Growth Sensitivity of Corynespora cassiicola to Thiophanate-methyl, Iprodione, and Fludioxonil

An African violet production facility has relied solely on thiophanate-methyl, a site-specific fungicide that possesses a high risk for pathogens to develop resistance, to manage a devastating Corynespora leaf spot problem. During a disease outbreak in September 2007, 325 isolates of C. cassiicola were collected and 40 isolates were randomly selected to determine the pathogen's sensitivity for mycelium growth on agar amended with various concentrations of thiophanate-methyl, iprodione, or fludioxonil. EC50 values, concentration resulting in a 50% reduction in mycelium growth, were determined and indicate a population that currently is sensitive to all three fungicides. Due to the high risk of the pathogen developing resistance to thiophanate-methyl and iprodione, a moderate risk for cross-sensitivity between iprodione and fludioxonil, and phytotoxicity and visual residue problems with protective fungicides such as chlorothalonil, a fungicide rotation is recommended with fludioxonil as the main chemical selection. Accepted for publication 22 July 2011. Published 26 September 2011.