The Relationship Between Soil Populations of Meloidogyne incognita and Yield Reduction of Soybean in the Coastal Plain.

In a replicated field plot experiment, the population density of Meloidogyne incognita was monitored biweekly through the overwintering period (December through April) between soybean crops. The population survived as second-stage juveniles whose numbers remained stable through the winter months and did not decline until February. The yields of plots planted with a M. incognita susceptible cultivar were negatively correlated with the numbers of juveniles recovered at all preplanting sampling dates. In the mid-winter period (December through February), a regression equation describing the relationship predicted a yield reduction (slope) equivalent to 5.36 kg/ha for each juvenile in a 10-cm(3) soil sample. In two subsequent field experiments, conducted in different sites and years, mid-winter (November) sampling gave yield reduction predictions of 4.65 and 6.69 kg/ha. Tests of the null hypothesis gave no evidence to indicate that the three slopes differed (P = 0.05). A regression analysis of combined data from the three experiments determined a mid-winter predictive yield reduction of 5.31 kg/ha for each juvenile in the 10-cm(3) sample. As the sampling time approached the planting date, there were changes in the predictive yield reductions due to each juvenile in a sample. These are best described by the equation, gamma (yield loss) = 54.47 - 0.67X + 0.0023X(2), where X equals the days remaining between sampling and planting. Soil sampling should be performed during mid-winter (November through January) for the most reliable prediction of soybean yield loss.