Populations and Biology of Pythium Species Associated with Snap Bean Roots and Soils in New York

Abstract

Soil populations of total Pythium spp. varied considerably between and within bean [Phaseolus vulgaris] fields in New York [USA]. Average counts of low- and high-temperature Pythium spp. ranged 133-1560 and 22-95 propagules/g oven-dry soil, respectively. P. ultimum, P. irregulare and unidentified sporangial-forming Pythium sp. isolates were recovered from bean soils at 21.degree. C and accounted for 80, 2 and 16% of the isolates, respectively, obtained from 1 field. However, the frequency of recovery of these species differed markedly between fields, although P. ultimum always was the most abundant pathogenic species. Only P. oligandrum was isolated from soil samples incubated at 37.degree. C. These same Pythium spp. also were isolated from roots and hypocotyls of naturally infected beans as early as 7 days after planting, and the percent recovery increased with plant age. Pythium spp. were recovered more frequently from bean roots than from hypocotyl tissues. Low- and high-temperature Pythium spp. were isolated from 55-92 and 0-28%, respectively, of the root systems of 8 wk old plants. In pathogenicity tests, P. ultimum and P. irregulare caused severe pre- and postemergence damping-off and root rot of beans, whereas P. oligandrum and the unidentified sporangial isolates produced limited or no damage. Under high moisture conditions, lesions incited by P. ultimum on bean tissues below the soil line continued to move upwards until the terminal bud was infected, resulting in plant death. A direct correlation was found between inoculum density of P. ultimum and root-rot severity on beans in pasteurized soil. Root-rot severity ratings were increased (1.8-4.0) and dry wt per plant was decreased (0.98-0.49 g) as inoculum density was increased from 1 to 500 propagules/g oven-dry soil. Four to 28 day old bean plants were equally susceptible to root rot incited by P. ultimum.