Plant-parasitic Nematodes in Loess Toposequences Planted with Corn.

Populations of plant-parasitic nematodes in an Iowa cornfield were studied along north- and west-facing toposequences. Samples were collected monthly during the growing season. The greatest biomass for Xiphinema americanum occurred at the footslope on the north face. Paratylenchus microdorus had its greatest biomass at the summit position, generally more in the west- than in the north-facing slope. Pratylenchus spp. in the roots peaked at the toeslope in the north-facing slope, but at the footslope in the west-facing slope. Helicotylenchus pseudorobustus peaked at the backslope and the toeslope along the north- and west-facing slopes, respectively. Diversity, as computed for each plot by the Shannon-Weiner diversity index, was highest at the backslope in both toposequences.