Weather conditions ideal for late blight

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After the heartbreaking late blight epidemic of 2009, I wanted to make readers aware that according to Penn State's vegetable pathologist, Beth Gugino, late blight has been confirmed in two Pennsylvania counties and is suspected in a third. We have been experiencing the right weather conditions for this devastating disease that strikes tomatoes and potatoes. The closest infection to Allegheny County to date is in a commercial tomato field in Blair County.

Warm (70-80 degrees), humid days followed by cool (40-60 degrees), foggy nights make for ideal late blight weather. Late blight is highly contagious and can wipe out tomato and potato crops in short order. Caused by the fungus Phytopthora infestans, late blight is the disease responsible for the potato famine in Ireland in the mid-1800s.

Epidemics often start in home gardens where fungicide applications are less likely to be made on a regular basis than in commercial plantings. Whenever the disease develops unchecked, large quantities of late blight spores are produced and released into the air. During moist weather, the spores can survive and be transported up to 50 miles on air currents to infect other plantings of tomatoes and potatoes. During favorable weather conditions, unprotected foliage can be infected in three to six hours; symptoms can appear within a week. Those symptoms can expand rapidly during cool, wet weather and cause entire plantings to die within two weeks of infection. Late blight is held in check by hot, dry weather.

The fungus overwinters in southern areas on winter-grown tomatoes and potatoes. In northern areas, it overwinters in commercial potato cull piles, compost piles and potato tubers overlooked during harvest. The late blight fungus requires live tissue to overwinter, so potato tubers are the most likely source in northern gardens. It can be introduced to the garden on infected tomato transplants or seed potatoes. This is one of the reasons it is so important to used certified seed potatoes rather than saving homegrown potatoes to grow the following season. Late blight spores are also carried north on air currents coming from the south.

Check for foliar symptoms on tomato and potato plants by examining vigorous new growth higher on the plant. This way you'll be less likely to confuse late blight symptoms with less serious problems such as early blight that often develop on the older lower leaves. Late blight symptoms first appear as somewhat circular, water-soaked spots near the edge of expanded leaflets. These spots expand rapidly during moist weather to form irregular brown, dead areas. There is often a light green margin between the dead tissue in the center of the spot and the normal green tissue outside the spot. The real diagnostic feature of late blight is the white, downy-looking mold that develops at the margin of the spot on the underside of the leaflet. If the white mold is not obvious, remove the suspicious leaflet and put it in a plastic bag with a moist (rung out well, not sopping wet) paper towel for 24 hours to see if this symptom develops. If it does not develop, late blight is probably not the cause of the leaf spots.

To protect your tomato plants, avoid growing tomatoes where potatoes were grown the previous year, or growing potatoes and tomatoes next to each other. Late blight is likely to start in the potatoes and spread to the tomatoes. Protective fungicide sprays are the only sure way to avoid this devastating disease. Chlorothalonil (Daconil 2787) and maneb are labeled to control late blight in home vegetable gardens. Organic gardeners can use copper in spray or dust form. Copper is not as effective as chlorothalonil or maneb, but it is better than doing nothing. Applications should continue as long as weather conditions favor blight development. Follow label directions as to how often the product you are using should be applied.

One of the difficult things about plant diseases is that fungicide sprays are most effective when they are used preventively. Once the disease is present, fungicide sprays will not necessarily "cure" the problem, especially one as severe as late blight.

Plants infected with late blight should be removed from the garden and destroyed. Do not compost them. Cut potato tubers from infected plants in half so that they decompose quickly. Bury infected plants rather than sending them out with the trash. The disease-causing spores could be released if they are exposed at a later date in the landfill.

Mature green fruits from infected tomatoes can be removed and stored for ripening. Avoid storing them under conditions of high humidity (plastic bags or containers) because this will promote spore production. Potato tubers from infected plants can be eaten but should not be stored for any length of time. Avoid using infected tubers for seed potatoes.


Send questions to Sandy Feather by email at or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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