Thielaviopsis basicola in San Joaquin Valley Soils and the Relationship Between Inoculum Density and Disease Severity of Cotton Seedlings. B. A. HOLTZ, Former graduate student researcher, Department of Plant Pathology, University of California, Berkeley 94720. A. R. WEINHOLD, professor, Department of Plant Pathology, University of California, Berkeley 94720. Plant Dis. 78:986-990. Accepted for publication 20 July 1994. Copyright 1994 The American Phytopathological Society 986. DOI: 10.1094/PD-78-0986.

Populations of Thielaviopsis hasicola in naturally infested cotton field soils in the San Joaquin Valley of California were determined in 1992 with modified Specht's T. ba.nro/fl-carrot-etridiazol-nystatin medium. In cotton fields in Kings County, CA, the pathogen was detected in 24 (88%) oi the 27 fields surveyed, with a mean population density of 77.6 cfu/g of soil and a range of I to 220 cfu/g of soil. Black root rot was detected in 79% of the fields where plants also were sampled. Disease severity was positively correlated with inoculum density, and pathogen populations were positively correlated with the number of years fields were planted to cotton. T. basicola was found less frequently and at lower population densities in fields where crop rotation or summer flooding had been practiced, compared with fields planted continuously to cotton.

Keyword(s): Chalara elegans