Home gardeners need to be on the lookout for a highly destructive and infectious fungal disease that’s destroying tomato and potato plants across the state and much of the Northeast. Called late blight, it’s the same disease that ravaged the Irish potato crop in the 1840s, which contributed to The Great Famine.

(photos courtesy of Meg McGrath)

Jack Algiere, the farm manager at the Stone Barns Center for Food and Agriculture in Pocantico Hills, said yesterday that he had already pulled and destroyed 400 infected tomato plants and that he expected to lose even more of the 1,500 outdoor tomato plants he planted in the spring. So far, the farm’s potato plants and the tomatoes in the greenhouse have not been affected.

(Click here to read this same story that ran on Page One of The Journal News and lohud.com, with another photo.)

This is the earliest late blight has been seen in the growing season — and it’s the most destructive infection in recent memory.

“Eastern New York is getting hit really bad,” Meg McGrath, a plant pathologist for Cornell University in Riverhead, N.Y., said yesterday. So far, Long Island has escaped serious problems. “It’s just a matter of time is what I’m worried about.”

Farmers are blaming the cool, wet June weather.

“This particular bout of late blight is definitely the worst I’ve ever seen,” Algiere said. “And all the other farmers I’ve talked to are saying the same thing.”

The fungus, Phytophthora infestans, is highly contagious because it spreads on airborne spores that can travel for miles.

“It’s everywhere,” said Dianne Olsen, horticultural educator for Cornell Cooperative Extension of Putnam, which has a plot in the community garden run by the county parks department. “All of our tomatoes have been lost: totally, completely gone. They die in a week – it’s just horrible. Once the leaves turn yellow, they’re goners.

“I’ve heard that those folks who bought tomato seedlings in big-box stores and those giant nurseries are having the biggest problems,” she said yesterday. Earlier this month, Bonnie Plants, a plant wholesaler based in Alabama, recalled about $1 million worth of possibly infected tomato plants from the stores it supplies, which include Wal-Mart, Lowe’s and The Home Depot. People who grew tomatoes from seed seem to have escaped late blight problems so far, Olsen said.

“Not every variety has been affected in the same way,” Algiere said. Some of the older heirloom varieties have shown surprisingly strong resistance to late blight, while newer varieties that he expected to be tougher have gone under.

Late blight symptoms on tomato leaves, stems or fruits are fairly dramatic and easy to spot. Look for nickel- to quarter-sized lesions that are olive-green or brown and look wet. Leaf or lesion edges may appear yellowish.

When the lesions dry out, they may appear lime green or beige. The edge of the water-soaked lesion will be covered with white fungal growth that contains the contagious spores.

In potatoes, the disease destroys both the leaves and the edible roots, or tubers, of the potato plant. The Irish Potato Famine was the worst in Europe in the 19th century. About 1 million people died of related diseases and up to 1.5 million more left Ireland.

Petunias are closely related to tomatoes and potatoes and may be infected, too, with similar symptoms.

It’s important for homeowners with infected plants to destroy them quickly to avoid affecting commercial growers.

“Let’s say you live in Yorktown near a farm that’s growing tomatoes for sale,” Olsen said. “The spores travel on the wind, so there you are.”

Algiere said he’s been pulling up any plants with signs of infection.

“Once it comes in on the wind, there’s no way to stop it,” he said. “Once it shows itself on the leaves, it’s going to spread.”

McGrath said better air circulation would help avoid the spread of the disease.

“In the typical home garden – and I can speak from experience – we tend to cram as many plants as we can into our gardens,” she said. “There’s not enough air circulation, and the humidity is higher than normal.”

The best way to stop the spread of late blight would be two weeks of dry weather with temperatures above 80 degrees, Algiere said. “Every time it rains or we have humidity above 50 percent, the incubation process happens quickly.”

Those with infected plants can take steps in August to avoid problems next year, Olsen said.

“Cover the area with black plastic and let it bake in the August sun,” she said. “This will help kill some of the organisms in the soil that were left over from the tomato plant roots.

“Next year, plant something else other than tomatoes or potatoes in the spot where you grew this year,” she said.

What you can do

– Closely inspect your tomato and potato plants at least once a week for lesions or other signs of late blight. – You can try using a preventive fungicide spray to treat your plants, which is what commercial growers are doing. Look for products that contain chlorothalonil. These sprays are not absorbed systemically by the plant, so they need to be applied thoroughly and then reapplied every five to seven days if there’s a lot of wet weather.
If you’re gardening organically, you can try a copper spray, but they are not considered very effective against late blight. – Be prepared to destroy your plants if you get an infestation. – Do not compost any diseased portions of your plants. Put them in plastic garbage bags. – Rotate your crops so you don’t grow tomatoes and potatoes in the same soil from year to year.

For more information
Call your county Cornell Cooperative Extension for more information – Westchester: 914-285-4620; Putnam: 845-278-6738; Rockland: 845-429-7085.