ID and Prevent 6 Common Tomato Diseases

Learn how to spot the symptoms of several tomato diseases, including early blight, late blight and nematodes, and adopt strategies to keep your plants healthy.


| February/March 2015



Preventing Spread of Tomato Diseases

Wide spacing between plants and applying thick mulch will help fend off the spread of tomato diseases. Also prune off low branches to reduce the number of early blight spores and improve air circulation around the base of your plants.


Illustration by Lyn Wellsand

Ah, tomatoes — revered by many as the quintessential home garden crop. Every gardener who grows them hankers for a hearty yield, but many may not realize that tomato diseases, which can strike quickly and be difficult to recognize, pose one of the biggest potential roadblocks to good production. Follow these instructions on how to recognize the telltale signs of common tomato diseases, and discover proactive steps you can take to help prevent pathogens from infecting your prized plants.

Early Blight

The fungus Alternaria solani, which is present worldwide wherever tomatoes or potatoes are grown, causes early blight. Prevalent throughout the United States, the fungus overwinters on tomato plant debris and perennial tomato-family weeds, including hairy nightshade, horsenettle, jimsonweed, sacred thorn-apple and silverleaf nightshade. Spores can then spread to tomatoes in spring via wind or splashing rain, but they need wet leaf surfaces to germinate and grow. Low leaves that drip with dew each morning provide perfect conditions for early blight.

In early summer, when tomato plants begin to set fruit, brown spots will develop on the lowest leaves of infected plants. As the spots expand and become more numerous, leaf tissues between the spots may turn yellow before the leaf eventually withers. Use a magnifying glass to examine spots that are about a quarter-inch across. Early blight causes spots with outer rings around a bull’s-eye center. In most cases, early blight damage will be limited to the lower third of tomato plants (where conditions are more damp), and these plants may manage to produce a good crop. In rainy years, however, early blight can take down entire plants.

If early blight is a threat in your area, rotate plants to a new patch of ground each year, and keep tomato foliage dry. Make sure plants aren’t crowded (space them far enough apart to prevent plants from touching), use stakes or cages to hold the foliage off the ground, and avoid watering plants late in the day. Apply a thick mulch to prevent mud and water from splashing onto plants, as moisture on lower leaves can activate the disease. Control tomato-family weeds that grow close to your garden, and think twice before adopting volunteer tomato plants that appear early in the season — they may be loaded with an invisible cargo of early blight spores. As soon as you see the first early blight leaf spots, use pruning shears to clip off all leaves within 12 to 18 inches of the ground, removing no more than 20 percent of the plant’s total leaf mass.

Resistant varieties are new and few, but they’re worth growing in areas where early blight is a severe problem. Try ‘Defiant,’ ‘Jasper’ or ‘Mountain Magic.’ (See Find Disease-Resistant Tomato Varieties.) Copper is an organic fungicide that helps prevent early blight, but it’s not especially safe. It’s “allowed with restrictions” by organic-certification rules. With repeated use, it can build up to levels in soil that can be toxic to earthworms.

Look-alikes: Verticillium wilt, fusarium wilt, septoria leaf spot and nutritional stress can each cause older leaves to yellow and wither, but these other problems don’t show the rings or halos around irregularly shaped leaf spots that are the signature of early blight. Late blight spots are much larger and faster to develop compared with those caused by early blight, and they are rarely limited to the bottom section of the plant.

amk1100
7/10/2015 9:09:25 AM

To save your plants from all these diseases just water them with garlic tea. Add 2 or 3 tsp. of minced garlic to one gallon of hot tap water, let it sit for 15 minutes, then water the tomato plants with it. Do this 3 times the first day, and one a day for a few more days. As the plant absorbs the garlic over the next 24 to 48 hours, the plant will get healthy again and develop its own immunity to the blight.


tammy
3/28/2015 5:33:31 AM

Hi in this article you recommend a diseases resistant tomato and rotate crops. I wonder, we use lemons to clean our kitchen and tea tree to clean our skin. Is there not a natural way to kill the bacteria in the soil. Maybe some thing that we can grow along side tomato's that may clean the soil also? or a tea we can make that may be as effective in bacteria growth in soil? Thank you






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