Recovery of Pythium arrhenomanes and its virulence to corn

Of four Pythium species isolated frequently from corn (Zea mays) roots in corn-soybean (Glycine max) rotation plots and in irrigated plots of continuous corn, only Pythium arrhenomanes was virulent to corn. P. arrhenomanes was isolated sporadically, usually from 5 to 10% of the plants sampled. However, on two dates, following 10.1 cm of rain and 5.8 cm of rain plus irrigation, the fungus was obtained from 75 and 50% of plants sampled, respectively. Twenty-two isolates of P. arrhenomanes varied in virulence from highly to weakly virulent, and the virulence of isolates remained consistent in subsequent tests on five additional hybrids. Although the incidence of the fungus could not be correlated with yield reduction, the presence of P. arrhenomanes strains with virulence to several corn hybrids indicates a possible association of the fungus with the lower yields obtained on poorly drained soil when continuous corn cropping and no tillage are practiced.