Disease Identification: Botrytis Leaf Blight

This article appeared in "Onion World," September/October 1999.

Causal Agent: Botrytis squamosa (teleomorph: Botryotinia squamosa)

Distribution: North America and Europe

Symptoms: The fungus primarily attacks the leaves. The first symptoms appear as small white spots surrounded by a greenish halo. Center of spots often are tan, making it difficult to distinguish between leaf blight and damage from insect feeding, mechanical damage, or herbicide injury. Lesions expand with age and, when numerous, may cause leaf tips to die back. Eventually, leaf death results, and severely affected onion fields develop a blighted appearance. Bulbs from infected plants may be small because growth is reduced by leaf loss.

Conditions for Disease Development: The fungus may overwinter in infected plant material, or may survive in the soil as small, dark-brown sclerotia. During moist periods with moderate temperatures, fungal spores that arise from sclerotia or infected leaves and debris are dispersed. These spores land on susceptible tissue, and infection occurs. This disease can spread rapidly when environmental conditions are favorable for development.

Control: A good preventive fungicide spray program (Rovral, Bravo, or Mancozeb) is important. Disease-forecasting systems have been developed for some areas, and these are very useful for determining the optimum timing for sprays. Destroying onion- or debris-cull piles will help reduce sources of inoculum. Orienting plant rows and spacing to maximize air movement helps reduce the time that leaves are wet, and results in less disease incidence and severity. Cultural practices, such as deep plowing and crop rotation, will help reduce numbers of sclerotia in the soil.