Fungi Associated with Postemergence Seedling Disease of Cotton in Three Soils

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Abstract
Soil samples were collected from 12 sites of 3 soils (Vicksburg fine sandy loam, Memphis silt loam and Dexter loam [in Tennessee, USA]) during a 12 mo. period. Fungi were isolated from discolored or necrotic hypocotyls of cotton seedlings grown in the 3 soils at 19.degree. C. More diseased hopocotyls occurred in soils not cropped to cotton than in soils cropped to cotton for 4 yr. The most frequently isolated fungi were Pythium spp., followed in descending order of frequency by Fusarium spp., Rhizoctonia solani and Thielaviopsis basicola. The Pythium spp. isolated, in order of their frequency, were P. ultimum, nonsporulating "mycelial" isolates, P. sylvaticum, P. irregulare and P. heterothallicum. In pathogenicity tests, isolates of Pythium caused the most severe disease on cotton hypocotyls, followed by isolates of Rhizoctonia, Thielaviopsis and Fusarium. Fewer diseased hypocotyls occurred on cotton grown in soils collected in Feb. than in June, Aug. or Nov. In Nov., Phythium spp. were most prevalent; Fusarium spp., R. solani and T. basicola were more prevalent in June and Aug. than in Feb. or Nov. Rhizoctonia solani and T. basicola were more prevalent in the Vicksburg fine sandy loam than in the Memphis or Dexter soils. Phythium spp. and Fusarium spp. were isolated more frequently from plants grown in soils not cropped to cotton, and R. solani and T. basicola in soils cropped to cotton.

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