Evaluation of Two Triazole Fungicides for Postinfection Control of Apple Scab

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Abstract
The triazole fungicides 1-((2-(2,4,-dichlorophenyl)-4-ethyl-1,3-dioxolan-2-y)methyl)-1H-1,2,4-triazole (CGA-64251) and bitertanol effectively controlled apple scab under greenhouse and orchard conditions. In greenhouse studies, postinfection scab control by CGA-64251 at 18.7 .mu.g/ml and bitertanol at 299.6 .mu.g/ml applied 2 and 3 days, respectively, after inoculation were similar to fenarimol at 41.9 .mu.g/ml, fenapanil at 617.9 .mu.g/ml and an organic mercury fungicide at 93.8 .mu.g/ml. Fungicides applied 4.0, 4.5, 5.0 and 5.5 days after inoculation did not prevent establishment of lesions, but did inhibit their development and sporulation from them. In orchard studies, 2 sprays 7 days apart of CGA-64251 at 18.7 .mu.g/ml or bitertanol at 299.6 .mu.g/ml, applied to lesions either late in the incubation period or starting 2 days after they were visible, prevented spore production in chlorotic lesions and suppressed conidial development in sporulating lesions. Fruit infections were also suppressed. Fungicides applied at 7 day intervals suppressed scab development better than those applied at 14 day intervals. Leaves sprayed with CGA-64251 were smaller, thicker, puckered, darker green, and had more layers of palisade cells than those from unsprayed trees; whether the net effect was detrimental or beneficial is undetermined. These therapeutic fungicides should be tested further in apple scab control experiments designed to identify infection periods and to suppress established epidemics of apple acab.