SENSITIVITY OF CLADOSPORIUM-CARYIGENUM TO PROPICONAZOLE AND FENBUCONAZOLE

Monoconidial isolates of the pecan scab fungus, Cladosporium caryigenu m, were obtained in 1993 and 1994 from one pecan orchard each in jeff Davis and Troup counties in Georgia, counties with no previous history of exposure to demethylation-inhibiting (DMI) fungicides. Isolates we re grown on potato dextrose agar (PDA) amended with propiconazole or f enbuconazole at 0, 0.0001, 0.0005, 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1.0, or 5.0 mu g ml(-1). After 4 to 5 weeks at 25 degrees C, the diam eter of each colony was measured. Relative growth was expressed as the colony diameter for each fungicide concentration as a percentage of t he diameter on unamended PDA. ED50 values for both fungicides in all y ears and locations were lognormally distributed. Mean ED50 values rang ed from 0.19 to 0.30 mu g ml(-1) for fenbuconazole and 0.12 to 0.17 mu g ml(-1) for propiconazole. There were highly significant positive co rrelations between sensitivity to propiconazole and sensitivity to fen buconazole at all locations and years, indicating significant potentia l for development of cross-resistance to these compounds. A discrimina tory concentration of 0.2 mu g ml(-1) propiconazole was selected for f urther monitoring of DMI sensitivity. With this discriminatory concent ration, a sample size of 50 isolates was estimated to be sufficient to detect a difference of 8.3% in mean relative growth between two popul ations.