Abstract

Fungicidal management of anthracnose leaf spot of euonymus (Euonymus fortunei ‘Emerald 'n Gold’ or ‘Emerald Gaiety’), caused by Colletotrichum gloeosporioides, was examined. Fungicide resistance was present in the pathogen population. Weekly applications to foliage for 12 weeks to the same plants in 1997 and 1998 of: thiophanate-methyl alone; or thiophanate-methyl, chlorothalonil, ethylenebis-dithiocarbamate and copper applied sequentially; or mixtures of thiophanate-methyl plus chlorothalonil alternated weekly with thiophanate-methyl plus ethylene-bis-dithiocarbamate were applied. Applications of the same treatments, azoxystrobin, or each of the above fungicides alone were applied at biweekly intervals in 1999. Unsprayed plants served as controls. Percent defoliation was lowest for fungicide mixtures (thiophanate-methyl plus chlorothalonil alternated with thiophanate-methyl plus ethylene-bis-dithiocarbamate) and was highest for unsprayed controls and plants treated with thiophanate-methyl alone. The average number of lesions per 100 leaves was lower for mixed and sequential fungicide programs than for untreated or thiophanate-methyl alone spray programs. Emerald 'n Gold had more lesions per leaf than Emerald Gaiety, but the level of defoliation was greater for Emerald Gaiety. Fungicide-resistant C. gloeosporioides isolates, which were recovered on media amended with thiophanate-methyl or with chlorothalonil, were in lowest frequency from plants treated with the mixed fungicide program. Management of euonymus anthracnose and fungicide resistance in C. gloeosporioides may be achieved by applying fungicide mixtures with different modes of action.