ARS | Publication request: Correlating reduction in fungicide sensitivity in C. beticola with loss of disease control

Submitted to: Journal of Sugar Beet Research
Publication Type: Abstract Only
Publication Acceptance Date: October 22, 2010
Publication Date: June 1, 2011
Citation: Secor, G.A., Bolton, M.D., Rivera, V.V., Khan, F.R. 2011. Correlating reduction in fungicide sensitivity in C. beticola with loss of disease control [abstract.] Journal of Sugar Beet Research. 48:89-90.

Technical Abstract: Practical resistance can be defined as a situation in which reduced sensitivity (resistance) of a fungal pathogen to a fungicide results in poor disease control in the field and the resistance is confirmed by laboratory testing. Although we have many years of data showing reduced sensitivity of C. beticola isolates to fungicides based on increased EC50 values, there is a paucity of data showing how this in vitro resistance correlates with actual loss of disease control. Ultimately, we want to be able to determine if decreased sensitivity to the fungicide detected by laboratory testing (EC50 values), results in reduced disease control in the field. Using greenhouse trials, sugarbeet plants susceptible to Cercospora leaf spot were treated with tenfold dilutions of the fungicides Eminent of Headline using a spray bar to insure uniform coverage. One day later, each group of fungicide treated plants was inoculated with spores from isolates designated as sensitive moderate, or resistant to the fungicide based on EC50 values and incubated for disease development. Disease severity was measured by counting the number of infection sites (spots) on four of the inoculated leaves of each plant and correlated with fungicide concentration and EC50 values of C. beticola isolates used for inoculum. Preliminary data shows that disease severity increases using isolates with reduced sensitivity (resistance) at comparable fungicide concentrations. There appears to be a correlation between decreased fungicide sensitivity and loss of disease control in greenhouse trials.