RESUMEN
Seasonal multiplication and overwinter survival are density-dependent in Heterodera glycines. At low to moderate population densities, the nematode is capable of large population increases on susceptible soybean cultivars and high rates of oversummer or overwinter survival in the absence of a host. To improve estimates of H. glycines multiplication and survival rates, egg densities were monitored for 12 cropping sequences across 10 years. Log-linear regression analysis was used to describe and compare density-dependent relationships. Growing-season change in H. glycines egg densities was density-dependent for all crops (susceptible soybean, resistant soybean, and nonhost), with slope estimates for the density-dependent relationship greater for susceptible soybean compared with a non-host crop. Overwinter population change also was density-dependent, with similar declines in survival rates observed for all crops as population densities increased. Survival was greater following susceptible soybean compared with resistant soybean, with an intermediate rate of survival associated with non-host crops. Survival estimates greater than 100% frequently were obtained at low population densities, despite attempts to account for sampling error. Rates of growing-season multiplication and survival, when standardized for population density, declined with year of the study. Standardized overwinter survival rates were inversely related to average daily minimum temperature and monthly snow cover.
RESUMEN
The effects of soybean maturity and determinacy on the host-parasite relationships of Heterodera glycines were investigated in a field microplot study over 2 years. Determinate and indeterminate isolines of the maturity group (MG) III cultivar Williams 82 and the MG V cultivar Essex were grown in microplots artificially infested with a race 3 isolate of H. glycines at three initial population (Pi) densities (0, 300, and 3,000 eggs/100 cm(3) soil). Soybean seed yields, nematode final population (Pf) densities and reproductive index (Pf/Pi), and root colonization by Macrophomina phaseolina, the causal agent of charcoal rot, were monitored in each year. Seed yields were reduced (P = 0.05) in the presence of H. glycines in both years, but losses were greater in 1996 in the absence of drought stress. Yield loss was lower (P = 0.06) for the determinate isoline of Essex than for the other cultivar-isoline treatments across years. Nematode reproduction was density-dependent in the more conducive environment of 1996 but was unaffected by soybean maturity or determinacy traits. Root colonization by M. phaseolina increased (P = 0.05) in the presence of high H. glycines densities on determinate, but not indeterminate, isolines. Differences in H. glycines-induced yield loss among cultivar-isoline treatments were not related to nematode reproduction, M. phaseolina colonization, or environmental stresses. These results indicate that the effects of soybean maturity and determinacy on H. glycines-soybean interactions are not independent and that their combined effects must be considered in geographic regions where both traits vary.
RESUMEN
The seasonal distribution of Pratylenchus spp. in seminal and adventitious roots and the relationship of maize yield variables to nematode densities were examined for irrigated maize in sandy soil in 1994 and 1995. Nematode populations in seminal roots were stable or declined (P = 0.05) during the growing season, whereas total numbers of nematodes in adventitious root systems increased in both years of the study. Late-season nematode densities in adventitious roots were better related to midseason densities in seminal than adventitious roots. Seed test weights were negatively related to Pratylenchus spp. densities in seminal roots in both years (P = 0.05) but inconsistently related to adventitious root populations. Maize yield was inversely related to early-season nematode densities in seminal roots in 1995 (P = 0.03). Regression analyses indicated a 1% loss in seed test weight for each 10-fold increase in nematode density and a 1% loss in seed yield for each 1,000 nematodes/g root.