Cornell offers tips to protect gardens from 'late blight'

You can share these with your customers to help them avoid problems with tomatoes and potatoes

April 21, 2010
Garden Center
Pest and disease

Gardeners throughout the Northeast can take steps to protect their cherished tomato and potato plants against the “late blight” that wiped out thousands of home tomato plants during the 2009 growing season.

Late blight, Phytophthora infestans, is the fungus-like pathogen that causes lesions and eventual die-off in tomatoes, potatoes and other tomato-family plants. This disease can be highly contagious among susceptible plants, and gardeners need to take steps to identify the disease and prevent it from spreading.

“Anyone growing susceptible plants needs to take responsibility to ensure they don’t become a ‘Typhoid Mary.’ We need to treat this like a community disease,” said Meg McGrath, a plant pathologist at Cornell University’s Long Island Horticulture Research and Extension Center. “If infested, even a small garden can have a devastating impact on other plantings.”

Here are 10 tips that gardeners can use to reduce the chance of spreading the late blight to other plants and gardens:

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

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

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, grow your own plants. (Late blight isn’t spread on tomato seeds.) Start seed about six to eight weeks before your last frost date.

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 the 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.  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.