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Disease Control and Pest Management

Chlorothalonil Residues on Field Tomatoes and Protection Against Alternaria solani. R. J. Lukens, Plant Pathologist, Department of Plant Pathology, The Connecticut Agricultural Experiment Station, New Haven 06504, Present address of senior author: Chevron Chemical Co., 940 Hensley St., Richmond, CA 94804; S. H. Ou, Plant Pathologist, Department of Plant Pathology, The Connecticut Agricultural Experiment Station, New Haven 06504. Present address: Department of Plant Pathology, International Rice Research Institute, Manila, The Philippines. Phytopathology 66:1018-1022. Accepted for publication 14 November 1975. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1018.

Leaf disk samples (1 cm in diameter) from sprayed plants of tomato cultivar New Yorker were analyzed for chlorothalonil residue and bioassayed for appressorial and lesion formation with spores of Alternaria 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 percent, 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 µg/cm², and against lesion formation, 0.6 µg/cm². On plants sprayed at weekly intervals, chlorothalonil exceeded the initial deposit on bottom leaves 2-5 weeks after treatments were commenced, whereas 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.