On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue. Damp, cool conditions and moisture on leaves are required for the fungus 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. The fungus apparently does not survive in the soil.
New races continue to occur as the fungus changes.
Within each pathotype, some isolates have developed a lack of sensitivity to some fungicides. However, disease will develop on these cultivars if the resistance genes do not match the pathotypes present, or if new pathotypes develop. If downy mildew has only infected the older wrapper leaves, these can be stripped off at harvest. Copper fungicides may be used, but in general these are marginally effective. Spray at 7-21 day intervals if necessary.