Late Blight

Late blight lesions can occur on all aboveground plant parts. On leaves, lesions typically first appear as small pale to dark green water-soaked spots that are irregular in shape and surrounded by a zone of yellowish tissue. Under conducive conditions, lesions expand rapidly and become brown to purplish black as tissue is killed. Under sufficient humidity, white sporulation of the fungus can be observed at the periphery of lesions, principally on the underside of leaves. On stems and petioles, lesions are brown to black and may also support sporulation of the fungus. Infected tubers develop a firm brown decay that starts on the outside and may later extend to include the outer 0.125 to 0.5 inch (3–12 mm) of tissues.

Late blight occurs commonly in coastal environments and in the southern San Joaquin Valley in California. In other California potato-growing areas its occurrence is sporadic, depending on the presence of the pathogen and cool, damp weather conditions. Inoculum of Phytophthora infestans can originate from seed tubers, cull piles, volunteers, closely related weed hosts such as nightshade, and adjacent plantings of potatoes or tomatoes that are affected. Late blight can develop and spread rapidly if inoculum is present and conditions are conducive. High humidity (above 90%) and average temperatures in the range of 50° to 78°F favor the disease.

Late blight is controlled by eliminating cull piles and volunteer potatoes, using proper harvesting and storage practices, and applying fungicides when necessary. Air drainage to facilitate the drying of foliage each day is important. Under marginal conditions, overhead sprinkler irrigation can favor late blight; in Tule Lake under solid set sprinklers, conditions conducive to late blight development are enhanced by day time irrigation but not night time irrigation.

Plant certified seed tubers. When late blight has developed on foliage and tubers are at risk of infection, make sure that vines are completely dead for 2 to 3 weeks before harvest. Phytophthora infestans does not survive very long in dead foliage.

In districts that are commonly subjected to outbreaks of late blight, preventive applications of fungicides are advised when environmental conditions are favorable for the disease. Continue fungicide applications at 7- to 10-day intervals as conditions require. Shorter intervals may be needed under cool, rainy conditions. In interior districts where late blight occurs sporadically, watch for disease symptoms during routine field monitoring, and record your results (example form—PDF). Apply fungicides when late blight lesions appear in the field or in nearby plantings.

Strains of Phytophthora infestans have developed resistance to mefenoxam (Ridomil Gold).

• Surveying foliage pests form (PDF)