Plant Disease (1997) 81, 163-166

Katherine L. Reynolds, Timothy B. Brenneman and Paul F. Bertrand (1997)
Sensitivity of Cladosporium caryigenum to propiconazole and fenbuconazole
Plant Disease 81 (2), 163-166
Abstract: Monoconidial isolates of the pecan scab fungus, Cladosporium caryigenum, 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 were grown on potato dextrose agar (PDA) amended with propiconazole or fenbuconazole at 0, 0.0001, 0.0005, 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1.0, or 5.0 µg ml^-1. After 4 to 5 weeks at 25°C, the diameter of each colony was measured. Relative growth was expressed as the colony diameter for each fungicide concentration as a percentage of the diameter on unamended PDA. ED₅₀ values for both fungicides in all years and locations were lognormally distributed. Mean ED₅₀ values ranged from 0.19 to 0.30 µg ml^-1 for fenbuconazole and 0.12 to 0.17 µg ml^-1 for propiconazole. There were highly significant positive correlations between sensitivity to propiconazole and sensitivity to fenbuconazole at all locations and years, indicating significant potential for development of cross-resistance to these compounds. A discriminatory concentration of 0.2 µg ml^-1 propiconazole was selected for further monitoring of DMI sensitivity. With this discriminatory concentration, a sample size of 50 isolates was estimated to be sufficient to detect a difference of 8.3% in mean relative growth between two populations.
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Research topic(s) for pests/diseases/weeds:
control - general
pesticide resistance of pest

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Venturia effusa	Pecan/hickory (Carya)	U.S.A. (SE)