How to Manage Pests
UC Pest Management Guidelines
Pathogen: Bremia lactucae
(Reviewed 4/17, updated 4/17)
In this Guideline:
Symptoms AND SIGNS
Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. The white fluffy growth of the pathogen develops on the lower sides of these spots. With time these lesions turn brown and dry up. Older leaves are attacked first. Severely infected leaves may die. On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue.
If downy mildew infects the cotyledons of young seedlings, the plants can die. Greenhouse-grown lettuce transplants can also be infected.
Comments on the Disease
Damp, cool conditions and moisture on leaves are required for the pathogen to infect lettuce and cause symptoms to develop. The short-lived spores are dispersed by winds during moist periods. Cultivated lettuce is the main host. Bremia lactucae apparently does not survive in the soil.
Bremia lactucae is a complex organism, consisting of multiple races (pathotypes) in California. New races continue to occur as the pathogen changes. B. lactucae has been reported to infect other plants such as artichoke, cornflower (Centaurea cyanus), and strawflower (Helichrysum bracteatum). Because B. lactucae isolates from these other hosts generally only infect the original host, B. lactucae probably consists of a number of host-specific forms. Within each pathotype, some isolates have developed a lack of sensitivity to some fungicides.
The most effective means of controlling downy mildew is to grow resistant cultivars. Some cultivars are resistant to most of the isolates of B. lactucea that are currently in California. However, such resistant cultivars are not available for all areas and seasons. Also, because the pathogen is highly variable and dynamic, resistant cultivars do not remain resistant indefinitely and can be overcome by new virulent isolates of B. lactucea.
In the absence of resistant cultivars, apply fungicides before the development of the disease. Protect greenhouse-grown transplants so downy mildew is not distributed and brought to the field on transplants.
Irrigation systems, such as drip irrigation, that reduce leaf wetness and humidity may reduce the severity of the disease but will not prevent it when weather conditions are conducive to epidemics.
Available fungicides are mostly protectants, and must be applied before infection occurs for best results.
UC IPM Pest Management Guidelines: Lettuce
S. T. Koike, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to Diseases:R. M. Davis, Plant Pathology, UC Davis