A lack of diversity and durability of resistant soybean varieties complicates management of the soybean cyst nematode (SCN), Heterodera glycines, exemplified by the current overdependence on the PI 88788 source of resistance. Of interest is the effect of adaptation of a SCN population to a source of resistance on its subsequent ability to develop on others. Female indices (FI) from virulence assays (race, HG Type and SCN Type tests) for SCN field populations and inbred lines were analyzed. Female indices on PI 88788, PI 209332 and PI 548316 were highly correlated, as were those of PI 548402, PI 90763, PI 89772 and PI 438489B. Previous studies on resistant SCN-infected soybean roots indicated that the cellular resistance response was similar within these two groups of soybean genotypes. In field populations, highly significant correlations were also found between FI on PI 88788 and PI 548402 and those on PI 89772 and PI 437654. In inbred lines, FI on PI 437654 were correlated with PI 90763 and PI 438489B. To avoid further adaptation, rotation of cultivars with resistance from these groups should be carefully monitored, including those from the most promising source of resistance, PI 437654, such as CystX. In a separate test, 10 soybean varieties developed from CystX were tested against HG Type 0, HG Type 2.5.7 and HG Type 1–7. Female development occurred in all tests but one. Although identification and deployment of unique resistance is needed, management strategies to prevent and detect adaptation should be emphasized.
Adaptation; genetic diversity; Heterodera glycines; management; resistance; soybean cyst nematode
The soybean cyst nematode Heterodera glycines (SCN) is of major economic importance and widely distributed throughout soybean production regions of the United States where different maturity groups with the same sources of SCN resistance are grown. The objective of this study was to assess SCN-resistant and -susceptible soybean yield responses in infested soils across the north-central region. In 1994 and 1995, eight SCN-resistant and eight SCN-susceptible public soybean cultivars representing maturity groups (MG) I to IV were planted in 63 fields, either infested or noninfested, in 10 states in the north-central United States. Soil samples were taken to determine initial SCN population density and race, and soil classification. Data were grouped for analysis by adaptation based on MG zones. Soybean yields were 658 to 3,840 kg/ha across the sites. Soybean cyst nematode-resistant cultivars yielded better at SCN-infested sites but lost this superiority to susceptible soybean cultivars at noninfested sites. Interactions were observed among initial SCN population density, cultivar, and location. This study showed that no region-wide predictive equations could be developed for yield loss based on initial nematode populations in the soil and that yield loss due to SCN in our region was greatly confounded by other stress factors, which included temperature and moisture extremes.
crop loss; crop rotation; Glycine max; Heterodera glycines; management; resistance; soybean; soybean cyst nematode; yield; soil type
The most effective management program for soybean cyst nematode, Heterodera glycines, is a crop rotation that uses nonhost crops and resistant soybean cultivars. However, little is known about the effects of rotation crops and overwintering on H. glycines biology. These experiments were initiated to determine the effects of seven alternative crops on H. glycines' ability to infect and mature on subsequent soybean crops, and to assess the viability of eggs during the overwintering months. Rotation studies were conducted for 2 years in each of two naturally infested fields, and overwintering tests were conducted in three consecutive growing seasons in one naturally infested field. Rotation crop and fallow treatments did not have a consistent effect on the ability of H. glycines to infect soybean or mature. Soybean yields were often higher following fallow or a nonhost crop than following soybean, although not usually significantly so. Heterodera glycines egg viability did not differ (P < 0.05) between overwintering months at 0-to-10 or 10-to-20-cm soil depths. These results suggest that H. glycines' ability to infect a subsequent soybean crop and develop to maturity is not diminished by nonhost crops or during the winter months.
Avena sativa; Bbrassica rapa; canola; corn; fallow; Glycine max; Heterodera glycines; grain sorghum; infectivity; nematode; nonhost; oat; overwintering; common red clover; rotation; sesame; Sesamum indicum; Sorghum bicolor; soybean; soybean cyst nematode; trifolium pratense; viability; Zea mays
To determine whether currently used sources of resistance (soybean Plant Introductions [PI] 548402, 88788, 90763, 437654, 209332, 89772, and 548316) influence sex ratios in H. glycines, four inbred lines of the nematode characterized by zero or high numbers of females on resistant soybean were used to observe the number of adult males produced. Nematodes were allowed to infect soybean roots for 5 days in pasteurized sand. Infected plants were washed and transferred to hydroponic culture tubes. Males were collected every 2 to 3 days up to 30 days after infestation (DAI), and females were collected at 30 DAI. Resistance that suppressed adult females also altered adult male numbers. On PI 548402, 90763, and 437654, male numbers were low and close to zero, whereas on PI 88788, male numbers were higher (α = 0.05). In a separate experiment, the same PIs were infected by an inbred line that tested as an HG Type 0 (i.e., the numbers of females that developed on each PI were less than 10% of the number that developed on the standard susceptible soybean cultivar Lee). In this experiment, male numbers were similar to female numbers on PI 548402, 90763, 437654, and 89772, whereas male numbers on PI 88788, 209332, and 548316 were higher than those of females (α = 0.05). In all experiments, the total number of adults that developed to maturity relative to the number of second-stage juveniles that initially penetrated the root was less on resistant than on susceptible soybean (P ≤ 0.05), indicating that resistance influenced H. glycines survival and not sexual development.
Glycine max; Heterodera glycines; resistance; sex determination; sex ratios; soybean cyst nematode
Heterodera glycines, the soybean cyst nematode, is a major yield-limiting pathogen in most soybean production areas worldwide. Field populations of H. glycines exhibit diversity in their ability to develop on resistant soybean cultivars. Since 1970, this diversity has been characterized by a bioassay used to assign a race classification to a population. The value of the race scheme is reflected in the number and quality of resistant soybean cultivars that have been developed and released by soybean breeders and nematologists working in concert. However, the race scheme also has been misapplied as a means of studying H. glycines genotypes, in part due to the use of the term "race." For fungal and bacterial pathogen species, "race" can theoretically be applied to individuals of a population, thus allowing inference of individual genotypes. Application of a race designation to an individual egg or second-stage juvenile (J2) of H. glycines is not possible because a single J2 cannot be tested on multiple hosts. For other nematode species, "race" is defined by host ranges involving different plant species, whereas the H. glycines race test involves a set of lines of the same plant species. Nonetheless, because H. glycines populations vary in genetic diversity, and this variation has implications for management strategies, a mechanism is needed for documenting and discussing population differences. The HG Type scheme described herein avoids the implication of genetic uniformity or predictability in contrast to the way the race scheme has been used.
Heterodera glycines; HG type; nematode; races; soybean cyst nematode
The purpose of this research was to compare the overwinter survival of populations of Heterodera glycines at different latitudes in the United States and the effect of changing latitudes before and after the initiation of dormancy. Soil samples infested with H. glycines were collected in August or October in 1992 to 1994 from soybean fields in two to four states (combinations of Arkansas, Florida, Minnesota, Missouri, and Wisconsin). The samples were mixed thoroughly, divided into subsamples, shipped to an overwinter location, and buried until time for processing. To determine survival, cysts, eggs, and second-stage juveniles were extracted from replicated subsamples and counted each month from December to May. Survival generally was between 50% and 100%, and often was best in the state of origin. In Florida, survival was at the 50 to 100% level in soil from most locations, and in Wisconsin was near 100%. Survival of H. glycines in Arkansas and Missouri varied more than at the other locations. In a separate test, survival in microplots in Arkansas, in a more natural environment than that of buried samples, was 70 to 94% for field isolates from Arkansas, Minnesota, and Missouri and 100% for isolates of races 1, 3, and 14 that had been maintained in a greenhouse for several years. Survival appears to be better than previous tests had indicated. High survival rates require cultivars with high levels of resistance and long-term rotations for management.
Heterodera glycines; nematode; overwintering; soybean cyst nematode; survival
High-performance liquid chromatography and Sinorhizobium fredii USDA191 nodC-lacZ gene fusion were used to monitor changes in the isoflavonoid content of soybean roots infected with Heterodera glycines isolate TN1. Isoflavonoid concentrations in infected roots of both H. glycines-resistant Hartwig and susceptible Essex soybean were two to four-fold higher than those of uninfected roots 2 and 3 days after inoculation. The isoflavonoids produced activated the transcription of nodC-lacZ fusion. The most abundant isoflavonoids were identified as daidzein and genistein by HPLC and GC/MS. Heterodera glycines increased the number of nodules formed on Essex roots inoculated with B. japonicum (USDA110) but reduced shoot weight and decreased the net nitrogenase activity of the nodules. Heterodera glycines infection of resistant Hartwig did not affect the total number of nodules or their nitrogen- fixing capacity.
β-galactosidase; Heterodera glycines; host-parasite relationship; HPLC; isoflavonoid; nematode; nodulation; soybean; soybean cyst nematode
Polyclonal antibodies were raised against Heterodera glycines eggshells to determine the feasibility of developing an immunoassay for H. glycines eggs. An indirect enzyme-linked immunosorbent assay (ELISA) was developed from anfisera collected 10 weeks after the initial injection. From serial dilutions of sonicated eggshells or whole eggs, a sensitivity of detection to 5 ng/ml sonicated eggshells or 1 egg of H. glycines was determined. The method of eggshell preparation had no effect on the antibodies produced; however, the antibodies cross-reacted with sonicated J2 of H. glycines and eggs of Meloidogyne incognita and H. schachtii. Most of the proteins in both life stages of H. glycines and eggs of M. incognita and H. schachtii had similar migration properties when separated on SDS-PAGE gels and stained with Coomassie blue. Western blot analysis, with antisera adsorbed with homogenized J2 of H. glycines, showed proteins that were specifically localized to eggshells of H. glycines. Monoclonal antibodies might provide a useful immunoassay where polyclonal antibodies lack sufficient specificity.
adsorbed antisera; ELISA; hatching; Heterodera glycines; immunoassay; nematode
Changes in the carbohydrate (glucose, trehalose, and glycogen) and total protein contents of eggs retained within Heterodera glycines cysts were monitored monthly in a field microplot experiment conducted from March 1993 to March 1995. Treatments included two near-isogenic lines of soybean cv. Clark differing for date of maturity, and one corn hybrid. The soybean lines were planted in microplots infested with H. glycines at a high average initial population density (Pi) (23,810 eggs/100 cm³ soil), and the corn was planted in microplots infested at high (24,640) and low (5,485) Pi. Soil temperatures at 15 cm depth and rainfall were monitored. Carbohydrate contents varied in the same pattern, with the highest levels measured before planting (May) and after harvest (October) in both years. Neither Pi nor soybean isoline had an effect on any measured response, but the carbohydrate contents of eggs from corn and soybean microplots differed during the overwinter (October-May) periods (P < 0.0001). Trehalose accumulation was negatively correlated with soil temperature (r = -0.78 and r = -0.84, P = 0.0001, July through November 1993 and 1994, respectively), which reflects its role as a cryoprotectant. In contrast to the pattern for carbohydrates, total protein was lowest before planting and after harvest, and highest (>20 μg/1,000 eggs) June through October. Protein content was unaffected by plant cultivar or species. Protein and carbohydrate levels in H. glycines eggs showed seasonal changes that appeared to be primarily temperature-dependent.
biochemistry; dormancy; Glycine max; hatching; Heterodera glycines; soybean cyst nematode; trehalose
A 2-year study was conducted in field microplots to determine the relative importance of soybean phenology and soil temperature on induction of dormancy in Heterodera glycines in Missouri. Four near-isogenic soybean lines differing for maturity date were planted in microplots infested with a race 5 isolate of H. glycines. Soil temperature was monitored at a depth of 15 cm. Eggs of H. glycines, extracted from cysts collected monthly from each microplot, were used in hatching tests and bioassays to determine dormancy. Egg hatching and second-stage juvenile (J2) infectivity rates decreased sharply from their highest levels in midsummer (July-August) to a low level by October of each year and remained low (< 10% hatching and < 0.2 J2/cm root) until May or June of the following year. The patterns of numbers of females and eggs in the bioassays were similar. The decreases were not related to soil temperature and did not differ consistently among soybean isolines. The monophasic changes in all nematode responses with peak midsummer rates suggest that H. glycines produces one primary generation per year in central Missouri. Changes in hatching rates and the timing of minimum and maximum rates suggested that H. glycines eggs exhibit more than one type of dormancy.
diapause; dormancy; Glycine max; hatching; infectivity; Heterodera glycines; nematode; soybean cyst nematode; survival; viability
Experiments were conducted to determine the effects of zinc fertilizers on hatching and soil population densities of Heterodera glycines. In vitro egg hatching in solutions of reagent-grade zinc sulfate and zinc chloride and fertilizer-grade zinc sulfate was significantly greater than hatching in deionized water, whereas zinc chelate fertilizer significantly inhibited egg hatching relative to deionized water. In greenhouse experiments, no differences in cumulative percentage egg hatch were detected in soil naturally infested with H. glycines amended with fertilizer-grade zinc sulfate and zinc chelate at rates equivalent to 0, 1.12, 11.2, and 112 kg Zn/ha and subsequently planted with corn (Zea mays L.). In a field experiment, no significant differences in H. glycines egg population densities and corn yields were detected among plots fertilized with 0, 11.2, and 22.4 kg Zn/ha rates of zinc chelate. Yields of H. glycines-susceptible soybean planted in plots 1 year after zinc fertilization of corn plots also were not significantly affected. Zinc compounds significandy affected H. glycines egg hatching in vitro, but had no effect on hatching in natural soils.
Glycine max; Heterodera glycines; hatching; nematode; soybean cyst nematode; zinc fertilizer
Greenhouse tests were conducted to determine the persistence of nonviable Heterodera glycines eggs in soil. Laboratory and greenhouse tests were conducted to determine the usefulness of the vital stains acridine orange and tetrazolium red for differentiating viable and nonviable eggs of H. glycines. Nonviable (boiled) egg preparations were compared with preparations that were not boiled for their persistence in methyl bromide-fumigated soil. Boiled eggs persisted longer (>200 days) than untreated eggs, perhaps due to disinfestation of the egg suspensions by boiling. Neither stain was a good indicator of egg viability as inferred from infectivity of juveniles in a bioassay of the same egg preparations exposed to the stains.
biological control; egg viability; Heterodera glycines; nematode; soybean cyst nematode; viability stain
A nondestructive technique was developed to characterize and separate eggs of soybean cyst nematode, Heterodera glycines, by developmental stage using flow cytometry. Eggs from cysts cultured on susceptible soybean roots were suspended in 0.1% xanthan gum or 59% sucrose and loaded into either a Coulter EPICS 752 or EPICS 753 flow cytometer. Eggs were analyzed and sorted according to forward angle and 90° light scatter, flow cytometric parameters that are relative measures of object size and granularity, respectively. Mature eggs containing vermiform juveniles were less granular and slightly larger than eggs in earlier stages of embryogeny, allowing for separation of mature eggs from immature eggs. The effectiveness of flow cytometric sorting was evaluated by comparing the developmental stages of subpopulations of unsorted and sorted eggs. Of a subpopulation of unsorted eggs, 62% contained vermiform juveniles, whereas 85 to 95% of sorted subpopulations of larger, less granular eggs contained vermiform juveniles. Suspending H. glycines eggs in 0.1% xanthan gum or 59% sucrose for flow cytometric analysis had no effect on subsequent egg hatch in vitro. This technique is an efficient and effective means to collect large, relatively homogeneous quantities of H. glycines eggs in early or late embryogeny, and would likely be useful for analyzing and sorting eggs of other nematode species for use in developmental, genetic, or physiological research, or for identification and collection of parasitized eggs.
egg development; embryogeny; flow cytometry; Heterodera glycines; method; nematode; nematode egg sorting; soybean cyst nematode
Field plots of tall rescue (Festuca arundinacea) at two locations on the same experimental farm in southwestern Missouri were sampled (one in 1987-88, the other in 1988-89) to inventory root-parasitic nematodes and to determine whether cultivars or endophyte (Acremonium coenophialum) infection frequencies (EIF) affected nematode population densities within single growing seasons. Plots were planted with seven tall rescue cultivars: Kentucky-31, Kenhy, Johnstone, Martin, Mozark, Missouri-96, and Forager. Kentucky-31 seed with high and low EIF were planted in separate plots. Plant-parasitic nematodes were extracted from soil samples, identified to genus, and enumerated four and three times per year for the 1987-1988 and 1988-1989 studies, respectively. Several plant-parasitic genera were identified from both fields, including Helicotylenchus, Heterodera, Hoplolaimus, Paratylenchus, Pratylenchus, Tylenchorhynchus, and members of genera grouped in the family Tylenchidae. Densities of five of these seven groups of nematodes differed among tall fescue cultivars in the 1987-88 study, but only two out of eight groups did so in the 1988-89 study. Irrespective of tall rescue cultivar, EIF had no consistent impact on nematode densities. The putative suppressive effect of endophyte infection on infection by plant-parasitic nematodes is not detectable within single growing seasons and deserves long-term study in field situations.
Acremonium coenophialum; endophyte; Festuca arundinacea; Helicotylenchus; Heterodera; Hoplolaimus; nematode; Paratylenchus; Pratylenchus; tall fescue; Tylenchorhynchus; Tylenchidae
Between May, 1988, and July, 1992, the University of Missouri Plant Nematode Diagnostic Laboratory collected data on Heterodera glycines populations in Missouri by offering diagnostic services to soybean farmers. A greenhouse bioassay (free), egg count ($10/sample), and race determination test ($25) were conducted by request on soil samples submitted to the lab. Each test was offered for a specific purpose: the bioassay for fields not known to be infested; the egg count for population monitoring in fields with known infestations; and the race test for fields with a history of resistant cultivars. Of 818 samples submitted for bioassay on a H. glycines-susceptible soybean cultivar, 13 (1.6%) contained brown cysts but no white females, and 364 (45%) contained white females after 35 days in the greenhouse. Of 6,193 egg counts, 39% were either free of H. glycines or contained fewer than 500 eggs/250 cm³ soil, the action threshold for Missouri. The remaining 61% ranged from 500-400,000 eggs/250 cm³ soil (mean = 10,617). Eleven races were detected, with races 3 (45%) and 1 (23%) the most common. The data show that H. glycines is widespread in Missouri (with confirmed infestations in 80 of 114 counties), that most infested fields have population densities above the action threshold, and that there is considerable genetic diversity among H. glycines field populations.
bioassay; distribution; Glycine max; Heterodera glycines; nematode; race; race determination; soybean; survey
During September 1990, 30 cotton fields in each of three Missouri counties were surveyed for plant-parasitic nematodes. Soil samples for nematode analysis consisted of a composite of 20 cores collected in a zig-zag pattern within a 1-ha block in each field. Cores were taken from within weed-free cotton rows. Nine genera of plant-parasitic nematodes were found (Rotylenchulus, Helicotylenchus, Hoplolaimus, Meloidogyne, Paratylenchus, Pratylenchus, Tylenchorhynchus, Heterodera, and Trichodorus), and five species were identified: Meloidogyne incognita, Rotylenchulus reniformis, Hoplolaimus galeatus, Pratylenchus vulnus, and P. scribneri. This is the first report of R. reniformis, H. galeatus, P. vulnus, and P. scribneri in Missouri cotton fields and the first report of R. reniformis and P. vulnus in Missouri. The known cotton pathogens M. incognita, R. reniformis, and H. galeatus were found in 30%, 3%, and 2% of the fields sampled, respectively. The correlation between sand content of the soil sample and the number of vermiform M. incognita in the sample was not significant, with r² = 0.13. Select fields where H. galeatus and R. reniformis were found in 1990 were sampled more intensely in 1991. The 1-ha block sampled in 1990 was sampled in 1991, along with three other 1-ha blocks uniformly distributed within the field. In addition, a 1-ha block was sampled in each of eight nearby fields, within 2 km of the first field. The nine plant-parasitic nematode genera identified in the 1990 survey were observed again in 1991. Within-field distribution of M. incognita, R. reniformis, and H. galeatus was not uniform. When M. incognita, R. reniformis, or H. galeatus were present in a field, the same species was found in 38%, 25%, or 50% of nearby fields, respectively.
cotton; Gossypium hirsutum; Meloidogyne incognita; Missouri; nematode; Rotylenchulus reniformis; Hoplolaimus galeatus; survey
Eighteen species in eight genera of plant-parasitic nematodes were identified from soil samples collected from soybean fields throughout Missouri. The genera represented were Helicotylenchus, Heterodera, Hoplolaimus, Meloidogyne, Paratylenchus, Pratylenchus, Tylenchorhynchus, and Xiphinema. Three fields, each with high densities of Helicotylenchus pseudorobustus, Pratylenchus hexincisus, or Paratylenchus projectus, were planted in 1989 with six soybean cultivars, with plots of each cultivar either not treated or treated with 5.43 kg/ha aldicarb, to determine whether economically important relationships existed. In none of the sites were nematode densities affected by either aldicarb treatment or cultivar, nor were seed yields related to nematode densities; however, mean seed yield was significantly lower in the P. projectus site. In 1990, seed yield was negatively correlated (r = -0.34, P < 0.05) with P. projectus density at planting. Based on the present and previous studies, H. pseudorobustus and P. hexincisus do not appear to be of economic interest on soybean, but P. projectus probably deserves more study.
Glycine max; Helicotylenchus; Heterodera glycines; lesion nematodes; nematode; Paratylenchus; pin nematode; plant-parasitic nematode; Pratylenchus; soybean; spiral nematode
Soybean cultivars in maturity groups III, IV, and V that were resistant or susceptible to Heterodera glycines were evaluated for their response to in-furrow applications (5.43 kg a.i./ha) of aldicarb. Tests were conducted over three years (1988-1990) in various H. glycines-infested locations in Missouri for a total of 16 environments. In 7 environments, overall yields were higher (P ≤ 0.05) in aldicarb-treated than in nontreated plots. In one environment, soybean yields in aldicarb-treated plots were lower (P ≤ 0.05) than in nontreated plots. Cultivar x aldicarb interactions were detected in only one environment. Resistant cultivars yielded higher than susceptible cultivars in 13 of 15 environments. Final populations (Pf) of H. glycines were generally unaffected by aldicarb treatment; however, in 3 environments Pf were significantly higher, whereas in 2 other environments Pf were significantly lower in aldicarb-treated plots than in nontreated plots.
aldicarb; chemical control; Heterodera glycines; management; nematicide; nematode; soybean cyst nematode
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.
soybean cyst nematode; root-knot nematode; Glycine max; interaction; microplot; Heterodera glycines; Meloidogyne incognita
Greenhouse and field microplot studies were conducted to compare soybean shoot and root growth responses to root penetration by Heterodera glycines (Hg) and Meloidogyne incognita (Mi) individually and in combination. Soybean cultivars Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were selected for study. In the greenhouse, pot size and number of plants per pot had no effect on Hg or Mi penetration of Coker 237 roots; root weight was higher in the presence of either nematode species compared with the noninoculated controls. In greenhouse studies using a sand or soil medium, and in field microplot studies, each cultivar was grown with increasing initial population densities (Pi) of Hg or Mi. Interactions between Hg and Mi did not affect early plant growth or number of nematodes penetrating roots. Root penetration was the only response related to Pi. Mi penetration was higher in sand than in soil, and higher in the greenhouse than in the field, whereas Hg penetration was similar under all conditions. At 14 days after planting, more second-stage juveniles were present in roots of susceptible than in roots of resistant plants. Roots continued to lengthen in the greenhouse in the presence of either Mi or Hg regardless of host genotype, but only in the presence of Mi in microplots; otherwise, responses in field and greenhouse studies were similar and differed only in magnitude and variability.
soybean cyst nematode; root-knot nematode; Glycine max; root penetration; root length; microplots; Heterodera glycines; Meloidogyne incognita
Five soybean cultivars (Braxton, Gordon, Jeff, Bragg, and Wright) resistant to Meloidogyne incognita (Mi) and three susceptible cultivars (Coker 156, GaSoy 17, and Coker 237) were grown at two locations for four seasons in microplots with increasing initial soil population densities (Pi) of Mi. The resistant cultivars and Coker 156 yielded better than GaSoy 17 and Coker 237 at all Pi. Yield response was dependent on environmental conditions and at one location was stimulated on Braxton, Gordon, Jeff, and Bragg by low Pi. Although Mi reproduced well on all cultivars, the pattern of reproduction differed. Population densities of Mi leveled off after 90 days on GaSoy 17 and Coker 237 but were still increasing after 120 days on the resistant cultivars; population densities were lower on resistant than on the susceptible cultivars. The population density of Mi on Coker 156 after 120 days was intermediate between those on the other susceptible and on the resistant cultivars. Mi population densities followed the same pattern under varying environmental conditions.
resistant cultivars; nematode reproduction; yield loss; population dynamics; nematode x environment interaction
Nursery blocks (48 dogwood, 27 maple, 17 peach) in 20 middle Tennessee nurseries were sampled for nematodes in March,July, and October 1981. Dogwoods and maples were grouped in three age classes: 1-2, 3-5, and 10+ years. Nematodes were extracted from soil samples, counted, and assigned to trophic groups as follows: plant parasites, microbivores, fungivores, predators, and omnivores (= Dorylaimida). Total nematode numbers per 200 cm s soil ranged from 52 to 9,166 (mean = 1,785 ± 1,420). Nematodes were more abundant in dogwood and maple than in peach blocks, and their numbers were significantly correlated with percentage of weed ground cover and number of weed species. Nematode numbers in dogwood sites were also correlated with dogwood age. Microbivores were the most abundant trophic group in all sites, followed by plant parasites, fungivores, omnivores, and predators. Nematode communities in nursery sites shared characteristics of both undisturbed and agricultural habitats. Degree and diversity of plant ground cover appeared to be the most important factors determining nematode community structure.
Cornus florida; Acer rubrum; Prunus persica; trophic groups
Nursery blocks (48 dogwood, 27 red maple, and 17 peach) distributed among 20 Tennessee nurseries were sampled for nematodes in March, July, and October 1981. Plant-parasitic nematodes were extracted from soil, counted by genera, and identified to species after fixation. A total of 57 species in 24 genera were found, with 1-16 species occurring in a site. The species most commonly detected were Paratylenchus projectus and Xiphinema americanum, which were found in 88% and 78% of the sites, respectively. Relationships existed between distribution and densities of some species present in more than 10% of the sites and certain soil factors (pH, bulk density, texture, and organic matter content). Plant-parasitic nematode community diversity was related to tree age, percentage of weed ground cover, and number of weed species. Site similarities in community ordinations were dependent on the individual nurseries sampled, tree age, and soil type, but clusters of sites of similar tree ages and soil types were not exclusive.
Cornus florida; Acer rubrum; Prunus persica; community ordination