Late Blight A Horror Show For Gardeners

By Nancy K. Crevier

Connecticut home and commercial tomato growers are fighting a potentially devastating disease this summer, in great part due to the excessive rain and below normal temperatures that have plagued the Northeast since May. “Late blight is a big problem throughout the Northeast this summer,” said Dr Sharon Douglas, plant pathologist with the Connecticut Agricultural Experiment Station in New Haven. Although late blight, caused by the fungus Phytophthora infestans is encountered sporadically every summer, this year’s outbreak is significantly greater, she said.

“In the 27-plus years that I have worked as a plant pathologist here, I have never seen an outbreak as extensive as it is this year,” said Dr Douglas.

The pathogen was introduced into the region through infected tomato seedlings distributed by growers in the southern United States, and sold at “big box” stores such as Home Depot, Agway, and Walmart. “You can imagine how many consumers buy at these big stores, and how many plants were sold,” she said. The infected plants, which may have not yet exhibited symptoms of the destructive disease when purchased, were then subjected to conditions ideal for the proliferation of the windborne pathogen: windy, cloudy, cool, conditions and wet days and nights. Spores normally killed by UV rays have not received enough sunlight to prevent the rampant spread of late blight.

This particular strain of late blight affects tomatoes and potatoes, and is the same organism responsible for the blight that caused the Irish Potato Famine in the 1840s, said Dr Douglas. “It still seems historical until you see it in today’s perspective,” she added.

The disease presents itself approximately three days after infection, most commonly on the leaves. Greasy-looking olive brown, nickel or quarter-sized lesions appear on the leaflets. Stems may present with dark brown to black lesions. The fruit of the tomato can also be affected by late blight. Dark brown lesions appear on the fruit or stem end and rapidly spread over the entire fruit.

The spores of late blight appear as a dusty white growth, visible after a rainfall. “Under the right conditions, late blight can kill the whole plant within two days of the appearance of the symptoms,” said Dr Douglas. According to Dr Douglas, 100,000 to 300,000 sproangia each day can be produced by just one lesion, each one capable of starting a new infection.

The spores can travel more than two miles by air, said Dr Douglas, which is how the disease is spread from garden to garden, and from tomato plants to potato plants. Potatoes are infected when the spores are washed by rain or overhead watering from the leaves into the soil and infect the eyes or cracks in a growing tuber. Soft rot bacteria sets in and quickly takes over the entire crop.

Seedlings purchased from small, local nurseries or self-grown are not immune to the threat of late blight, because of the pathogen’s spread via the wind. “At this point, there are so many hot spots throughout the state all of the tomato and potato plants in Connecticut are at risk,” said Dr Douglas. “There are already a lot of disappointed homeowners who have had to take action against late blight,” she said, “some with 100 to 300 plants in a garden.”

Sandi Wilson is the Master Gardener Program Coordinator at the Fairfield County Extension Service located at 67 Stony Hill Road in Bethel. In the ten years that she has been with the service, this is the first year that she recalls people bringing in samples of late blight. “One gentleman who brought in a diseased plant said that he had been growing tomatoes for 30 years and didn’t recognize the fungus on his plants. It was late blight,” said Ms Wilson.

Another man brought in two plants on a Friday afternoon that were diagnosed with late blight. “He said his other plants seemed fine. But he called on the following Monday, and all 30 of them were infected by then. It’s sad,” said Ms Wilson, “because we’ve had so many people calling [the extension service] for advice on starting their first-ever vegetable garden, and it has turned out to be such a bad year for gardens. It could dissuade some from continuing next year, I’m afraid.”

Master gardeners on staff at the extension service have identified late blight on plants from several area towns, including Danbury, New Fairfield, and Newtown. “It is prevalent throughout all of the counties in Connecticut,” said Ms Wilson. “We’ve had cases in every extension service.”

Take Immediate Action

If late blight is discovered on a plant, immediate action must be taken. “Get a large, heavy plastic garbage bag, and pull and place in that bag every plant that appears to be infected,” said Dr Douglas. Do not compost the plants, as active spores can still be carried afar by the wind from a compost heap.

Tomato plants not yet showing signs of infection can be protected by making sure that each plant has good air movement around it and that the soil is well-drained. Remove any “volunteer” plants from the garden, as well as any weeds in the Solanaceous family, such as bittersweet or hairy nightshade, which are also susceptible to this strain of late blight.

Eggplant, pepper plants, and hybrid petunias should be inspected for signs of late blight, as there is some indication that they can also fall victim to this late blight. Any infected plants must be removed and destroyed to prevent further spread of late blight.

Avoidance of overhead watering, staking plants, and mulching can reduce the incidence of late blight.

Fungicide sprays applied regularly and applied according to directions may be necessary to prevent late blight in plants not yet affected. In Connecticut, homeowners may apply the fungicides chlorothalonil and copper.

“You must go by the directions on the fungicide,” cautioned Ms Wilson. “As with any pesticide or fungicide, wear protection,” she said. When fungicide is applied — and it could take several regular applications — there is a period of time following the application in which it is not safe to harvest and consume treated produce.

“Look for the ‘days to harvest’ information on the label. It is different for every one, so you must go by the label,” warned Ms Wilson.

Because late blight spreads so rapidly, though, under optimal weather conditions such as those encountered so far this summer, even fungicide application is no guarantee that a crop will be saved.

Fall cleanup should be scrupulous this year, said Ms Wilson, but the good news is that the pathogen does not live in the soil and should not overwinter, unless infected tubers are left in the ground.

Not all leaf spots and leaf drops indicate late blight. Septoria leaf spot and early blight are two fungal diseases that are frequently mistaken for late blight. Septoria leaf spot develops first on lower leaves of the tomato plant, appearing as small, circular spots that turn from gray to tan, with dark brown margins. The leaves will then turn yellow.

Early blight appears as dark brown to black, dead spots ranging from pinpoint size to ½ inch in diameter. Concentric rings may form as the spots get bigger, giving a “bull’s-eye” appearance.

Gardeners must remain vigilant to prevent further spread of the destructive pathogen that has already adversely affected tomato production in the state. “It certainly will have an impact on the availability and cost of local tomatoes this summer,” said Dr Douglas, “although it is hard to say just how much of an impact, yet.”

For answers to questions or for assistance in diagnosing late blight, contact the Agricultural Experiment Station’s plant disease information office at 974-8601, or 877-855-2237. Or contact the local extension service located at 67 Stony Hill Road, Bethel, 207-8440. Gardeners may also bring samples of suspect plants to the Master Gardeners’ Fair at the Fairfield County Extension Service on Saturday, August 15, when master gardeners will be on hand to answer questions and identify problems from 12:30 to 5 pm.