Physical, chemical, and biological characteristics of cotton soils and their relationship to seedling disease of cotton

Cotton (Gossypium hirsutum) seedling disease is responsible for considerable reductions of cotton yields in Tennessee each year. The complex of causal organisms, in order of pathogenicity, includes species of Pythium, Rhizoctonia, Thielaviopsis, and Fusarium. Attempts to control the disease have been mostly unsuccessful, especially during cool, wet periods which favor disease development. Because of the lack of a good control, a bioassay technique was tested for accuracy in predicting occurrence and severity of cotton seedling disease. It was hoped that by using this technique better recommendations could be made on selecting fields for cotton culture. A quantitative method for isolating Pythium from soil on a selective medium was compared to the bioassay technique for effectiveness in predicting the disease.

Forty-five soil samples were collected at 15 sites in West Tennessee in the fall of 1982 and again in the fall of 1983. The bioassay procedure consisted of planting Stoneville 213 cotton seeds in pots filled with each soil sample. The seedlings that emerged were each evaluated for disease severity and the causal organism was isolated from each diseased plant. A soil assay for Pythium was performed on each soil sample using gallic acid medium. In addition, each soil sample was analyzed for pH, texture, and organic matter content. A field test was conducted in 1984 which consisted of planting Stoneville 213 cotton seed at the sites where soil samples were taken the previous fall. Emerged seedlings were evaluated for disease severity as in the bioassay procedure. Percent stand and percent post-emergence mortality were also determined.

Correlation coefficients were calculated each year between all the variables that were evaluated. There was a definite negative relationship between the percent clay in the soil and the disease severity of seedlings grown in the field. Disease severity of the seedlings grown during the bioassay procedure and numbers of seedlings infected with Pythium and/or Rhizoctonia did not correlate with the disease severity of the seedlings in the field. Also, disease severity correlated positively with soil pH.

Significant correlations occurred both years between percent clay and the disease indexes from the bioassay, the number of Fusarium isolates and disease indexes from the bioassay, pH and the number of Rhizoctonia isolates, and percent clay and the number of Fusarium isolates. There was no correlation between the numbers of Pythium colonies isolated on gallic acid medium and any of the other variables either year.