Chlorothalonil Residues on Field Tomatoes and Protection Against Alternaria solani

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Abstract
Leaf disk samples (1 cm in diameter) from sprayed plants of tomato [Lycopersicon esculentum] cv. ''New Yorker'' were analyzed for chlorothalonil residue and bioassayed for appressorial and lesion formation with spores of A. solani. Reduction of residue levels of chlorothalonil and percent inhibition of appressorial formation were linear with time. Protection, defined as inhibition of appressorial formation greater than 50%, persisted 11 days for top and middle leaves and 13 days for bottom leaves. The loss of residue and protection was linear with time and failed to coincide with rainfall, plant growth or crop age. The ED50 concentration for inhibition of appressorial formation by chlorothalonil at all leaf levels averaged 1.2 .mu.g/cm2, and against lesion formation, 0.6 .mu.g/cm2. On plants sprayed at weekly intervals, chlorothalonil exceeded the initial deposit on bottom leaves 2-5 wk after treatments were commenced; no accumulation was noted on upper and middle leaves. Evidently, a 10 day spray schedule with chlorothalonil at the standard rate for Bravo 6F would provide protection against early blight disease of tomato.

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