Cool, damp conditions in Florida this winter have meant an increase in lettuce downy mildew, caused by the oomycete pathogen Bremia lactucae. Primarily a foliar disease, it has a direct effect on yield and quality as it affects the marketable portion of the crop.
Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. White mycelial growth of the pathogen develops on the under sides of these spots. Over time, these lesions turn brown and dry up. Severely infected leaves may die. In some instances, the pathogen can become systemic, causing discoloration of stem tissue.
Transplants might be infected in the greenhouse. If downy mildew infects the cotyledons of young seedlings, the plants might die. Although yield losses in the field may be substantial, downy mildew’s impact often is accentuated by significant postharvest losses during transit or storage.
Management Methods
Cultivar resistance when available is the most economical form of downy mildew control. Since many Florida cultivars are susceptible, downy mildew is extremely difficult to control once an outbreak has occurred. For this reason, prevention and early detection are important.
Establishment of a lettuce-free period, crop rotation, the destruction of weed hosts, and rapid incorporation of crop residues also are recommended control measures.
In the absence of resistant cultivars, the alternative is to apply fungicides before the development of the disease. Greenhouse-grown transplants should especially be protected so that downy mildew is not distributed and brought to the field on transplants.
Use of drip or furrow irrigation can help reduce leaf wetness and humidity and reduce the severity of disease but will not prevent disease when weather conditions are conducive to epidemics.
Source: growingproduce.com