Late Blight Alert! Disease of Tomatoes, Potatoes Arrives in New Hampshire

earliest appearance on record; devastating disease spreads fast, kills quickly

Late blight, a devastating fungal disease that infects tomatoes and potatoes, has been confirmed in Maine, upstate New York, Pennsylvania and in several areas of New Hampshire.

Left unchecked, the fungus can quickly wipe out plantings of these popular vegetables.

Premature arrival, probably on infected tomato seedlings
"Late blight usually doesn't strike the Northeast until August," says Extension Plant Health Specialist Cheryl Smith. "Rainy, overcast weather has provided very favorable conditions for development and spread of the disease."

"Some large, nationwide retail stores have apparently sold infected tomato seedlings. If you bought tomato seedlings at one of these stores, check your plants and keep on checking," Smith says.

Symptoms
"Classic symptoms include large, irregularly-shaped, water-soaked, olive-green-to-brown spots on leaves. Under wet or very humid conditions, a slightly fuzzy, white fungal growth may be visible on the underside of the leaf," says Smith." "Leaf lesions begin as tiny, irregularly-shaped dark green or brown spots. Brown to blackish irregular lesions also develop on upper stems. Firm, brown spots develop on tomato fruit, and infected fruit often looks bumpy".

Advice to home gardeners
"The late blight fungus produces many spores, which can travel long distances through the air. It's crucial that everyone who grows potatoes or tomatoes, including home gardeners, is monitoring for late blight to avoid being a source of spores that move on to infect potatoes and tomatoes in neighboring gardens and commercial fields.

"There's no need to take action if your plants show no signs of infection," Smith says. But she urges home gardeners to heed this advice:

• Thoroughly inspect potato and tomato plantings on a daily basis, because late blight moves fast and can be difficult to control once established in a planting.
• Fungicides containing the active ingredient chlorothalonil are fairly effective in protecting plants from infection. Although copper fungicides are an option for organic gardeners, copper is not highly effective. Gardeners don't have access to fungicides effective for controlling the disease once plants are infected.
• Don't attempt to treat infected plants with fungicides, even those labeled for late blight. Fungicides available to home gardeners can't cure plants that are already infected.
• If you see signs of infection, pull all infected plants from the ground, bag them up, and dispose of the bags in the trash. Do not put them in the compost or in a refuse heap.
• Put a few samples (include several stems plus leaves and/or fruit) into a plastic bag and bring it to your County Extension office, but don't wait for confirmation to pull out the infected plants.
• Don't touch healthy plants after handling infected plants until you've scrubbed your hands with soap and water thoroughly.

Guidance for commercial growers Detailed information for commercial growers, who have access to methods and materials for controlling late-blight infected plantings not available to home gardeners.

Cornell University's late-blight photo gallery

Late blight/early blight photos Photos compare late blight with early blight.

Cornell University vegetable disease updates Frequent updates about vegetable diseases in the Northeast, including late blight.

Photo of late blight on tomato leaves by Eric Sideman, Maine Organic Farmers and Gardeners Association.

Posted June 26, 2009