Upland Cotton Cultivar Evaluation for Reaction to Corynespora cassiicola
Type of DegreeMaster's Thesis
Crop Soils and Environmental Sciences
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Target spot of cotton (Gossypium hirsutum L.) is caused by the fungus Corynespora cassiicola (Berk. & Curt. C. T. Wei.) and has become a recent problem in cotton in the humid, southeastern states. Primarily occurring in the tropics and subtropics, the disease is characterized by leaf lesions, starting small then enlarging with a concentric ring appearance, thus the name target spot. High levels of defoliation can occur and yield loss estimates vary. The causal organism has been found to be extremely variable and can infect a wide range of crops, including soybean (Glycine max L. Merr.), sesame (Sesamum indicum L.), cucumber (Cucumis sativus L.), tomato (Solanum lycopersicum L.) and cotton. With the recent appearance in cotton in local areas of the southeast, it is important that resistant lines are identified. Some limited data has been reported on cotton genotypic response to C. cassiicola based on field observations. In 2012, CC made a natural appearance in the Regional Breeders Testing Network (RBTN) at the Tallassee, Alabama location. Lines were rated and the results indicate a differential response among lines with regard to symptom development. A greenhouse protocol was implemented to evaluate genotypes under controlled conditions, and to compare the results to that of their field performance. While we were able to induce high levels of disease in the greenhouse, we were not able to significantly differentiate disease levels among genotypes. In field evaluations of the Alabama Cotton Variety test locations, we were able to differentiate among genotypes in only a very limited number of cases. In addition, the greenhouse ratings for disease severity did not correlate with our field ratings. Thus it appears that there is a minimal amount of genetic variation for resistance to target spot in upland cotton.