Aggressiveness of isolates of five Pythium species on seeds and seedlings of six North American soybean cultivars

Several species of Pythium cause seed rot and damping-off of soybean in the United States and Canada. The aggressiveness of 14 isolates representing five Pythium species was evaluated on six North American soybean (Glycine max) cultivars based on seed disease in a Petri plate assay and on seedling emergence, plant and root weights, and root rot severity in temperature-controlled greenhouse experiments. In the in vitro assay, the P. aphanidermatum, P. ultimum var. ultimum and P. spinosum isolates caused higher disease severity on germinating seeds than the P. sylvaticum and P. irregulare isolates. In the greenhouse inoculum layer assays, all isolates reduced emergence and tissue weights of at least some cultivars, with significant isolate x cultivar interactions. The aggressiveness of isolates within each species varied significantly based on all or most of the disease parameters measured. At least one isolate from each species reduced emergence by at least 50% on some cultivars, but Pythium ultimum var. ultimum isolate PU 350 caused the most damping-off, stunting and root rot overall. There was a significant negative correlation between root rot severity or seedling emergence (r = -0.93, P < 0.001). 'Archer' and 'Maple Glen' were more resistant overall than 'Conrad', 'Maple Isle', 'Sloan', 'Williams', but some isolates were not very aggressive on any of the cultivars. This study revealed significant variation in the relative ability of the 14 isolates to cause disease and illustrated the importance of identifying aggressive isolates of Pythium species for applications like resistance screening or genetic mapping studies.