Apparently, lightning can strike twice: A case of late blight, the rare plague that devastated tomato and potato crops across Long Island last year, was found in a community garden in Setauket on Thursday. Meg McGrath, a plant pathologist at Cornell University’s Long Island Horticulture Research and Extension Center in Riverhead, personally confirmed the disease on Saturday.
Late blight is caused by a fungus-like pathogen that thrives on moisture. Last year's epidemic was created when the pathogen went crazy, fueled by excessive spring rains and cool temperatures. Here's how it works: An infected plant can release up to a million spores a day. Those spores hang overhead, and when it rains, they are spread all over the region with the precipitation. This is not just a concern to your garden or your neighborhood. Theoretically, your one infected plant in, say, Floral Park, could wipe out a farm in Riverhead. It's that serious.
“Anyone growing susceptible plants needs to take responsibility to ensure they don’t become a ‘Typhoid Mary,’” McGrath said. “We need to treat this like a community disease,” she adds. “If infested, even a small garden can have a devastating impact on other plantings.”
The Setauket plants are being disposed of, McGrath told me, "but I'm concerned that conditions were likely also favorable in other gardens, so wherever the pathogen is present, late blight is probably now occurring," she said.
This isn't good, but because McGrath gave me an immediate heads-up, we can keep on top of this thing so it doesn't spread and devastate gardens and farms all over the region. Right now, go outside and check your tomato and potato plants. Inspect them thoroughly. Look for elongated dark lesions on stems or wet-looking grayish blotches on leaves.
There is no silver bullet, unfortunately, but there are some steps you should take if you suspect late blight on plants growing in your garden. First, bring the plant or affected plant part to the Suffolk County Cornell Cooperative Extension office at 423 Griffing Ave., Riverhead, for diagnosis. (call 631-727-4126 for help).
If late blight is present in your garden, remove the plant, roots and all, before the next rain to minimize the chances it will spread to other gardens and farms. Immediately bag the plant tightly in plastic and set in the sun for a few days until the plant dies off completely. Then dispose of in the trash. Do not compost. Do not leave it lying on the ground for later.
If you're not sure -- and chances are you won't be unless you get a professional diagnosis -- place suspect leaves in a sealed zipper-top plastic bag overnight. Late blight spores will develop into a fuzzy mass on the underside of the affected foliage.
Here are 10 things McGrath recommends you do to minimize late blight in your garden.
1. Kill volunteer potatoes. Dig up, bag and trash any potato plants that sprout from tubers left in the ground or compost pile last year. It may take repeated efforts to get them all.
2. Buy healthy tomato plants. Learn what late blight looks like. (See photo above) If you spot any infected plants while shopping, alert store management and your local Cooperative Extension office, and buy your plants somewhere else. Or you can grow your own plants. (Late blight isn’t spread on tomato seeds.) Start seed about 6 to 8 weeks before your last frost date.
3. Use certified seed potatoes. Don’t use leftovers from last year’s garden or table stock from the grocery store.
4. Keep plants dry. The late blight pathogen thrives in cool, wet weather. That’s because it requires moisture to infect plants, grows best when it’s cool, and clouds protect spores from lethal UV radiation when they are dispersed by wind. Even in absence of rain, the pathogen can infect plants if the relative humidity is 90 percent or more. If plants need watering, water the soil – not the foliage.
5. Be vigilant. Inspect plants at least once a week – more often if weather is cool and wet. Immediately remove and bag foliage you suspect might be infected. While late blight symptoms are distinctive – dark brown lesions on stems and leaves with white fungal-like growth developing under moist conditions – it’s possible to confuse it with other diseases. Your local Cooperative Extension office can help you with identification.
6. Act quickly. If symptoms continue despite removing infected foliage, consider removing plants entirely – sooner rather than later. "It is rarely possible to control late blight just by removing affected tissue," McGrath said. "The longer you wait to remove plants, the more spores your garden sends to the wind to infect other gardens and farm fields."
7. Sound the alert. If you find late blight in your garden, let your gardening neighbors and local Cooperative Extension staff know so they can warn others and be on the lookout for additional infestations. Make sure your neighbors know how to spot late blight in their own gardens.
8. Dispose of plants properly. To reduce disease spread, remove infected plants during the middle of a sunny day after leaves have dried, if possible. But don’t wait for these conditions. Seal plants in garbage bags and leave them in the sun for a few days to kill plants and the pathogen quickly before placing in the trash or burying underground or deep in a compost pile. Don’t just leave plants on the ground or on top of the compost pile where they will continue to be a source of spores until the plant tissue dies. With a large number of plants, you can build a pile on the ground and cover securely with a tarp until the plants die.
9. Keep an eye on other tomato-family plants. Some strains of late blight can infect other tomato-family plants, including weeds such as hairy nightshade and bittersweet nightshade. Control them early so that late blight on these plants doesn’t go unnoticed. Petunias and tomatillos are also vulnerable to attack.
10. Use fungicides with care. Fungicides can control late blight. (Chlorothalonil and copper-based products are both available to home gardeners.) But if you wait until late blight symptoms appear, it might be too late to rescue plants. For fungicides to work effectively on late blight requires a regular preventive spray schedule and thorough spray coverage. Follow all label directions, including use of respirator, waterproof gloves and protective eyewear.
The video at the bottom of this page is from last year's outbreak.