Symptoms and Signs
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.
Comments on the Disease
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.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Merivon Xemium)||8–11 fl oz||12||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitors (7)/quinone outside inhibitors (11)|
|COMMENTS: To limit the development of resistance, do not make more than three applications per season, do not apply more than 33 fl oz per acre per year, and do not make more than two sequential applications before rotating to a fungicide with a different mode-of-action group number.|
|(Rovral Brand 4 Flowable)||1.5 pt||24||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2)|
|COMMENTS: Registered for control of Botrytis neck and bulb rot in dry bulb onion only. Apply as a foliar spray as soon as conditions become favorable for disease development.|
|(Bravo Weather Stik, Echo 720)||Label rates||12||See label|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)|
|COMMENTS: See label for information on suppression of bulb neck rot during storage. Chlorothalonil is a contact-only fungicide; therefore coverage is critical for control of Botrytis neck and bulb rot.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 7, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different group number.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment until the harvest may take place. In some cases, the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest.|