Spot the symptoms
Powdery mildew appears as white and powdery spots, mostly on the upper side of the leaves. The disease appears first on the older, lower leaves of lettuce heads (Fig. 1). As the disease advances, hyphae and “powdery” conidia cover the leaves, resulting in slower plant growth and yield reduction. The conidia represents the symptom and sign of the pathogen due to its visibility to the naked eye. Conidia on the plant surfaces germinate, penetrate the tissue and form haustoria. Infected lettuce leaves turn chlorotic and eventually take on a brown, dried appearance. Occasionally, survival structures, cleistothecia, will emerge as visible dark brown spots.
Know the enemy
Golovinomyces cichoracearum (formerly known as Erysiphe cichoracearum) is the name of the fungus that causes powdery mildew on lettuce. This pathogen is distributed worldwide. This type of powdery mildew affects most types of lettuce species (e.g. lettuce butterhead, crisphead, romaine, etc.) and related crops (e.g. chicory). Other strains of this same pathogen cause diseases on greenhouse ornamentals in the Asteraceae family.
This fungus is an obligate parasite, which means that it requires plant tissue in order to complete its life cycle. Resting structures such as cleistothecia and thick-walled mycelium in crop residues are suspected to be the main survival stages between crops. Therefore, removal of infected plants and debris becomes very important for preventing disease spread.
Spore dissemination is favored by air flow in the greenhouse. Spores disperse to very long distances. The time from when a spore lands on the surface of a leaf to the time it produces new conidia can be as short as 72 hours but is most likely to occur between 5 to 7 days. Unlike other fungal diseases, free moisture on the leaves will inhibit germination of spores.
Powdery mildew thrives in dry and warm environments but can also persist in cooler periods. Once infection has begun, the mycelium continues to spread on the plant surface regardless of the moisture conditions in the atmosphere. Maximum germination of conidia occurs at 96 to 98 percent relative humidity (RH).
Reduce the risk of disease and control the problem
Maintain adequate ventilation. Maintain low relative humidity levels by properly timing ventilation and heating. Maximum spore germination occurs at RH higher than 96 percent. Spatial distribution between lettuce plants is important to increase air flow between plants and also for uniform coverage for spraying fungicides.
Remove infected plants. Scout plants and remove symptomatic plants quickly. Collect symptomatic plants in a plastic bag to avoid spore dispersal. Sanitize all greenhouse equipment and production areas to avoid the presence of plant residues with the disease re-infecting the greenhouse. After each crop cycle, remove all excess plant debris. Inspect new plant material such as seedlings, cuttings or transplants coming into the greenhouse.
Use resistant cultivars. Keep resistant varieties of lettuce in mind that are less susceptible to powdery mildew. These varieties include Jericho, Sandy, Red Salad Bowl, Loma and Regal Oak. You can visit vegetablemdonline.ppath.cornell.edu to stay up to date on resistant varieties.
Choose the right fungicide. Preventative fungicides are recommended if powdery mildew has been an issue in the past or the environmental conditions favor the disease. Corrective fungicides should be applied at first sight of powdery mildew. Rotating out fungicides helps avoid the risk of the powdery mildew becoming resistant to the chemical or antagonistic organism. Attention to spray distribution and coverage is also important for control. Table 1 includes a summary of products available for control of powdery mildew.