Location: Southern Horticultural ResearchTitle: Growth sensitivity of Corynespora cassiicola to Thiophanate-methyl, Iprodione, and Fludioxonil) Author
Submitted to: Plant Health Progress
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/22/2011
Publication Date: 9/26/2011
Citation: Clark, J.S., Blythe, E.K., Copes, W.E., Windham, A.S., Bost, S.C., Windham, M.T. 2011. Growth sensitivity of Corynespora cassiicola to Thiophanate-methyl, Iprodione, and Fludioxonil. Plant Health Progress. doi:10.1094/PHP-2011-0926-03-RS. Interpretive Summary: Corynespora leafspot is a devastating disease problem affecting African violet production. Because control information is limited, producers have relied on a single fungicide (thiophanate-methyl) that could become ineffective if the pathogen develops resistance. Sensitivity profiles were developed for forty Corynespora isolates. Growth of the isolates was stopped by rates of thiophanate-methyl, the fungicide of concern, and two other fungicides, iprodione and fludioxonil;, therefore, chemical resistance has not developed to date. A chemical use strategy was suggested to control the disease and reduce the likelihood of the pathogen population developing resistance. The chemical use information will be immediately useful to producers of African violets, extension specialists, and crop advisors. The sensitivity profiles will be useful to research scientists and extension specialists for determining if changes in the pathogen’s sensitivity to the chemicals occur in the future.
Technical Abstract: Corynespora cassiicola (Berk. & M.A. Curtis) C.T. Wei, causal agent of Corynespora leaf spot, can cause devastating epidemics in African violet (Saintpaulia ionantha H. Wendl.) production facilities. Because of phytotoxicity and visible residue issues with some fungicides such as chlorothalonil, production facilities have relied on thiophanate-methyl, a site-specific fungicide that possesses a high risk of development of resistance by pathogen populations. During a disease outbreak at a commercial African violet production facility in September 2007, 325 isolates of C. cassiicola were collected and 40 isolates were randomly selected for in vitro testing to determine EC50 values on agar amended with concentrations of thiophanate-methyl, iprodione and fludioxonil. EC50 values for thiophanate-methyl and iprodione ranged from 0.0157 to 0.1539 µg mL-1 and 0.0833 to 0.6478 µg mL-1 with mean values of 0.0553 µg mL-1 and 0.2828 µg mL-1, respectively. Fludioxonil EC50 values ranged from 0.0013 to 0.0103 µg mL-1 with a mean value of 0.0075 µg mL-1. All isolates were considered sensitive to the fungicides tested. The resistance factors, defined as the ratio of an insensitive isolate’s EC50 value to the mean EC50 for the sensitive isolates, were 2.8, 2.3, and 1.4, respectively, for thiophanate methyl, iprodione, and fludioxonil. All results indicate a population that currently exhibits no evidence of being insensitive for all three fungicides. Correlation coefficients were calculated to determine cross-sensitivity between fungicides. Among the three fungicides, iprodione and fludioxonil had a moderately significant correlation (r = 0.38686; p = 0.0125). Development of baseline sensitivity and resistance probability data is the first step in determining fungal population sensitivity shifts.