In April and May, onion producers will begin to look forward to harvesting their crops prior to the summer. There are several diseases of onion bulbs that may occur this time of year, each having different management options. Post-harvest bulb diseases may often be a result of unnoticed field infections that have continued to develop after onions are moved into storage. Late season diseases that may be encountered include slippery skin, Botrytis bulb and neck rot, black mold, and blue mold. Optimal conditions favoring development of these diseases and management considerations will be discussed.
Onion slippery skin is a bacterial disease, caused by Burkholderia gladioli pv. allicola (Syn. Pseudomonas gladioli pv. allicola). Foliar symptoms often appear as a soft neck and one or two wilted leaves in the center of the leaf cluster. These leaves will turn pale-yellow, and die back from the tip. However, the older and younger leaves maintain a healthy green appearance. The most definitive symptoms of the bacterial infection are more obvious after cutting open the bulb.
The inner scales will appear watery or cooked, eventually leading to browning of the inner scales. The disease spreads in a longitudinal direction from top to base of each scale, and then out to nearby scales, rather than across the scales as the disease progresses. The outer scales will eventually dry out. Squeezing the onion bulb will cause the rotten interior to ‘slip out'- hence the name slippery skin.
The bacterium requires moist conditions for disease development. Rainfall, overhead sprinklers, or water splashing favor disease. The bacterium is splashed from the soil environment up onto the neck where it enters through wounds and begins to grow. Optimum air temperatures for bacterial growth are within 41 F to 105 F. As the plant matures, the plant becomes more susceptible to the disease. At temperatures of 86 F, the bulb can decay completely within 10 days. In storage, the cool temperature can delay decay for up to three weeks (UC-IPM).
Control: Onions should be harvested when they reach full maturity, and should be properly dried prior to moving into storage. Avoiding overhead sprinkler irrigation of mature plants may prevent spread of the disease. It is prudent to avoid bruising and injuring bulbs, and maintaining cool temperatures during transit to storage.
Botrytis bulb and neck rot, is a fungal disease caused by Botrytis alli. The symptoms of the disease appear as a soft brown water-soaked area on the onion neck that will eventually move into the bulb. The bulb will exhibit water-soaked, and opaque scales that may develop grey spores between scales if humidity is high.
Eventually, the scales will turn dry and necrotic. Infection may occur when the weather is cool (50 F to 75 F) and wet.
Control: Avoid damage to bulbs throughout the growing season. Harvest mature onions with dried necks and bulbs.
Before putting the bulbs into storage, make sure that the onion bulbs are adequately dried. Avoid heavy or late applications of nitrogen fertilizer. Avoid irrigating late in the season, so that the onions are dried prior to harvest. Storage temperature should be at 41°F (5°C) or below with low relative humidity and good air circulation.
Black mold, caused by the fungus Aspergillus niger is a soilborne pathogen that invades the bulb of injured onions. Evidence of the fungus can first be seen on the top or sides of the bulb as a black growth under the outer layer of scales. The fungus develops well under warm and dry conditions. Once the fungus has invaded the onion, it may allow the entrance of secondary soft rot pathogens. The fungus survives on plant debris in the field.
Control: Currently, there are no available chemical control options for black mold. However, research has indicated that a good foliar fungicide control program throughout the season will reduce the incidence of the disease (UC-IPM).
Maintaining storage and transit temperatures between 55 F to 33 F may help suppress black mold development.
Blue mold rot, is a fungal disease caused by Penicillium spp. Penicillium is a common soil fungus, that survives on dead or dying plant debris. The fungus invades the onion bulb through wounds on the bulb or uncured neck tissue. Once the fungus has invaded the bulb it grows profusely, and may sporulate on the surface of the wound, appearing as a blue-green furry coating. Optimum temperatures for fungal growth are between 70 F to 77 F, with high relative humidity.
Control: Avoid insect or mechanical damage to the bulb, as this pathogen gains entry through open wounds. Make sure to properly dry the onions prior to storage. Store the bulbs at 41 F or less with low relative humidity (UC-IPM).