HPIPM:Onion White Rot

Author: Howard F. Schwartz, David H. Gent, and Michael E. Bartolo

Identification and Life Cycle

White rot is caused by the soil-borne fungus Sclerotium cepivorum. White rot has not been identified in Colorado, and likely would not be of concern to onion production because spring and summer temperatures are not favorable to the pathogen. White rot is favored by cool weather (below 68°F). Only Allium spp. such as onion, leek, and shallot are attacked. Once the pathogen is introduced into a field, it is very difficult to grow onions successfully. Sclerotia allow the pathogen to survive for more than 15 years in soil until susceptible host root exudates stimulate them to germinate. Sclerotia are spread within and between fields by cultivation equipment, irrigation water, on infected transplants, grazing animals, and movement of infested soil.

Plant Response and Damage

White rot appears on isolated plants, usually at bulb formation. However, young plants also may be affected. Symptoms become obvious as older leaves yellow, wilt, and fall to the ground. A semi-watery soft rot gradually destroys the bulb and roots. The pathogen can continue to develop and spread between bulbs in storage. Affected plant parts are covered with a thick white mycelial mat bearing numerous small (0.03 inch diameter) black sclerotia.

Biological Control

Biological controls have not been developed for white rot.

Cultural Control

Practice crop rotations of at least five years. Only high quality transplants free from the white rot pathogen should be planted. Select fields with adequate and promote thorough drainage. Avoid over-wintering onion production. Thoroughly wash equipment to prevent moving contaminated soil into new fields.

Chemical Control

Planting time treatments can reduce white rot losses, but white rot occurs so infrequently in the High Plains that chemical controls are not necessary.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.