How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS AND SIGNS

In onions, Botrytis bulb rot generally appears during storage, although infection originates in the field. Initial symptoms usually begin at the neck, where affected tissue softens, becomes watersoaked, and turns brown. In a humid atmosphere, a gray feltlike growth appears on rotting scales and mycelia may develop between scales. Sclerotia may eventually develop in the neck and sometimes between scales. In garlic, symptoms appear either in the field towards the end of the season or during storage. Plants infected in the field may be stunted with dead and dying outer leaves. Affected tissue is initially watersoaked but later turns dry and necrotic. Sclerotia form in the neck or adhere to the rotten outer scales of the bulb. In both onion and garlic, initial infections may be latent and symptoms develop only when leaves senesce and become necrotic.

COMMENTS ON THE DISEASE

Bulb rot affects garlic, onions, leek, and shallots. The fungus persists on dead onion and garlic tissue and for long periods as sclerotia in the soil. The sclerotia germinate in moist weather and produce airborne conidia, which land on tissue, germinate, and infect when conditions are favorable. The greatest incidence of infection occurs when cool 50° to 75°F (10° to 24°C) and moist weather prevail. The fungus is associated with garlic and onions wherever they are grown and is a common colonizer of senescent tissue.

MANAGEMENT

During the growing season, minimize damage to bulbs caused by insects and diseases. Avoid heavy or late applications of nitrogen fertilizer. Harvest onions and garlic only when the crop is mature and necks are well cured. Handle the crop with a minimum of bruising or wounding. Avoid late-season irrigation to allow the tissue to dry before harvest. The neck tissue must be well-cured before the crop is stored. Healthy onions that are properly stored are seldom affected. Store bulbs at temperatures of 41°F (5°C) or less with low relative humidity and good circulation.

PUBLICATION

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Diseases
R. M. Davis, Plant Pathology, UC Davis
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Diseases:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara/San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis

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