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Late blight update

Late blight lesion on tomato leaf.  Click for larger view.

Late blight lesion on tomato leaf. Click for larger view.

On May 18, New York State Agriculture Commissioner Darrel J. Aubertine alerted home gardeners and commercial growers of the potential introduction of late blight this growing season. “The exceptionally cool, damp spring we are experiencing throughout New York State this year heightens our concern for late blight,” the Commissioner said. (View news release.)

Cool wet conditions, coupled with the presence of the disease led to a quick and devastating spread of the disease in 2009. This season, late blight has already been detected at three sites in Michigan, Connecticut and Maine.

Gardeners and growers need to be on full alert to detect the disease early. Be sure to scout greenhouse and high tunnel tomatoes and early potatoes. If you think you may have late blight, contact your local Extension educator.

Knowing where late blight has been found is critical for making the best management decisions. A new national website USAblight is mapping reported outbreaks online. (Reports will not pinpoint your farm or garden, just the county.) View disease occurrence map.

Some resources to help you identify the disease and monitor the situation include:

10 tips for gardeners to beat late blight

So what can gardeners do to minimize late blight in their own gardens – as well as reduce the chance of spreading it to others?

Kill volunteer potatoes. Dig up, bag and trash any potato plants that pop up in your garden or compost pile. It may take repeated efforts to get them all.

Buy healthy tomato plants. Learn what late blight looks like. 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.

Use certified seed potatoes. Don’t use leftovers from last year’s garden or table stock from the grocery store.

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.

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.

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,” says McGrath. “The longer you wait to remove plants, the more spores your garden sends to the wind to infect other gardens and farm fields.”

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.

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.

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.

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.

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