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Backyard Gardener: Watch for late blight in tomatoes

Hello Mid-Ohio Valley farmers and gardeners! I hope everyone’s garden is off to a great start for the summer of 2021. We started off unusually hot and dry, but are now receiving some rainy and cool weather to give those plants a break. If you need a WVU Extension garden calendar, we still have some available at the Extension office.

I want to send a Thank You to our United States Military. Sunday, June 6, marked the 77th Anniversary of D-Day, when 150,000 Allied troops stormed the beaches at Normandy, France, to liberate Europe from Nazi control. The sacrifice of the fallen is not forgotten. We salute you for fighting for freedom.

This is a great week to seed sweet corn, in addition to cucumbers and squash. The price of food is on the rise in the grocery store, so homegrown vegetables from the garden are more valuable than ever. This week I want to talk tomatoes, specifically a disease which wreaks havoc on gardens in our area called late blight.

Late blight (Phytophthora infestans) is a significant fungal disease that can affect tomatoes. Late blight is favored by cool, wet weather and will cycle repeatedly if weather conditions are favorable.

Late blight had a significant impact on history, especially if your ancestors immigrated to the United States from Ireland. The Irish Potato Famine of the 1840’s was caused by the late blight pathogen, destroying the entire potato crop for several years. Many families starved and their only recourse was to leave Ireland for America.

This disease also spurred research into a new scientific field, plant pathology. Unfortunately, several factors enhanced the devastation of this disease on the Irish. The entire crop was all the same variety, so there was no resistance to the disease. In addition, many people depended on potatoes not only to feed their families but to sell to pay the bills.

Late blight is spread from infected transplants, volunteer potato or tomato plants, and certain weeds which are botanically related to tomatoes. Spores of this fungus can become airborne and travel great distances in storms. Rain from these storms drop spores onto plants, resulting in infection.

Late blight symptoms include leaf, stem, and fruit lesions that have a water-soaked appearance. The lesions eventually turn brown and the plant looks like it has been frost-damaged or blasted by a blowtorch. Unlike early blight, which typically begins infection on the lower leaves of the tomato plant, late blight infections seem to move from the outside of the canopy inward.

Late blight favors cool, wet weather for infection, so cultural practices have a relatively small effect on controlling this disease. However, there are some management practices gardeners can utilize which can help prevent this disease. Plants should be spaced at least 24 inches apart in the row and keep rows relatively wide (3 feet apart) to facilitate air movement around plants. Staking or caging tomatoes will also result in faster drying of the foliage. Plants should not be handled when the foliage is wet.

Watering at the root level with garden hoses, soaker hoses or drip irrigation will be beneficial. This keeps soil from being splashed onto the plant leaves.

Mulching with plastic or an organic material will reduce the development of early blight more than late blight, but these diseases often work in tandem to destroy tomato plants. Before planting, inspect transplants for any symptoms of disease. Avoid planting on sites where tomatoes or potatoes were grown previously.

You can also select resistant tomato varieties if they are available or start your own seeds. Mountain Magic, Mountain Merit, Legend, Defiant PHR and Plum Regal have excellent resistance to late blight. Jasper, Red Pearl and Matts Wild Cherry are small fruited tomatoes with good resistance. Some heirloom tomato varieties have good tolerance to late blight.

Cornell University has developed a tomato variety called Iron Lady which is triple resistant. This variety has resistance to early blight, late blight and Septoria leaf spot.

Spraying fungicides is the most effective way to prevent late blight. For conventional gardeners and commercial producers, protectant fungicides such as chlorothalonil, (Bravo, Echo, Equus or Daconil) and Mancozeb (Manzate) can be used. Read the label on any fungicide before application.

Fixed copper products can be used by organic gardeners to prevent late blight. Copper products are most effective if applied before initial infection. They have a protective, disease-slowing effect, so repeat sprays are necessary.

If conditions favor late blight, start a weekly spray application immediately after transplanting. Otherwise, apply protective fungicides beginning at flowering for control of late blight and other diseases.

Late blight will eventually kill plants. This depends upon how many points of infection the plant received, the variety (some are more susceptible than others), use of protectant fungicides (such as copper), and on the weather.

Hot, dry, sunny weather typically slows late blight. Cool, rainy, overcast weather will cause late blight to progress rapidly killing the plant in seven to 10 days.

Healthy-appearing, unblemished fruit from late blight-infected tomato plants are safe to eat. However, do not let these tomatoes sit around long. If they have been infected but are not showing symptoms, they will not keep in storage. Late blight infected fruit should not be canned or frozen. It can raise the pH of the canning solution and promote further growth of microorganisms.

The late blight pathogen has the potential to infect other plants in the Solanaceae family potatoes, peppers, eggplants, and nightshade weeds. This is why we recommend you do not compost late blight infected tomatoes or potatoes, and purchase seed potatoes from a certified clean source.

Contact me at the Wood County WVU Extension Office 304-424-1960 or e-mail me at jj.barrett@mail.wvu.edu with questions. Good Luck and Happy Gardening!

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