DYNAMICS OF SELECTED NEMATODES ASSOCIATED WITH SOYBEANS

Field: Agricultural and Biological Sciences (Life Sciences)

Examination of dispersional characteristics of Pratylenchus scribneri and Hoplolaimus galeatus indicated that there were patches within soybean fields in which both survival and reproduction were enhanced in spite of apparent homogeneity of soil type and topography. Carbofuran treatment reduced the patchiness (or increased the dispersion) for H. galeatus while it had the opposite effect for P. scribneri. P. scribneri was less highly dispersed in conventional tillage plots than in the zero tillage plots. Plot sets contained entirely within the patches could be described by the normal distribution (in the case of P. scribneri) or by the Poisson distribution (in the case of H. galeatus), while plot sets contained entirely outside the patches could be described by the Poisson distribution for both nematodes. None of the distributions tested gave an adequate fit when plot sets from both inside and outside the patches were considered together. In all instances log(,10) and ln transformations reduced the goodness of fit of the data to all of the distributions tested. Parametric statistics were not appropriate for analysis of data in most instances, even with logarithmic transformations. Nematode communities in soybean field soil were studied by means of principal components analysis. Analysis of early season data from conventional tillage and zero tillage plots showed that the difference in tillage treatments (over several years) did not result in clear separation of the plot groups. Data taken later in the season, however, produced clear separation, probably as a result of secondary effects of the two different tillage treatments on plant growth and/or soil environment. Principal components analysis of data from carbofuran test plots and checks did not result in clear groupings (which corresponded to treatments) until late in the growing season, indicating a delay in the full expression of the treatment on nematode community structure. A computer simulation model for the Pratylenchus scribneri-soybean system was formulated using the GASP IV simulation language. The model is driven by cumulative degree days. Growth in root mass and nematode population levels is affected by a modification of the exponential equation with variable reproductive or growth rates. The model was constructed so that it could be modified easily to fit other systems involving a migratory endoparasitic nematode species. Both the median and the mean were simulated. When the median was used in the simulation and compared to medians of the validation data, the model appeared to be more reliable than when means were used. Dispersional characteristics were evaluated for six species of ectoparasitic or non-parasitic nematodes in soybean fields under different management practices. Nematodes tended to become less highly dispersed as soil counts increased and more highly dispersed as soil counts decreased. Carbofuran treatment tended to make the dispersional level more stable, although the timing of this effect varied with species. Dorylaimid and mononchid nematodes exhibited more volatile dispersional levels in the conventional tillage plots while the rhabditid nematodes exhibited more volatile dispersional levels in the zero tillage plots.