Botrytis Neck and Bulb Rot

Botrytis neck and bulb rot is a disease of both onion and garlic, as well as leek and shallot.

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 water-soaked, and turns brown. In a humid atmosphere, a gray and feltlike growth (where spores are produced) appears on rotting scales, and mycelia may develop between scales. Dark-brown-to-black sclerotia (the resting bodies of the pathogen) may eventually develop in the neck or 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 water-soaked, 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 not be noticeable, and symptoms may develop only when leaves senesce and become necrotic.

Botrytis allii and Botrytis aclada cause this disease in onion, whereas B. porri causes it in garlic. These fungi persist on dead onion and garlic tissue and persist as sclerotia in the soil for long periods. The sclerotia germinate in moist weather and produce airborne conidia (spores), which land on tissue, germinate, and infect when conditions are favorable.

The greatest incidence of infection occurs in persistent cool (50° to 75°F) and moist weather. Infections are favored by excessive use of nitrogen fertilizer, applications of nitrogen after bulb initiation, and poor drying of the tops and curing of the necks.

The fungi are associated with garlic and onions wherever they are grown and are common colonizers of senescent tissue. Infection can also occur through wound sites on the bulb.

Cultural control (especially methods that reduce crop injury) is the primary way to avoid damage from Botrytis neck and bulb rot. Healthy onions that are stored properly and harvested at the right time are seldom affected. To prevent infection:

• Harvest onions and garlic only when the crop is mature and necks are well cured.
• Minimize damage to bulbs from insects and diseases during the growing season.
• Avoid excessive or late applications of nitrogen fertilizer.
• Avoid late-season irrigation to allow the foliage and neck to dry before harvest. The neck tissue must be well cured before the crop is stored.
• Handle the crop to minimize bruising or wounding.
• Store bulbs at temperatures of 41°F or below.
• Minimize relative humidity and promote good air circulation during storage.
• Maintain stable temperatures when moving bulbs into and out of storage, as well as shipping to markets. Wide fluctuations in temperature can cause condensation on the bulbs, which favors the development of the disease.

Seed and bulb treatment with fungicides prior to planting can reduce incidence of Botrytis neck rot. However, late in-season fungicide applications during the last month of harvest are the most effective at preventing infection of the necks, since late-season infection poses the greatest risk for bulb rot in storage. Chemical control may not be effective once the symptoms develop, and cultural control is the most reliable way to avoid infection.

Since Botrytis-specific fungicides are highly prone to resistance development, it is especially important to rotate fungicides with different modes of action.