Late blight can cause tomato fruit rot, which is blotchy and dark brown but starts out relatively firm.

Late blight can cause tomato fruit rot, which is blotchy and dark brown but starts out relatively firm.
Image Credit: Photo courtesy of Howard Hart

BEAUFORT COUNTY — Late blight disease, the most destructive and infectious bane of tomatoes and Irish potatoes, was reported this week on tomatoes in a home garden in Beaufort County.

The diseased plants have been destroyed, but further spread of late blight to other areas of South Carolina is likely if the fungal-like pathogen arrived via spores blown up from Florida. To make matters worse, the cool, cloudy and wet weather that affected large portions of the state created ideal conditions for the potential of escalation.

Clemson Cooperative Extension vegetable pathologist Anthony Keinath and county agent Zachary Snipes recommend that all commercial growers and home gardeners carefully check these crops and — if the disease is discovered — treat them immediately. Growers along the coast should preemptively treat regardless. Large commercial growers already have started their overall spray programs, but some smaller growers might not have.

Symptoms of late blight appear as large dark-green or gray spots on leaves surrounded by a lighter halo or ring. The halo shows where the fungus is active, and the spots are visible on both sides of the leaves. In addition, white-to-gray fungus growth develops on the undersides of the leaves at the edges of the spots.

Symptoms of late blight appear as large dark-green or gray spots on leaves, surrounded by a lighter halo or ring.

Symptoms of late blight appear as large dark-green or gray spots on leaves, surrounded by a lighter halo or ring.
Image Credit: Photo courtesy of Howard Hart

Dark brown spots also appear on the stems and petioles (leaf stems). This symptom is fairly typical of late blight on tomatoes. The disease can cause tomato fruit rot, which is blotchy and dark brown but starts out relatively firm. Late Blight can even damage white potato tubers. Though relatively rare, the disease can affect eggplants and petunias, which are related to tomatoes and potatoes. Peppers, which are also in the same family, are not susceptible.

“Late blight can spread rapidly from home gardens to commercial fields, which is why we’re putting out this information immediately,” said Keinath, who is based at Clemson University’s Coastal Research and Education Center in Charleston. “It is considered to be the most destructive disease for tomatoes and Irish potatoes. The fungus is a diploid organism, which makes it very diverse and allows it to create new variances in its population. And it can become resistant to fungicides very quickly, as well.”

However, a number of chemicals that effectively control late blight are available to commercial growers. An extensive list can be found on page 231 of the 2016 Vegetable Crop Handbook for the Southeastern United States. Keinath and Snipes said that the most efficient fungicides are Presidio; Revus Top; or Ariston, a new fungicide that is a combination of chlorothalonil and cymoxanil. Growers should check the labels for the specific directions on rotating and tank-mixing.

“If growers discover a heavy infestation, they will see some reduction in yield,” Keinath said. “But if they treat quickly enough and with the proper fungicides, they should be able to save most of their crop.”

Home gardeners do not have access to the same array of products as commercial growers do, so they must rely on mancozeb, plus copper or chlorothalonil.

Late-blight disease was reported this week on tomatoes in a home garden in Beaufort County.

Late blight disease was reported this week on these tomato plants in a home garden in Beaufort County.
Image Credit: Photo supplied by homeowner

“The fungicides available to home gardeners are not as effective as the ones available to commercial growers,” Keinath said. “The home garden fungicides work as protectants. But for plants that are heavily diseased, home gardeners will not be able to control the infestation. So we are asking them to destroy, bag and carefully discard plants that have become diseased.”

Because late blight is closely related to algae, wet conditions allow the spores to grow quickly after they land on leaves. Temperatures around 70 degrees are ideal for the organism’s ability to thrive and spread. Clouds protect the spores from the sun’s damaging UV light.

Late blight triggered the Irish potato famine of the 1840s. It is caused by the fungus-like oomycete pathogen Phytophthora infestans. It was the first plant disease that was determined to be caused by a microorganism, which gave birth to plant pathology as a science.

“There was also a famous late blight pandemic in 2009 in the northeastern United States, where the disease was found on tomato transplants that were being sold to home gardeners,” Keinath said. “And it literally spread over several states during a cloudy, rainy summer. So this is not a disease that should be taken lightly in South Carolina, or anywhere.”

Potatoes are usually planted in February and March and harvested in late May and early June in South Carolina, while tomatoes are typically planted in March and April and harvested in June and early July.

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