Detection and quantification of Thielaviopsis basicola in diseased cotton roots and San Joaquin Valley field soils

In this study an enzyme linked immunosorbent assay (EIA) and a selective medium were used to quantify Thielaviopsis basicola (Berkeley and Broome) Ferraris (syn. Chalara elegans Nag Raj and Kendrick) in diseased cotton (Gossypium hirsutum L.) roots and cotton field soils.The EIA detected both brown and gray cultural types of T. basicola and had negligible crossreactivity with other soilborne fungi commonly found in San Joaquin Valley cotton field soils. T. basicola could be detected in cotton roots 2 days after inoculation. At this time initial symptoms were apparent.The T. basicola-carrot-etridiazol-nystatin-pentachloronitro-benezene (TB-CENP) selective medium was developed and used to quantitatively determine pathogen populations in naturally infested cotton field soils. In an extensive survey of cotton fields in Kings County California, the pathogen was detected in 23 (88%) of the 26 fields surveyed with populations ranging from 1 to 220 cfu/g of soil. Black root rot was detected in 17 (66%) of the surveyed fields. Disease severity was positively correlated with inoculum density, and pathogen populations were positively correlated with the number of years cotton had been grown. T. basicola was found less frequently and in lower inoculum densities in fields in which crop rotation or summer flooding had been practiced as a control measure.In another survey for T. basicola over 500 single spore isolates were obtained from 40 San Joaquin Valley cotton field soils. The isolates were identified as either brown and gray wild types based on their initial growth in culture. Both brown and gray wild types were isolated from 8 fields whereas only gray types were isolated from 17 fields and only brown types were isolated from 11 fields. In only 4 fields were we unable to isolate T. basicola. In green house experiments the gray isolates were found to be more pathogenic on cotton than the brown isolates at 5 inoculum densities ranging from 100 to 5,000 cfu/g of soil. At inoculum densities below 100 cfu/g no difference in pathogenicity could be observed between the two wild types. The gray isolates were found to produce significantly more chlamydospores and endoconidia in culture than the brown isolates. After 15 to 20 days of growth on PDA cultural mutants began to appear. (Abstract shortened by UMI.)...