Symptoms and Signs
This disease begins with the rotting of the basal plate, which prevents water and nutrients from being transported to the foliage. Symptoms caused by this rotting include yellowing of the foliage and leaf dieback from the tips at early or intermediate stages of crop development. Rotted areas of the bulb progress from the basal plate towards the neck of the bulb. Affected roots become dark brown to dark pink, and a white fungal growth is sometimes evident at the base of infected bulbs.
When cut vertically, an infected bulb will show brown discoloration of the stem plate tissue. Later, the stem plate tissue becomes pitted and shows a dry rot. The stem plate and dry outer scales crack open under dry conditions.
Fusarium basal rot can continue to develop in storage, and bulb mites are quick to colonize basal plates affected by the disease.
Comments on the Disease
The fungus can survive indefinitely in soil. Infection occurs through wounds or in the vicinity of old root scars at the base of the bulb.
Insect damage to roots and bulbs can increase the occurrence of Fusarium basal rot. The disease can occur when soil temperatures range from 59° to 90°F, with optimal temperatures for disease development ranging from 77° to 82°F. Basal rot is more prevalent in transplanted onions than in direct-seeded onions.
Some rotational crops and grasses (sudangrass and oat) support the growth of the fungus even though they are symptomless. Black beans supported greater growth of this pathogen than onion, while corn, cowpea, tomato, sunflower, and wheat were similar to the onion host based on a greenhouse study.
Plant resistant onion cultivars. Avoid fields with a history of Fusarium basal rot problems, and rotate 3 to 4 years out of onion, garlic, leek, and other crops that favor growth of the fungus, such as corn, tomato, and sunflower. Manage soil insects and foliar diseases, and cure bulbs properly before storage. To avoid favorable conditions for infection, store bulbs at temperatures no warmer than 39°F and at low relative humidity (about 70%).