EVOLVING CONCEPTS CONCERNING THE NATURE AND CONTROL OF BOTRYTIS SPECIES PATHOGENIC TO ONION

James W. Lorbeer

Department of Plant Pathology, Cornell University, Ithaca, New York 14853, USA, E-mail: JWL5@cornell.edu

Botrytis squamosa causing Botrytis leaf blight and Botrytis allii (B. aclada) causing Botrytis neck rot (grey mold) are two species of Botrytis pathogenic to onions that have been studied extensively. Botrytis byssoidea causes mycelial neck rot. Botrytis cinerea causes Botrytis brown stain of onion bulbs and is one of the causes of Botrytis flower blight along with B. squamosa and B. allii. Three predictive systems for forecasting the occurrence of Botrytis leaf blight developed in North America (Michigan, New York, Ontario) presently are utilized commercially. Recent research conducted in Quebec, Canada indicates that epidemics of Botrytis leaf blight can begin when airborne levels of B. squamosa conidia reach 10-15 conidia per m3 of air. Using either this concentration or an average sampling of 5 lesions on the intermediate leaf of an onion plant and depending upon which came first as the trigger mechanism for fungicide application, the number of fungicide applications were reduced experimentally by 75 and 56% in 2002 and 2003. Failure of mancozeb to control Botrytis leaf blight occurs annually in Orange County, New York. The failure appears to be related to the occurrence of the sexual stage of the fungus (Bortyotinia squamosa) in the area. Fungicide mixtures are utilized in the area to simultaneously control Botrytis leaf blight and other fungal leaf diseases of onions. Programs in New York and elsewhere currently are underway to develop onions resistant to B. squamosa and enhancement of knowledge concerning the biology and control of B. allii as an onion pathogen.

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