproduced on bean cultivars Saginaw, Gratiot, and Kentwood, but did not decrease bean yields, suggesting that these cultivars were tolerant to this nematode.     

Crop Rotation and Races of Meloidogyne incognita in Cotton Root-knot Management

The influence o f various crop rotations and nematode inoculum levels on subsequent population densities of Meloidogyne incognita races 1 and 3 were studied in microplots. Ten different 3-year sequences o f cotton, corn, peanut, or soybean, all with cotton as the 3rd-year crop, were grown in microplots infested with each race. Cotton monoculture, two seasons o f corn, or cotton followed by corn resulted in high race 3 population densities and severe root galling on cotton the 3rd year. Peanut for 2 years preceding cotton most effectively decreased the race 3 population and root galls on cotton the 3rd year. Race 1 did not significantly influence cotton growth or yield at initial populations of up to 5,000 eggs/500 cm³ soil. At 5,000 eggs/500 cm³, cotton growth was suppressed by race 3 but yield was not affected.     

Probability of Globodera rostochiensis Spread on Equipment and Potato Tubers

The probability of spreading cysts of Globodera rostochiensis on farming equipment and potato tubers was investigated in naturally infested field plots. The number of cysts recovered from soil that adhered to equipment differed significantly between different pieces of equipment. These differences were related to initial nematode density and, in most cases, to the volume of soil that adhered to the equipment. At an initial density of 0.04 egg/cm³ of soil, significantly more cysts were recovered from a potato digger than from a potato hiller, cultivator, or plow. At an initial density of 0.90 egg/cm³ of soil, significantly more cysts were recovered from the plow than from the other equipment. Although the population density was 22 times greater, only 10 times more cysts adhered 3 to equipment used in soil with a density of 0.90 egg/cm³ of soil than when used in soil infested at 0.04 egg/cm³. The number of potato tuber samples (4.5 kg) that contained cysts with viable eggs was positively correlated with the initial densities of G. rostochiensis in soil in which they were produced. The percentage of tuber samples with cysts containing viable eggs was 10-12% for tubers harvested from soil with densities less than 1 egg/cm³ and 30-76% for tubers harvested from soil with densities greater than 4 eggs/cm³ of soil.     

Meloidogyne incognita Infested Soil Amended With Chicken Litter

The effects of chicken litter on Meloidogyne incognita in cotton, Gossypium hirsutum cv. DPL50 were determined in field microplots. Litters (manure and pine-shaving bedding) from a research facility and a commercial broiler house were used. Treatments consisted of 0.25%, 0.5%, and 1% litter by dry weight of soil for each kind of litter. Three control treatments consisted of soil not amended with litter, with and without nematodes, and one treatment to which mineral fertilizer was added at a nitrogen rate equivalent to that of the 0.5% litter rate, with nematodes. Microplots were inoculated at planting with 900 eggs/100 cm³ soil in 1993 and 1,000 eggs/100 cm³ soil in 1994. At 92 and 184 days after planting, nematode population densities decreased linearly with increasing rates of litter. Nematode numbers at midseason were larger in plots treated with mineral fertilizer than in plots treated with a rate of litter equivalent to the 0.5% rate. Fungal and bacterial population densities fluctuated throughout the growing season. Bacterial numbers had a positive linear relationship, with increasing rates of litter only in October 1993; however, significant positive relationships were observed throughout the 1994 growing season. In 1994, nematode population density at 92 days after planting decreased linearly with increasing bacterial numbers 30 days after planting. No other significant relationships between nematode densities and microbial densities were observed. Fungi and bacteria isolated from the litter and litter-amended soil were identified. Fungal genera isolated included Acremonium, Aspergillus, Eurotium, Paecilomyces, Petriella, and Scopulariopsis, whereas bacteria genera included Arthrobacter, Bacillus, and Pseudomonus.     

Control of Globodera rostochiensis in Relation to Method of Applying Nematodes

Soaking potato tuber pieces for 15 min in 8,000 μg/ml of oxamyl just before planting reduced the number of Globodera rostochiensis cysts that developed on potato roots, but this treatment was phytotoxic. Five foliar applications of 1.12 kg a.i./ha of oxamyl or carbofuran at 10-day intervals beginning when 90% of the plants had emerged suppressed increase in G. rostochiensis densities. Similar foliar applications of phenamiphos were ineffective in controlling G. rostochiensis. Soil applications (in the row at planting) of aldicarb, carbofuran, phenamiphos, ethoprop, and oxamyl at 5.6 kg a.i./ha reduced the numbers of white females that developed on potato roots, but only those treatments involving aldicarb and oxamyl suppressed G. rostochiensis population increase. Combined soil and foliar treatments did not provide any advantage over soil treatment alone, as soil applications of 5.6 kg a.i./ha alone were equal to, or better than, combined soil (3.4 kg a.i./ha) and foliar (2.2 kg a.i./ha) applications in controlling G. rostochiensis.     

Reproduction of Heterodera schachtii Schmidt on Resistant Mustard,Radish, and Sugar Beet Cultivars

The reproduction of a Wyoming population of Heterodera schachtii was determined for resistant trap crop radish (Raphanus sativus) and mustard (Sinapis alba) cultivars, and resistant and susceptible sugar beet (Beta vulgaris) cultivars in a greenhouse (21 °C/16 °C) and a growth chamber study (25 °C). Oil radish cultivars also were field tested in 2000 and 2001. In the greenhouse study, reproduction was suppressed similarly by the resistant sugar beet cultivar Nematop and all trap crop cultivars (P ≤ 0.05). In the growth chamber study, the radish cultivars were superior to most of the mustard cultivars in reducing nematode populations. All trap crops showed less reproduction than Nematop (P ≤ 0.05). In both studies, Nematop and all trap crops had lower Pf than susceptible sugar beet cultivars HH50 and HM9155 (P ≤ 0.05). In field studies, Rf values of radish cultivars decreased with increasing Pi of H. schachtii (r² = 0.59 in 2000 and r² = 0.26 in 2001). In 2000, trap crop radish cv. Colonel (Rf = 0.89) reduced nematode populations more than cv. Adagio (Rf = 4.67) and cv. Rimbo (Rf = 13.23) (P ≤ 0.05) when Pi was lower than 2.5 H. schachtii eggs and J2/cm³ soil. There were no differences in reproductive factors for radish cultivars in 2001 (P ≤ 0.05); Rf ranged from 0.23 for Adagio to 1.31 for Commodore for all Pi.     

Influence of Meloidogyne incognita on Resistant and Susceptible Sweet Potato Cultivars

Effects of several population densities ofMeloidogyne incognita on the sweet potato cultivars Centennial (susceptible) and Jasper (moderately resistant) were studied. Field plots were infested with initial levels (Pi) of 0, 10, 100, 1,000, 5,000, and 10,000 eggs and juveniles/500 cm³ soil in 1980 and 0, 100, 1,000, 2,000, 3,000, 4,000, and 5,000 in 1981. M. incognita population development trends were similar on both cultivars; however, at high Pi, more eggs and juveniles were recovered from Centennial than from Jasper. The highest Pi did not result in the highest mid-season (Pm) counts. Pi was negatively correlated with the number of marketable roots and root weight but positively correlated with total cracked roots, percentage of cracked roots, and cracking severity. Jasper tolerated higher Pi with greater yields and better root quality than Centennial. Cracking of fleshy roots occurred with both cultivars at low Pi.     

Effects of Interactions among Heterodera glycines,Meloidogyne incognita,and Host Genotype on Soybean Yield and Nematode Population Densities

The effects of host genotype and initial nematode population densities (Pi) on yield of soybean and soil population densities of Heterodera glycines (Hg) race 3 and Meloidogyne incognita (Mi) race 3 were studied in a greenhouse and field microplots in 1983 and 1984. Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were planted in soil infested with 0, 31, or 124 eggs of Hg and Mi, individually and in all combinations, per 100 cm³ soil. Yield responses of the soybean cultivars to individual and combined infestations of Hg and Mi were primarily dependent on soybean resistance or susceptibility to each species separately. Yield of Centennial was stimulated or unaffected by nematode treatments, yield of Braxton was suppressed by Hg only, and yield suppressions caused by Hg and Mi were additive and dependent on Pi for Coker 237. Other plant responses to nematodes were also dependent on host resistance or susceptibility. Population densities of Mi second-stage juveniles (J2) in soil were related to Mi Pi and remained constant in the presence of Hg for all three cultivars. Population densities of Hg J2 on the two Hg-susceptible Cultivars, Braxton and Coker 237, were suppressed in the presence of Mi at low Hg Pi.     

Effect of Criconemella onoensis on Potato

Criconemella onoensis (Luc) Luc and Raski increased to high (458-1,290/100 cm³) soil population densities in four fields planted to cover crops of sorghum-sudangrass (Sorghum bicolor (L.) Moench × S. arundinaceum (Desv.) Stapf var. sudanense (Stapf) Hitchc. ''Funk FP-4'') during the summer of 1984 in southeastern Florida. Three pathogenicity tests conducted in the greenhouse with C. onoensis on potato (Solanum tuberosum L. ''La Rouge'') using three different methods (inoculation, chemical treatment of infested soil, or pasteurization of infested soil) revealed no significant (P = 0.10) differences in plant growth, despite significant (P = 0.05) differences in population densities of C. onoensis between treated and control pots in each test. In these three tests, the maximum initial density of C. onoensis used was 720/100 cm³ soil and the maximum final density was 686/100 cm³ soil. Application of 933 liters/ha of Vapam to a field site with a pretreatment density of 1,120 C. onoensis/100 cm³ soil significantly (P = 0.05) reduced populations compared with untreated control plots, but yields remained higher in control plots. Apparently C. onoensis has no significant effect on potato growth at the population densities tested.