Farm Nightmares: All About Late Blight

Images courtesy of USABlight.org.

Images courtesy of USABlight.org.

Days like today — wet, cold, and windy — keep farmers up at night. Early in the season, a rainy day is welcomed; a good rain can re-hydrate a field, warm the frost from the soil, and cut down on the farm’s water consumption for new transplants. Later in the season, though, there’s much more danger in a blustery day. Lurking in the rain and the wind is the threat of late blight, a water mold that can wipe out a whole field of tomatoes in less than a week. Spores of the late blight disease can travel 30 to 40 miles on a wet and windy day, spreading like wildfire through farms all over the northeast.

Late blight is a variation of the same blight that caused the Great Irish Potato Famine in 1845 and primarily affects tomatoes and potatoes. The scientific name for the disease is Phytophthora infestans, which translates to “attack of the plant destroyer.” The water mold thrives in damp, cool conditions. When a plant becomes infected with late blight, wet-looking olive green lesions begin to form on the leaves. The lesions begin to encompass the leaf, turning brown with white, fuzzy spore growth visible on the underside of the leaves. The infected leaves become necrotic and the plant defoliates quickly. If the plant doesn’t die quickly from this, the blight can spread to the stalk and cripple the plant’s ability to absorb nutrients from the soil. Even if you harvest fine-looking tomatoes from a plant infected with blight, they turn black within days.

Late blight didn’t exist in the northeast U.S. until 2009, when a commercial greenhouse distributed infected transplants to big box stores in the area. That summer was also one of the coldest, wettest summers on record, which caused rapid spread of infection. Millions of dollars in failed tomato and potato crops were lost in the northeast that year and caused some farms to shut down all together. Since then, it’s been the stuff of farmer nightmares. 2011 showed no sign of blight until Hurricane Irene brought it up from hundreds of miles away. This year, the first reported case of blight in the U.S. was in Wisconsin on July 18. Since then, it has spread all the way from northern Maine to southern Florida to the coast of Oregon.

Classic early late blight.

Classic early late blight.

In the Northeast, blight often starts in Long Island, Pennsylvania, and northern Maine: all areas with large potato farms. It’s a community disease, a shared disease. If one farm spots blight, it’s not long before others in the region are infected. For this reason, farmers report blight as soon as it’s seen (in our case, to UMass) and, all season, keep a trained eye on forums and maps for new and encroaching outbreaks. At Land’s Sake, we spotted our first signs of blight on Sunday, August 10 and were one of the first two farms in Massachusetts to report an outbreak. With our fields in different locations around Weston, it’s possible that not all of our tomatoes will become infected. So far, our main tomato crop is fine. Because the disease is wind-borne, though, it can be nearly impossible to predict. After a day like today, we could be fine or we could be facing major devastation in our tomato fields.

The only organic preventative measure against late blight is a copper hydroxide fungicide spray. It’s certified for organic growers and non-toxic. The manufacturer suggests a spraying regiment of every 3 – 10 days; with the wet days and cold nights we’ve been having, Farmer Erik has been spraying our tomato fields on an approximate 7 day schedule. The spray will help prevent the disease, but there’s no cure once a plant has been infected. At that point, you have to rip out all the infected plants and hope it didn’t spread. Unfortunately, it can take up to a week to spot symptoms–but the plant is contagious from the moment of infection as the spores quickly multiply on damp leaves.

“It’s terrible,” Erik says. “It’s making growing potatoes and tomatoes organically very difficult. You spend all your time worrying about having another year like 2009. It’ll destroy your fields in a week, and there’s nothing you can do.”

Identifying Blight (left) vs. Drought Damage (right): check for fuzzy white spore growth to ID blight.

Identifying Blight (left) vs. Drought Damage (right): check for fuzzy white spore growth to ID blight.

If you’re worried about blight in your home garden, you can buy copper hydroxide at many local garden stores. To help prevent blight in your garden, give your plants plenty of space to encourage good air flow and trellis tomatoes to keep the leaves off the ground and dry. Try to avoid getting the leaves wet when you’re watering. If you spot late blight in your garden, destroy the infected tomato or potato plants. To destroy the plant, rip it up at the root, secure in a trash bag, and throw it out. Overwintering isn’t a problem for tomatoes, because the disease needs living plant tissue to survive, but it can survive the winter in potatoes left in the ground If potato plants pop up in the spring, they should be destroyed. For more information about Late Blight in both tomatoes and potatoes, visit USABlight.org.

Posted in Farm.