Post-harvest Storage Diseases
Botrytis Neck Rot of Onion
Botrytis neck rot may occur in the bulb if the foliage is still moist when it is cut from the bulb at harvest. The rot moves quickly from the neck region (top) to the center of the bulb, and is active at temperatures of 35°F and above.
In Utah, Botrytis neck rot of onion is caused by the fungi Botrytis aclada and B. allii. The onion bulbs are infected through the neck area during the growing season, but infections are latent, meaning that no visible symptoms occur, and the pathogen remains in the neck area. The fungus is only able to grow into the bulb soon after harvest if the foliage is not allowed to completely dry before it is cut from the bulb.
In optimal harvest practices, onions are undercut when the bulbs are mature and when at least 25% of the crop's foliage has fallen over. The onions are then left in the field to cure until the necks are dry, after which the foliage is removed. If the foliage is cut from the bulb before the neck is dry, the latent Botrytis infection can start to invade the bulb through the soft, juicy neck. Once onion bulbs are placed in storage at temperatures above 35°F, the fungus starts rotting the bulbs from the neck down. Botrytis can sporulate heavily, infecting neighboring bulbs in storage bins. In severe cases, losses to Botrytis neck rot can reach 100%.
Management of Botrytis neck rot starts at harvest. Onions should be undercut when they are fully mature (and the foliage has fallen over) and the necks should be fully dry before cutting the foliage. If this is not an option for weather-related reasons, onions should be stored at 33-34°F to prevent activity of the fungus.
Rhizopus Mold of Winter Squash
Rhizopus mold of winter squash causes a soft rot. The white fluffy mass at the stem is the fungus sporulating.
Rhizopus stolonifer is a mold fungus that is ubiquitous: in the air, soil, and on plant material. If fruits or vegetables are damaged in the field by insects, hail, blowing sand, or strong winds, the fungus can invade the tissue through even the tiniest wounds, causing a soft rot. After infection, the fungus produces abundant spores within a few days. In storage, the spores are released and can infect nearby fruit. Under favorable conditions, the time from infection of the fruit to a complete soft rot is about 4 to 5 days.
Damaged fruit will not hold up in storage. The fungus sporulates at temperatures above 46°F, but temperatures below 50°F injure squash fruit. The best management tool is to prevent damage to the fruit in the field by harvesting carefully and avoiding insect damage. If strong winds or even hail damage fruit, it may be best to use the squash that is salvageable as quickly as possible and not keep it in storage.
-Claudia Nischwitz, Extension Plant Pathologist