Fight the blight: quick action needed to prevent fungal resurgence

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This potato plant was killed by the “late blight,” a strain known as Phytophthora infestans.

Editor’s Note: this article has been updated to correct the date of the Irish “Great Famine.” The former date referred to the 1740’s “Irish Famine.”

FARMINGTON – If you were growing tomatoes or potatoes this year, and they did well, consider yourself lucky.

A powerful strain of blight, known to gardeners and farmers as “late blight,” has spread to potato and tomato plants throughout the county, state and country, withering plants and leaving many unusable for consumption.

Next year, one local expert warns, could be even worse unless people begin taking precautions now.

“My concern is that there was a big brouhaha when it hit,” Amy LeBlanc says, in a sunlit storage area. “But there hasn’t been anything since then.”

She’s surrounded by tomatoes bearing the tell-tale blue residue of copper hydroxide fungicides; LeBlanc has taken to using the chemical to try and stave off the blight’s advance. LeBlanc isn’t recommending the use of the sprays for the average garden-grower, the growing season is fast coming to an end for one, and she’s a professional.

LeBlanc has operated a seed and seedling business, specializing in tomatoes, for 20 years. She has grown more than 200 varieties of tomatoes at White Hill Farm, through a program she calls “Tomato Lover’s Paradise.” She’s a Maine certified organic grower and attends the Northeast Organic Farmers Association summer conference held in Amherst, MA every August.

A NOFA summit held this year, following breakouts of blight across the northeast, featured a presentation by researchers at University of New Hampshire, University of Massachusetts and Cornell University. Cornell researchers had been tracking the spread of Phytophthora infestans, a particularly nasty, systemic form of blight. Blight is a fungal infection which isn’t harmful to humans, but primarily spreads and affects tomato and potato plants.

Researchers were disturbed by the spread of Phytophthora infestans, a form of “late blight,” a collection of strains which spread easily, kill quickly and are difficult to eradicate. This strain was the same one which triggered the Irish “Great Famine” through the destruction of the potato crop in the 1840’s and 1850’s, killing more than a million people.

After some backtracking, Cornell researchers were able to trace the spread of Phytophthora infestans to a number of tomato seedlings sold at several large retail stores. This led investigators to a professional growing operation in the southern United States, which had unknowingly sold “hundreds of thousands” of pre-infected plants to stores across the eastern seaboard, from Maryland to Maine.

The state of Maine then suffered through the wettest summer on record, ideal growing conditions for several fungi as well as Phytophthora infestans.

The blight moved through Franklin County rapidly, spores carried through the wind. While it skipped a few pockets, LeBlanc said, it wiped out some gardens completely. She recalled a young couple who had started their first garden, only to see their plants wither and die. They pulled up the plants, completely cleared the ground and then burned their clothing and shoes before going to see LeBlanc.

Other fungi that can affect tomato plants. The top picture is Septoria Leaf Spot, while the bottom picture is a plant suffering from the controllable “early blight.”

Late blight starts on leaves, spreading after spores land on the plant. The brown and black blight spreads like an infection in the human body, moving along the veins of the leaf and into the stem of the plant. The blight causes stems to “bruise,” leaving discolored rings. Late blight also affects the tomato fruit, creating lesions which become hard, unlike most conditions which renders the fruit mushy or rotten.

Late blight should not be mistaken for the more common and less devastating “early blight” and Septoria Leaf Spot, which are easy to find in gardens every year.

Early blight is a common, annually-occurring problem faced by tomato growers, and is characterized by a slow advance of dying leaves and blight moving from the bottom of the plant toward the top. Early blight does not affect the fruit of the plant, except in extreme cases, and isn’t typically much of an issue. It can be controlled with some basic pruning and cleaning gardens out in the fall.

Septoria Leaf Spot, which can be identified by small yellow and brown spots appearing on leaves, can spread quickly, but usually leaves the plant relatively good shape. It does not affect the fruit. Again, carefully controlling the plant’s environment, ensuring that the growing area is completely cleaned out in the fall, can prevent the spread of Septoria Leaf Spot.

This growing season is nearly over, with harvests coming in. The focus now, LeBlanc says, needs to be on making sure that 2009 is late blight’s last year in Franklin County.

Three tell-tale signs of late blight. An unripe tomato with a blight lesion, which will be hard to the touch, not mushy. A leaf affected by the blight, with the characteristic cross-vein spread of the affected area. A stem, bearing a dark ring of affected tissue. While the affected stems can be pruned, preventing the spread of late blight is difficult.

The primary concern is potato plants. The plants typically die off late in the year anyway, making it difficult to distinguish infected plants from healthy ones. To be safe, experts recommend, the dead plants should be removed and taken to transfer stations.

Plants should not be composted this year. Some compost piles don’t freeze over the course of the winter, and can then reseed soil with blight spores when reused next year.

After removing all potato plants and disposing them correctly, the potato roots should be left in the ground for two weeks prior to harvesting. This prevents the tubers from being exposed to the blight, which could happen if they were dug up immediately.

Potatoes should be left in the ground two weeks after the plant is removed.

Once harvested, the potatoes should be stored in a cool dry place, which is typical, away from other vegetables. The gardener should then carefully inspect the potatoes, looking for signs of mold, white rings of spores or anything unusual. Any odd potatoes should be remove and disposed of at the dump.

Next spring, the potato fields should be watched carefully. Even a thorough gardener is likely to miss a few potatoes come harvest time, with those tubers sprouting again next year.

“There’s always the one you miss,” LeBlanc said. “The sprouts must be removed.”

A missed potato sprouting a plant in the spring, experts say, represents the most likely vector for late blight to make a resurgence next year. One missed plant can spread spores up to 30 miles away, depending on wind and atmospheric conditions. Enough missed sprouts could easily start the cycle all over again.

Tomato plants, which completely die off during the winter, are less of a threat. Still, the plants should be disposed of correctly and not composted.

LeBlanc encourages gardeners to choose tomato seedlings from small, local garden centers and farms, which cannot be pre-infected with the blight. Potato seeds should be certified “clean seed potatoes.” Most stores and businesses will be able to tell gardeners if they carry certified clean seed. Using seed potatoes from the 2009 crop should be avoided.

LeBlanc has been aggressively pruning, segregating and spraying her plants to try and stay ahead of the blight and maintain a decent harvest. This isn’t her first encounter with the strain; a particularly virulent version spread up along Franklin County valleys in 2000, carried by the wind. LeBlanc had 300 plants go down in three days.

Careful harvest, disposal and prevention can help it from happening again.

Amy LeBlanc stands in one of her gardens.

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  1. This is exactly what I have seen happening to my tomato plants. I have a tiny garden with six tomato plants of 3 different varities and was so looking forward to sharing with my family and friends. I also got just one acorn squash but beautiful plants and blossoms…lack of bees?

  2. Excellent article with a slight alteration. An extraordinary climatic shock “The Great Frost” struck Ireland and the rest of Europe between December 1739 and September 1741, after a decade of relatively mild winters. Its cause remains unknown. The crisis of 1740-1741 should not be confused with the equally devastating Great Potato Famine in Ireland of the 1840s.

  3. You are quite correct Mel, the date in the article should have been in the 19th century. We’ll make the change.

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