August is the month when growers normally have to keep watch for the disease late blight, which kills potato and tomato plants. But this year, late blight made an unusually early appearance and has been devastating crops in the Northeast and mid-Atlantic for more than a month. The experience of farmers and gardeners in those regions may provide guidance to the rest of the country as the regular late blight season approaches.
The most important thing to know about late blight is that its spores travel long distances on the wind, so any infected plants in an area can cause a widespread problem in a short period of time. That makes the disease a community problem, not just an individual farm’s problem. Farm groups and Extension services are working to educate backyard gardeners as well as commercial growers about the disease, in the hope of reducing its incidence now and next year.

“It’s been really upsetting, to be honest,” said Claire Parde, a community gardening specialist with Cornell Cooperative Extension in Greene County, New York. “I’ve had to instruct people to pull out all their tomato plants. Most community gardeners are new to gardening, so it’s very disappointing and discouraging for them. There’s such a strong temptation to try to maintain plants that aren’t clearly infected and see if they come through. But I tell them that it poses a risk to everyone, including people whose livelihoods depend on it.”

Late blight is caused by the fungus Phytophthora infestans, which was responsible for the Irish Potato Famine of the 1840s. It continues to be one of the most devastating diseases of plants worldwide. This year, it was found on Long Island in mid-June, the earliest ever detected in the Northeast, and it was quickly reported over a wide area from Maine to South Carolina and west to Ohio. Cornell traced the first occurrences of the disease to tomato transplants sold at big-box garden centers such as Wal-Mart and Lowe’s. Persistent cold, rainy weather created the perfect environment for the spread of the fungus.
Organically grown tomatoes and potatoes are at particular risk because there is no organic cure for late blight. The most organic growers can do is spray products that may protect the spores from attaching to the leaf surfaces.

“I have no symptoms in the field, but am shaking in my booties, to say the least,” said Dan Pratt, a market farmer in Hadley, Massachusetts, and frequent GFM contributor. “I may begin a five day schedule of copper, but it seems like whistling in the park to keep the lions away.”
Paul Arnold of Argyle, New York, said: “We don’t have it, but quite a few people around us do, both conventional and organic. We’re spraying copper and Serenade (together) every five days, or between rain storms.”

In northern Maine, Jim Gerritsen of Wood Prairie Farm, a top producer of organic seed potatoes, said the disease is not as prevalent in his area as it is farther south. But he has started a seven-day spray schedule with copper hydroxide. And he sprays OxiDate in between copper sprayings. It takes five hours to spray his crop each time, but he says it’s worth it. So far, he has seen no signs of the disease in his potatoes.

How to prepare
If you aren’t familiar with the symptoms of late blight, there is a wealth of images on university web sites and in vegetable pest books. One of the best web sites is from Cornell’s Long Island Horticultural Research and Extension Center in Riverhead, New York: http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm.

The primary symptoms of late blight are stem lesions and white fungal growth, which is sporulation of the pathogen, on stems and leaves. The disease also can affect other plants in the Solanaceae family, including petunias and nightshade weeds.

The second step is to find out if your state’s Extension service offers a late blight alert and reporting service. Call you county agent to find out how to stay informed about the incidence of late blight in your area. In New England, Extension has been reporting occurrences by county or township, without identifying farms.
If the disease is present anywhere nearby, especially upwind, start a spray program. There are several commercial fungicides labeled for tomato and potato late blight for non-organic growers. For organic growers, the most effective products are copper, which has to cover all above-ground plant surfaces to protect them from the spores. It should be sprayed every five days and after heavy rains. Other products that may provide protection contain the biological fungicide Bacillus subtilis (e.g. Serenade, Rhapsody) and OxiDate, which uses peroxygen chemistry – rapid oxidation that kills a wide range of bacteria and fungi on contact.
Scout daily for symptoms and if you find any, pull up the entire plant and all the plants around it. Do not compost them. Either bury them, or put them in a plastic bag and remove them from the farm. Cornell recommends this, but advises there is no evidence that it will stop the spread of the disease. That’s because infection may have occurred a week or more before symptoms appear.

If an entire field shows signs of infection, it’s pointless to try to save it, the experts say. The plants are sure to die within a few days and, in the meantime, the disease will be producing a huge number of spores that will blow into other areas. Potatoes of infected plants may still be harvestable, if the plants are killed and the tubers left to harden underground for a few days.

It is believed that the disease needs living plant tissue to survive over the winter, so growers with infected crops are urged to kill the plants by mowing or flaming, and then disk them in so they are covered with soil.
For more on organic control of late blight, you can find information and numerous links to Extension sites on the ATTRA web site, www.attra.org, or phone 800-346-9140 to request printed copies of the information.