Ecology and Epidemiology of Pythium Species in Field Soil

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Abstract
The ecology of several species of Pythium was studied for 2 yr in a field plot at the University of Maryland Vegetable Research Farm, Salisbury [USA]. Three components of a Pythium disease complex of bean [Phaseolus vulgaris] were identified and studied: damping-off and blight of seedlings caused by P. aphanidermatum, damping-off and predominantly seedling blight caused P. myriotylum and preemergence damping-off or seed rot caused by P. ultimum and several other low-temperature Pythium spp. P. myriotylum was most destructive. The population of P. aphanidermatum was highest at the beginning of the study in Dec. 1971, and lowest in the spring of 1972, following incorporation of a rye cover crop into the soil. The population remained low thereafter. Populations of the low temperature Pythium spp. also dropped sharply in the spring of 1972; they increased again in the fall of 1972 and decreased in the spring of 1973. Increases in populations of these species were attributed to colonization of bean refuse in the fall. The failure of P. aphanidermatum to increase after incorporation of bean refuse was thought to reflect its greater sensitivity to a lowered soil pH; oospores of P. aphanidermatum did not germinate in soil at pH 5.6; those of P. ultimum did. Populations of P. myriotylum in soil could not be followed, but greenhouse bioassay for bean blight caused by P. myriotylum suggested that this species was uniformly distributed and present in sufficient numbers at each sampling date to cause a constant high rate of plant infection. In contrast, P. aphanidermatum was localized and persisted best in a more fertile portion of the field where bean growth was best. The field soil ecological relationships of the 3 groups of Pythium spp. that make up the bean disease complex were significantly different enough to constitute separate and distinct disease problems.

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