How to cope with tomato blight

By Jim Janke | May 06, 2014

Did you have problems with your tomato plants last year? Many of my gardening friends had a total crop failure. The problem typically was early or late blight, made more vicious than normal by the excessive rainfall. Here are some ideas to prevent blight from attacking your garden.

Early blight (Alternaria solani) can appear as early as mid-May. Tiny dark brown spots appear on the lower leaves that grow to more than a half-inch in size, then spread to stems and fruit. Eventually the leaves turn yellow, then brown, and drop off. Spores that mature on plant surfaces then spread the infection further.

Late blight (Phytophthora infestans) is soil-born, but can also be on dust that wind or insects bring into the garden. The cool, moist nights and moderately warm days of mid to late summer are conditions this fungus prefers. Infections show up on both the leaves and fruit as wet-looking spots.

Cultural practices that help control blight:

— Rotate crops. Blight fungi can overwinter in the soil and show up the following year. Late blight also infects potatoes, so don’t grow them close to your tomatoes.

— Grow resistant varieties. Heirloom types are typically more susceptible to blight and other pests.

— Don’t save seeds from diseased plants; the blight fungus can survive on seeds.

— Space plants to allow good air circulation. Weed the garden regularly.

— Water only early in the day so leaves will have a chance to dry before dusk. Don’t water overhead; you may cause the fungus to spread from leaf to leaf or by splashing soil on to the plant. Work in the garden only when plants are dry.

— Mulch, cage or trellis each plant to keep leaves off the ground, and remove the lower 8 to 12 inches of leaves even if they are healthy.

— Keep your plants strong, and they’ll be less likely to succumb to the fungi. Low nitrogen levels particularly encourage early blight. But don’t overdo the fertilizer: too much nitrogen will encourage stem growth at the expense of fruit. A water soluble fertilizer (like MiracleGro or Peters) applied monthly works well.

— Remove any infected leaves or fruit as soon as you see them. Wash your hands after touching diseased leaves to prevent spreading the fungus. Remove plant debris promptly in the fall, but don’t compost diseased material. The average home compost pile doesn’t get hot enough to kill the fungus.

Treatment — Plants can survive early blight if treated quickly. Spray a listed fungicide on all plant surfaces at seven to 10 day intervals.

Once a late blight infection occurs, the only cure is to remove all sick plant parts and pray for the best. But you can prevent late blight by spraying a fungicide every seven to 10 days on the soil (not the plant) starting in early July.

Effective fungicides for blight contain chlorothalonil, mancozeb, or maneb. Popular brands include Daconil, Bonide and Dithane. Sulfur based products will not kill the fungus. Be sure to read and understand the label of any chemical you use in the garden, and follow those instructions.

For more information, call the Haywood County Extension Center at 456-3575. © 2014 NC State University.

 

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