In the Garden:
New England
March, 2010
Regional Report

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The yellow flowers of vernal witch hazel are one of the first harbingers of spring.

Will Late Blight Be A Problem Again?

Last summer gardeners all over New England were devastated to find their once healthy tomato plants dying within a matter of days. What started as a few water-soaked spots on the leaves and stems spread rapidly until the entire plant collapsed in a heap.

The cause of all this devastation was late blight, a disease caused by the fungus-like pathogen Phytopthera infestans, which strikes potatoes as well. It was late blight that decimated the potato crops in Ireland in the mid-1800's, leading to the famine that caused so much suffering and led to the immigration of so many of the Irish to our shores.

While we may not face the threat of starvation from the loss of our tomatoes and potatoes, it is still a huge disappointment to see our carefully tended crops turn into heaps of rotting tissue. What caused this disease to be such a problem last summer and will it strike our gardens again this year?

Usually late blight is not a major problem for home gardeners in our part of the country, appearing only sporadically. But cool, wet weather favors the spread of this disease and that is exactly what we had almost all season long, especially in the early summer. The seemingly endless days of rain set the stage for late blight to reach epidemic proportions. While the disease only survives in living plant tissue and does not persist in the soil over the winter, its spores can be carried long distances by wind. So if spores arrive in an area on infected tomato plants or potato tubers, or blow in from other areas, and the weather conditions are right, late blight rears its ugly head.

With luck, we won't have such a rainy early summer again this year. But gardeners should be on the lookout for late blight in their gardens and act quickly if they see problems developing.

How do I know if my tomatoes and potatoes are infected with late blight?
The first sign is the appearance of dark, water-soaked, irregularly shaped spots, about the size of a nickel or a quarter, on the leaves. These spots become covered with a fuzzy white mold on the undersides of the leaves. They enlarge quickly, turn black and kill the entire leaf. The infection then spreads to the leafstalks and main stem, eventually causing the entire plant to collapse and die. Tomato fruits and potato tubers can also be affected.

There are a couple other common diseases that cause leaf spotting on tomatoes and potatoes. Both early blight and Septoria leaf spot cause brown spots, often with yellow borders, usually infecting the lower leaves first. These diseases are usually less severe than late blight and don't kill the entire plant. Botyrtis gray mold can cause fuzzy white spots on leaves. If you aren't sure whether your plants have late blight, contact your local County Extension Service or Master Gardener program for help. Or go to www.hort.cornell.edu/lateblight for information and great color pictures of symptoms.

Can I save my plants if they become infected with late blight?
If you see spots on just a few leaves of tomatoes or potatoes, you can pick these off, put them in plastic bags and toss them in the garbage. But if lots of leaves or the stems are infected, it's best to destroy the whole top of the plant to prevent the spread of spores to uninfected plants. If the foliage on your potato plants is only lightly infected, you can cut it off and destroy the tops. This will help prevent the spores from washing down and infecting the tubers. Wait at least 2 to 3 weeks or until any remaining potato foliage is completely dead before digging the tubers.

Can I put my infected tomato or potato plants in my compost pile?
The pathogen that causes late blight only survives in living plant tissue, so the answer to this question is yes, if your compost pile heats up properly. But this is often a big "if" for home composters, so the safest strategy is to bag infected plants in plastic. Set the bags out where they can heat up in the sun for a few days to kill the plant debris inside, then put them in your trash. Also make sure you don't leave any unharvested potatoes in the ground over the winter, as the late blight pathogen can survive on them.

Can late blight survive on my tomato cages, stakes and garden tools?
No, late blight will only survive on living plant material.

Should I use a fungicide to prevent late blight on my plants?
The thing to remember about fungicides is that they are not a cure for an infected plant. They prevent a fungal infection from spreading to uninfected tissue. If your plants are not yet infected, or at the very first signs of the disease, spraying with a fungicide may prevent further infection. But you'll need to make sure you completely cover the plant with spray and reapply every 5-7 days when the weather is suitable for the spread of the disease. Make sure you use a fungicide labeled to control late blight on the crop in question and read and follow all label instructions and precautions.

I save my own tomato seeds. Can late blight survive in them? What about on my stored potatoes?
No, the pathogen that causes late blight can't survive the winter in or on your seeds. So go ahead and start seedlings from your saved seeds. Stored potatoes are another matter. Any potatoes left unharvested in the soil can serve as a reservoir for the disease, so clean up your potato patch well at the end of the season. Don't wash the potatoes you harvest to store until you use them, as this could spread disease among them. And check your stored tubers frequently and discard any that show signs of infection- brownish purple spots that become a wet or dry rot. If you buy seed potatoes in the spring, make sure they are certified disease-free.

Are other members of the tomato family (Solanaceae) at risk for late blight?
Eggplant and peppers are related to tomatoes and potatoes, but don't show the same susceptibility to this disease.


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