APS 2009 Annual Meeting Abstract of Presentation

A root rot of soybean (Glycine max) caused by Phytophthora sansomeana sp nov.
P. REESER (1), E. Hansen (1)
(1) Oregon State University, Corvallis, OR, USA
Phytopathology 99:S107

In 1990 a serious root disease of soybean in Indiana was associated with an unknown Phytophthora species, now named P. sansomeana. Plants in affected fields suffered severe yellowing and stunting after a period of unusually cool, wet weather in July. Symptoms were severe on varieties with the Rps1-k gene for race-specific resistance to P. sojae. Some isolates from diseased plants could not be assigned to race because of non-specific response in a set of 8 differential cultivars used for P. sojae race-classification, and showed certain morphological differences from P. sojae, but shared similarities with an unknown Phytophthora spp. isolated from Douglas-fir seedling roots in the Pacific Northwest and from herbaceous weeds in New England states. Soybeans grown at 15 and 20°C in sand infested with soybean isolates of P. sansomeana, and subjected to periodic flooding, showed yellowing, stunting, and root disease compared with un-infested controls. Soybeans grown in naturally infested field plots developed yellowing, stunting and root disease, compared with Ridomil treated plots. In all cases P. sansomeana was recovered from diseased roots. Stem cankers usually characteristic of infection with P. sojae were not common. Soybean root disease caused by P. sansomeana appears to be favored by cool soil temperatures and flooding, and is not affected by race-specific resistance to P. sojae.