Developing a Greenhouse Protocol for Evaluating Resistance to Corynespora cassiicola in Cotton (Gossypium hirsutum)

Target spot, caused by Corynespora cassiicola, has reemerged as a disease of economic importance in cotton (Gossypium hirsutum L.) in the southeastern United States. Although this pathogen affects other economically important crops, relatively little is known about C. cassiicola on cotton, especially with regard to conditions conducive for disease development and sources of genetic resistance. Therefore, to more efficiently screen cotton genotypes for resistance, a greenhouse protocol is needed. Optimum temperature and conducive leaf wetness duration were determined in growth chamber trials. Temperature range for disease onset and greatest lesion counts occurred between 20 and 28°C. Generally, with ≥24 h of leaf wetness at these temperatures, disease onset was noted as rapidly as 1 day after inoculation on a cultivar that was previously determined to be susceptible to target spot. A mist irrigation system was used to maintain prolonged periods of leaf wetness in the greenhouse. In greenhouse trials, inoculation of cotyledons with 4 × 10⁴ conidia/ml allowed differentiation of five selected genotypes with disease reactions that reflected their field rankings. The current protocol will be useful for evaluating cotton breeding lines for resistance to target spot.