Onion white rot is caused by a highly persistent soil dwelling fungus. The resting bodies (sclerotia) can survive for 15 years in the soil without a suitable host plant. The fungus is active when the soil temperature is around 50-68 F(10-20C). Autumn planted garlic and winter onions are particularly susceptible to onion white rot, from mid to late spring when soil temperatures are ideal. By this time they have a well-developed root system, extensive enough to stimulate the sclerotia into germination.

Susceptible Plants- Members of the Allaceae family


Symptoms- Plants inflected with onion white rot start to die. Older leaves turn yellow, roots become stunted or rotten. Seedlings keel over, larger plants can be easily pulled out of the ground, and garlic stems easily pull away from the bulb.

A few plants in a small batch may be affected first, and then a whole row may show signs as the onion white rot disease spreads. As the onion rot progresses, a white cottony like fungal growth will be seen around the base and up the side of the bulbs. Tiny black globules which resemble poppy seeds, reside along the fungus. These are the fungal resting bodies of onion white rot, known as sclerotia.

Prevention and Control- Grow onion from seeds, not sets, so the root system will be small when disease activity is at its highest. Grow garlic from reputable sources and avoid infecting clean ground. Clean tools and shoes well after cultivating contaminated soils. Use a strict crop rotation. If onion white rot is present, the rotation should be at least 8 years. Space plants widely when stimulated by nearby plants, onion white rot can spread sideways through the soil and intertwined roots will also move infection along a row. Clumps of multi sown onions should be 12in(30cm) apart.

If the area of infection is small, remove and dispose of affected and adjacent plants, and the surrounding soil. In areas known to be infected with onion white rot, try growing garlic in 4in(10 cm) diameter holes filled with uncontaminated soil.. Leeks are worth trying even on badly contaminated soil. Asof now there is really no effective fungicide for onion white rot.