Downy Mildew of Lettuce

Downy Mildew caused by Bremia lactucae is the most important foliar disease of lettuce and all lettuce types are susceptible. In cool, moist regions losses can be severe due to decreased quality and increased need for trimming of diseased leaves. Symptoms consist of light green to yellow, angular spots on the upper sides of leaves and white, fluffy growth on leaf undersides. Older lesions become brown and necrotic. Infection usually occurs first on lower leaves and lesions may be delimited by leaf veins. Extensive areas of leaf tissue can be destroyed. The pathogen can become systemic and result in dark streaking of pith and vascular tissues. If Downy Mildew attacks young plants, seedlings may be killed.

B. lactucae survives between crops in plant debris, in wild lettuce hosts, and within seed. Secondary spread is by airborne sporangia which are short lived, require high relative humidity, and are inactivated by exposure to bright sunlight. B. lactucae is a complex pathogen that exists in multiple pathotypes (races).

Management:

• Start with certified, disease free seed or treat seed with hot water.
• Plant resistant cultivars. Strains of the pathogen may arise that overcome resistance.
• Avoid sprinkler irrigation and use subsurface drip irrigation where available.
• Protect greenhouse transplants so that Downy Mildew is not introduced into fields.
• Apply fungicides prior to the development of disease. Resistance to metalaxyl and foestyl-Al has been reported in Downy Mildew populations. Apply fungicides in rotation of active ingredient to prevent resistance development.
• Fungicides registered for Downy Mildew include azoxystrobin (Quadris) and famoxadone plus cymoxanil (Tanos).