Late Blight, a very very serious plant disease (cause of the Irish Potato Famine serious) that doesn’t usually appear until later in the summer if at all, has been confirmed in several places in New Hampshire and Maine. Unfortunately, there are no easy “cures” for this plant disease, but home gardeners alongside farmers can help stave off the spread and respond to instances. The photos links to at the bottom are especially helpful!
UNH Cooperative Extension has put together excellent resources for diagnosing late blight and then formulating a plan of action if you do discover late blight:
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.
“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.
Late blight/early blight photos Photos compare late blight with early blight.
An article on blight from the New York Times explains a bit more:
A highly contagious fungus that destroys tomato plants has quickly spread to nearly every state in the Northeast and the mid-Atlantic, and the weather over the next week may determine whether the outbreak abates or whether tomato crops are ruined, according to federal and state agriculture officials.
The spores of the fungus, called late blight, are often present in the soil, and small outbreaks are not uncommon in August and September. But the cool, wet weather in June and the aggressively infectious nature of the pathogen have combined to produce what Martin A. Draper, a senior plant pathologist at the United States Department of Agriculture, described as an explosive rate of infection.
William Fry, a professor of plant pathology at Cornell, said, Ive never seen this on such a wide scale.
A strain of the fungus was responsible for the Irish potato famine of the mid-19th century. The current outbreak is believed to have spread from plants in garden stores to backyard gardens and commercial fields. If it continues, there could be widespread destruction of tomato crops, especially organic ones, and higher prices at the market.