Late blight is one of the most destructive diseases of potato and tomato in the world, and serious losses may result. Although it develops regularly in western Washington, the disease occurs sporadically east of the Cascade Mountains.

Late blight is caused by Phytophthora infestans, a fungus common on potato and tomato in wet years. Moisture is more important than temperature during late blight outbreaks as the disease generally is favored by a wide range of temperatures.

Infected potato plants develop brown, purple, or black streaks, spots (lesions) or blotches on the leaves and stems. The lesions often develop where moisture collects—on the tips, margins or undersides of leaflets or along the stems. The size and shape of the lesions is very dependent on weather conditions. During periods of high humidity from fog, drizzle, dew, rain or sprinkler irrigation, the lesions are covered by a white, downy fungal growth containing spores. Lower leaves and stems may be attacked first, but with additional moisture, the disease rapidly damages or kills entire plants. Dry weather stops the development of the disease and the white fungal growth disappears; however, the disease will resume activity when moist conditions return.

Damage to potato tubers usually is apparent when the tubers are dug, but sometimes does not show until the tubers have been in storage for several months. Affected tubers have brown to purple, metallic-appearing areas in the skin. When these areas are cut open, a copper-colored discoloration is visible, extending about 1/4 to 1/2 inch deep. This discolored area is usually firm, but may become mushy when rotted by bacteria.

Infected tomato plants develop lesions on the leaves and stems similar to those on potato plants. The lesions enlarge under moist conditions and the leaves may die and drop. The entire plant may die. Tomato fruits can also become infected. At first, diseased fruits have watersoaked areas on the surface. These areas later become enlarged, brown, and somewhat wrinkled or corrugated. The rotted areas usually remain firm, but may become mushy, and often are located on the upper part of the fruit. The margin of the decayed area is more or less definite and slightly sunken. Both green and ripe fruits can be infected. Green fruit that is picked in early fall to ripen indoors may develop symptoms before it is ready to eat.

The late blight fungus survives the winter in diseased potato tubers including those infected during the growing season and left in the soil, those discarded and not destroyed, or those stored for seedpieces the next spring. Potato tubers and tomato fruits discarded in late winter or early spring can be especially troublesome sources of the fungus because they do not necessarily freeze, so provide the spores that can infect the next crop. The presence of late blight one year seems to favor the occurrence of the disease the next year, probably because of these overwintering sources.

Spores produced on overwintering diseased tissue usually become airborne to infect new plants. Spores may be blown for distances up to several miles, especially in wind-blown rain. The fungus can also be transported via tomato transplants and seed potatoes. Spores formed on foliage of potato plants can be washed into the soil by rain or irrigation where they may infect tubers. Tubers nearest the soil surface are most likely to be infected and develop rot. More importantly, tubers which are harvested while the infected tops still are alive, even when only slightly infected, can become infected and rot in storage.

Strict sanitation and cultural practices are used to minimize late blight in the home garden. Sometimes, chemical control measures are used as well.

Sanitation and Cultural Controls

1. It is important to destroy the means by which the late blight fungus survives and/ or increases, i.e., on discarded potatoes and tomatoes, volunteer potato and tomato plants, infected vines, and weeds (related to potatoes and tomatoes) like hairy nightshade. Destroy all infected plant material, including tubers, by allowing them to freeze during the winter, burning, or burying deeply. Do this by early spring to avoid infection of the newly planted crop.

2. Always plant certified potato seedpieces or tomato seedlings. Do not plant tubers with rots or any skin lesions.

3. Avoid overfertilization with nitrogen because excess nitrogen causes dense plant growth. Dense foliage favors retention of moisture, exposing plants to periods when relative humidity is greater than 90 percent. Such humidity is favorable for late blight. Dense foliage also dries out less quickly by sunlight and air movement.

4. Adequately hill tubers as they form to minimize the opportunity for spores to wash from foliage through cracks in the soil to the tubers.

5. Irrigate to minimize periods of leaf wetness. Either avoid overhead watering or water only in the middle of the day so foliage will dry before evening. Also, avoid irrigation during rainy weather or cool cloudy weather that follows prolonged rainy weather. Remember, no disease should develop if the plants can be kept dry.

NOTE: Plants can also be shielded from rain by planting in a protected area such as next to a wall under extended eaves. Clear polyvinyl shelters can be constructed to offer protection from rain, but should also be designed to allow air circulation from the sides. These shelters are most practical for tomato production, and with some modification can extend the growing and harvesting season and also protect plants from frost damage in the spring and fall. Close sides to adequately protect from frost. If favorable weather for late blight has occurred, closely monitor plants and remove any suspicious foliage or shoots.

6. Remove and destroy severely infected or dead plants or plant parts during the season. Greenhouse tomato growers also should properly dispose of infected plants and plant parts.

7. Potato vines should be completely dead before tubers are harvested. In a home garden vines can be killed by cutting the tops an inch or so below the soil surface or waiting for frost. Tubers harvested 2-3 weeks after all tops have been removed or killed develop less storage rot. Also, stop irrigation about two weeks or so before the tubers are harvested. Burn, bury deeply, or freeze the potato tops. Do not take them to the dump or infection may spread to the surrounding area. Do not place them on the compost pile (back yard compost piles may not reach sufficient temperatures to kill specialized survival spores called oospores, if they are present). Follow the same procedures for infected tomato plants and fruit.

8. Harvest tubers when the soil is fairly dry and after the skins are set. Try to harvest all of the tubers. Handle potato tubers carefully to prevent injury, since the fungus readily infects injured areas. Inspect potato tubers and tomato fruits at harvest and destroy those that are infected. Keep the potato tubers under observation in storage and immediately destroy any suspected of infection. Store in a dark, cool, humid place such as a basement or, ideally, a root cellar.

9.The many races of the fungus have made it impossible to recommend potato or tomato cultivars which will always be resistant to late blight. However, there are a few cultivars which show some resistance, and home gardeners may want to contact local extension offices about those that seem to do best in their area.

Chemical Control

Fungicide applications are made to protect leaves, vines, and fruit from infection. During summers with extended periods of dry weather, usually no applications will be necessary. However, rainy days and heavy dews during mid-July, August, and September (in western Washington) can encourage disease development. Anticipation of these conditions by monitoring weather forecasts will allow the application of protectant type fungicides before the occurrence of disease. Fungicide applications will not eradicate the fungus from infected tissue; they will only provide protection. If late blight becomes established, immediate removal of diseased leaves, followed by an appropriate fungicide application, is recommended (see PNW Plant Disease Management Handbook for current recommendations). If the disease has spread to much of the plant before it is noticed, fungicide applications will be of little value. Continued close monitoring of plants after application is necessary, too, since infections may be present but may not have developed enough to show visible symptoms, or may develop on new foliage of growing plants. Continue surveillance and spray until the disease is under control. For the fungicides selected, make certain that the plant name and late blight are indicated on the label. Read and follow all label directions prior to any use!

Prepared by Debra Ann Inglis, Extension Plant Pathologist, WSU Mount Vernon; Dennis Johnson, Extension Plant Pathologist, WSU Pullman; and Ralph Byther, Extension Plant Pathologist (retired), WSU Puyallup.

Warning. Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Revised April 1996. Subject codes 270, 274. A. EB0958