HPIPM:Onion Botrytis Disease

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Leotiomycetes
Subclass	Leotiomycetidae
Order	Helotiales
Family	Sclerotiniaceae
Genus	Botrytis

Scientific Name

Botrytis aclada

Scientific Name Synonyms

Botrytis allii

Botrytis alli

Common Name

Author: Howard F. Schwartz and Michael E. Bartolo

These diseases may be caused by one or more of the following pathogens Botrytis allii Munn or B. aclada, B. squamosa J. C. Walker, and B. cinerea Pers. The first fungus is the principal species affecting onions in western states. Neck rot is primarily a storage disease, although infection originates in the field as leaves and necks mature or are injured, and become infected by spores blown from infested onion debris and improperly disposed cull piles. Colorado State University studies during 1988-89 recorded an average of six to 23 Botrytis spores/hour while monitoring diseased culls from June through September. A leaf blight may occur as leaf tips die and small oval, whitish to yellowish spots form on leaves (blast). A soil-line rot and scape blight can also occur on onions grown in western regions. The soil line rot appears to be more severe on transplanted onions and during periods of cool (less than 75° F), moist weather. Blighting and girdling of onion scapes (seed stalks) shows up as whitish, necrotic, shrivelled tissue usually with sporulation on the surface or below the epidermis of the lesion.

Storage symptoms appear as a softening of the tissue in the upper part of the bulb, especially around the neck area, and progresses downward toward the basal plate. Infected tissue is brownish and soft, and often exhibits gray mycelium and thin black sclerotia on and between infected scales. Larger sclerotia may form on the surface of the neck and outer scales. Infection is favored by high moisture conditions near maturity. The fungus may spread to other bulbs in storage under moist conditions. Germinating spores (of Botrytis cinerea Pers. ex. Fr.) also can induce a superficial discoloration (brown stain) of dry, outer scales with no further development.

Disease management recommendations include crop rotation out of onions for three years; sanitation of cull pile and onion debris; use of clean seed and transplants; planting early maturing varieties at moderate plant densities; no application of nitrogen fertilizer after bulbing; apply effective fungicides such as mancozeb (Manzate, Penncozeb), chlorothalonil (Bravo), and iprodione (Rovral) after bulbing begins and weather conditions favor infection; undercut onion roots or harvest at full maturity when necks top over naturally and ideally during dry weather; air-dry and/or heat treat (at 90 to 95° F) bulbs before storage to heal injuries and cure the neck region; store at 32° F with a relative humidity less than 70 percent.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.