In November of last year, a grower brought onion samples to my attention. The samples had been stored on a greenhouse bench after harvest. A black mold had developed on many of the onions. Some of the onions appeared to have developed rotten areas on the top of the onion at the neck (Figure 1). In the laboratory, I could observe a common soil fungus on the onions. Further, I was able to isolate the same fungus in the laboratory, thereby confirming the disease as black mold of onion.
Figure 1. Black rot of onions often results from improper storage conditions.
The causal fungus, Aspergillus niger, is very common in the soil. Black mold may occur where there is injury especially at the neck of the onion. Black mold is most common when storage temperatures are higher than 86°F in the field or higher than 75°F in storage. Free moisture for 6 hours or more is needed for infection to occur.
Fungicides may lessen the severity of black mold when applied to seeds, seedlings or bulbs. But most control measures will involve storage conditions. Onion bulbs should be stored cool and dry. Avoid any type of injury at or after harvest. The grower reported that there were differences in varieties affected. This may be due to maturity date. If onions are harvested before mature, this may make them more prone to disease. Likewise, if the onions are left well past maturity, post-harvest disease may become a problem.
While it may seem too early to talk about an onion storage disease, the proper measures to avoid such problems is during the season.