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Onion Diseases - Botrytis Neck Rot


Figure 7.21Early stage of botrytis neck rot. 

Figure 7.22Advanced decay by botrytis neck rot. 

Figure 7.23White mycelium of Botrytis growing between scales. 

Figure 7.24
"Sclerotia" spores produced by Botrytis on onion bulbs. 

Figure 7.25
Grey Botrytis spores produced on onion.

Botrytis aclada and B. allii cause postharvest storage disease. Severe infections can lead to over 60% loss.

SYMPTOMS:

The first symptoms of botrytis neck rot are seen in the neck area during storage. The neck may appear sunken and scales may have a water-soaked appearance that turn gray to dark brown. Over time, the decay will move through the entire bulb. Sometimes white to gray mycelium can be seen developing between scales.

DISEASE CYCLE:

Botrytis overwinters in the soil and in plant debris left behind in the fields or in cull piles. The fungus produces overwintering structures called sclerotia that can survive for several years until a suitable host (onion) is planted again. It also produces spores on plant debris that are blown by wind to the onion fields from miles away. Occasionally, the disease can be seedborne. Leaf tissue and bulbs can be infected in the field from soil or seedborne inoculum without showing any symptoms during the growing season. Most infections occur when onions are harvested before the leaves and necks are dry. The fungus infects the green neck area after the leaves are cut off. Symptoms usually don’t develop until onions have been in storage for one to two months.

MANAGEMENT:

      • Store onions with dry, well cured necks. The fungus is unable to penetrate and infect a dry neck.
      • Ensure proper curing. Proper curing can be achieved by undercutting onions at maturity, severing all roots, refraining from applying any nitrogen fertilizer once bulbs have been initiated, and planting at the correct plant density in the field. If it is difficult to dry onions due to environmental conditions, forced air (93°F) at 0.06 m3 per minute per 0.03 m3 of bulbs can reduce losses to neck rot.
      • Maintain proper storage conditions. The best storage conditions include air movement, temperatures between 33-34°F, and 70-75% relative humidity. Air movement must be monitored to avoid condensation on bulbs.