Distribution of Phytophthora and Pythium species in soils in the Continental United States

Abstract

The Pythium flora of 2132 soil samples from all sections of the United States was determined using an apple trapping method and two selective media. Isolates of the P. irregulare – P. debaryanum complex were obtained from 90% of the samples, and made up 55% of the 10 566 isolates examined. Isolates referable to the P. dissotocum – P. perniciosum complex, P. spinosum, P. splendens, P. ultimum, P. vexans, and heterothallic forms including P. sylvaticum, P. heterothallicum, and P. intermedium were obtained from 5 to 15% of the samples. Pythium afertile, P. aphanidermatum, P. dissotocum – P. perniciosum, P. irregulare – P. debaryanum, P. spinosum, P. torulosum, P. ultimum, and the heterothallic species were found in soil samples from 60% or more of the sections of the country sampled. The following species were isolated infrequently: P. acanthicum – P. oligandrum complex, P. catenulatum, P. helicoides – P. oedochilum complex, P. mamillatum, P. middletoni, P. myriotylum, P. paroecandrum, P. periplocum, P. prolatum, P. pulchrum, P. rostratum, P. vexans, Phytophthora cinnamomi, and P. parasitica.