Importance of Pythium ultimum in a disease syndrome of cv. Essex soybean

Abstract

In Virginia, a disease syndrome characterized by pre-emergence damping-off, delayed emergence, fine root necrosis, stunting, and, under dry conditions, by chlorosis, wilting, and plant death is common on cultivar Essex soybean. In growth chamber studies with naturally infested soil, frequency of isolation of Pythium ultimum from cotyledons and hypocotyls of seedlings was high (72-79%) and significantly greater from susceptible cultivar Essex than from cultivar Dare and V81-141, both resistant to the disease syndrome. Treatment of seeds of cultivar Essex with metalaxyl significantly increased seedling emergence in the growth chamber and field and decreased the disease severity index. In the field, the inoculum density of P. ultimum was greater in an area with a high level of disease than in an area with a low level of disease. In areas where the early phase of the syndrome (delayed emergence and damping-off) was severe, the late phase of the syndrome (stunting and wilt) was also severe. A low percentage (generally less than 8%) of the fine root segments of stunted plants of cultivar Essex assayed over 3 years was colonized by P. ultimum, except in early 1989 when late-phase disease was most severe. Pythium ultimum appears to be the main component of a fungal complex associated with the early phase of the disease and a contributor to the late phase of the disease.