Comparison of Predictive Systems for Timing the Initial Fungicide Application to Control Botrytis Leaf Blight of Onion

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Abstract
Experiments were conducted in a commercial onion field during 1984-1986 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 (1985-1986) 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 1-3 wk later than did BOTCAST, resulting in a savings of 1-3 fungicide applications. Lesion levels were slightly, but significantly, higher in the CDL plots in 1985, but no differences was observed in onion yields of the treatments in any of the experiments. Although the CDL system was 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.