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Timing Initial Fungicide Application to Control Botrytis Leaf Blight Epidemics on Onions. P. B. Shoemaker, Former Research Assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Senior author is presently Extension Associate Professor, Department of Plant Pathology, North Carolina State University, Mountain Horticultural Crops Research Station, Fletcher, NC 28732; J. W. Lorbeer, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 67:409-414. Accepted for publication 7 September 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-409.

In preliminary studies, the initiation of fungicide application when Botrytis squamosa lesions averaged one per 10 leaves resulted in higher yields of onions than when applications were commenced before or after this time. This disease level accordingly was utilized as the critical disease level (CDL) for timing the initiation of weekly fungicide sprays in subsequent studies. Yields from fields in which spray treatments were initiated at or soon after the CDL were equal to yields where weekly sprays were initiated 1-2 wk earlier. Use of the CDL method reduced by one to three the eight (12-38%) recommended spray applications during 3 years of testing. Disease progress curves generally showed slower rates of increase when fungicide sprays were initiated prior to detection of the CDL, but appeared to converge with time for treatments initially sprayed prior to, at, and past detection of the CDL. Analyses of disease progress curves from unsprayed plots in different fields indicated that leaf blight epidemics differed with respect to starting times and rates of development in the same as well as in different years. Botrytis squamosa conidia were trapped in a Hirst spore trap 2 wk prior to detection of the CDL and may be useful as an alternative method for timing fungicide initiation.

Additional keywords: epidemiology, onion leaf blight, pest management, disease threshold.