Abstract

Fifty-five isolates of C. gloeosporioides recovered from euonymus leaf spot and stem lesions from four nurseries in Connecticut were tested in vitro for fungicide resistance. Commercially formulated fungicides were added to potato dextrose agar at 0, 1, 10, 100, and 1,000 μg ai/ml, and the regression of normalized growth rate was used to determine the EC₅₀. Isolates exhibited a range of sensitivity to benzimidazoles, chlorothalonil, and iprodione. Resistance to multiple fungicides was common, and fungicide resistant isolates were recovered from all nurseries. Forty-four of forty-eight isolates were resistant to thiophanate-methyl; twenty-nine had EC₅₀ values over 1,000 μg ai/ml. Thirty-four isolates had EC₅₀ values greater than 1,000 μg ai/ml chlorothalonil; only eight were less than 100 μg ai/ml. Only two isolates of C. gloeosporioides were sensitive to 10 μg ai/ml iprodione. Over 91% of isolates had EC₅₀ values of less than 500 μg ai/ml for copper hydroxide, but the growth of four was not totally inhibited by concentrations of 1,000 μg ai/ml. All isolates tested had EC₅₀ values less than 500 μg ai/ml ethylene-bis-dithiocarbamate (EBDC), and 94.5% were sensitive to less than 100 μg ai/ml of this fungicide. Inhibition zones developed only around disks amended with 10,000 μg ai/ml EBDC. Conidia in contact with disks amended with 10,000 μg g ai/ml EBDC for 48 hr did not germinate. After transfer to unamended media, fungal growth was inhibited after contact with 10,000 μg g ai/ml thiophanate-methyl or iprodione, but not with copper hydroxide. Management programs for euonymus anthracnose need to be developed to control both the disease and the selection of fungicide resistant isolates of C. gloeosporioides.