All along the east coast, gardeners are concerned about late blight affecting their tomato or potato plants. Usually this disease begins appearing in August but in 2009 the cool, wet spring rains brought it on early. Normally it is too dry in midsummer for this to occur. Late blight mainly affects tomatoes and potatoes. It also affects other nightshade family plants such as peppers and eggplant, but not as severely. This is the disease that wiped out crops in the Irish potato famine and you should know what to watch for in case it infects your garden.
Late blight first showed up during the Maryland 2009 garden season in the middle of June, when a Howard County gardener brought his sick plant to experts to see what was wrong with it. Big box retailers sold many of the infected plants, so there are fears that the blight could be spread over many different areas. There are concerns that inexperienced home gardeners could aid the fungus with improper care because they may not be aware of how persistent the disease can be.
The late blight fungus usually starts in the leaves, with small brown or gray lesions that quickly grow and spread over the entire leaf. These look like brown or gray water soaking into the leaf. The fungus spreads to the stems, which end up with dark brown lesions, and then the tomatoes themselves rot after turning brown. Rain can gather the spores from the late blight fungus and carry it miles away.
Humans are not generally affected by the blight, but beware of spreading the fungus if you have to remove plants. Destroy the affected plants and don’t try to save them. Seal the plants well in a plastic garbage bag and put them out with the trash. If you compost tomato plants with late blight, the fungus will stick around and spread. For chemical control try cholorothalonil but for organic gardening try neem oil , which needs to be reapplied every week. But, fungicides are only for the remaining plants and a preventative. They cannot reverse the damage on an already sick plant.
Prevention will help. In the future, try purchasing tomato plants from small growers or farmer’s markets. Keep the plants dry and do not overcrowd them. Keep them properly staked and pruned. Make sure the plant gets plenty of air in between the leaves and branches. Check the plants for late blight symptoms at least once a week, more often in humid or wet weather. Water the plant at the base instead of misting from overhead. Mulch to control water from splashing up. Don’t plant problem vegetables in the same spot from year to year. With a few precautions you can prevent this fungus from spreading to the rest of your tomato or potatoes plants. For more pictures of how the fungus affects plants, see the article “Late Blight of Potatoes and Tomatoes” by William E. Fry.
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