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Comparison of Predictive Systems for Timing the Initial Fungicide Application to Control Botrytis Leaf Blight of Onion. P. C. Vincelli, Research Assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. J. W. Lorbeer, Professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Plant Dis. 72:632-635. Accepted for publication 10 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0632.

Experiments were conducted in a commercial onion field during 19841986 to compare two predictive systems that time the initiation of a fungicide spray program for controlling Botrytis leaf blight of onion. Using the critical disease level (CDL) system, the first fungicide application was called for when lesion counts exceeded 1.0 lesion/leaf. Using BOTCAST, the first application was made when 35 (1984) or 25 (19851986) disease severity units had accumulated and rain was indicated in the weather forecast. During the 3 years of field experiments, the CDL system called for the first fungicide application 13 wk later than did BOTCAST, resulting in a savings of 13 fungicide applications. Lesion levels were slightly, but significantly, higher in the CDL plots in 1985, but no difference was observed in onion yields of the treatments in any of the experiments. Although the CDL system was the more sensitive method for initiating fungicide applications for effective control of Botrytis leaf blight, a combination of the CDL system and BOTCAST may be most efficient in order to reduce grower reliance on intensive field scouting.

Keyword(s): Allium cepa, Botrytis squamosa, integrated pest management.