Identifying and Treating Tomato Plant Diseases

Protect Your Tomato Crop

Tomato blight
Photo courtesy of Tony Austin

While it's very possible to produce a huge crop of delicious tomatoes, it's also likely that at least a few of your plants will fall victim to one or another of the diseases that plague these popular plants. Fortunately, there are steps you can take to prevent and treat most of these problems.

Tomato Diseases to Treat

Several kinds of diseases can plague tomato plants. If you keep a close eye on your plants' leaf health, watering status, and growth patterns, there's a good chance you'll be able to catch the disease early to treat or eradicate it.

  • Blight: Two fungal diseases are known as blight: Alternaria solani or early blight, and Phytophthora infestans or late blight. Early blight begins earlier in the season and creates target-shaped ring spots on the leaves, usually on the lower portion of the plant first. Late blight causes irregular blotchy spots on the leaves and fruit. Controlling the environment to prevent excessive heat, moisture, and crowding helps to inhibit fungal growth. Remove affected leaves and adjust care as necessary.
  • Leaf Spots: You’ll notice leaf spots in the center of leaves, a black or gray spot with a light center. Once the spots settle in, the leaf will turn yellow, then brown, and wither away. Leaf spots are caused by a fungus, encouraged by excess heat and moisture, so removing the affected leaves and adjusting the environment will help to stop the spread.
  • Bacterial Infection: Exposed to the plant through a cut or damage in the vine or plant, bacterial diseases can wreak havoc on your tomato plants. If your plants are experiencing bacterial infection, you'll notice spots and blotches. To treat the problem, remove the infected areas as soon as you see them.
  • Mosaic Viruses: Because tomatoes are in the same plant family as tobacco (nightshades), tobacco users can transmit a mosaic virus to their tomato plants simply by touching them. While mosaic viruses won’t kill your plant, they will weaken them and reduce your crop, which is almost as bad. You can spot a mosaic virus by the mottled coloring on the leaves or fruit, with raised almost blister-like spots. Don’t allow smoking near your garden, and wash your hands or glove them before tending tomatoes if you are a smoker.
  • Verticulum Wilt: Sneaky and devastating, tomato wilt begins with sad, wilted looking leaves in the heat of the day that perk up later on, but then progresses to complete wilting and loss of the plant. It is caused by fungi that contaminate the root of the plant and block water and nutrients. There is no way to treat it, so when a plant dies from verticulum wilt, remove it completely and destroy it.

Preventing Tomato Disease

There is no better way to protect your plant's health than to prevent disease before it starts. When we take the time to plant carefully and tend properly from the get-go, we can—in most cases—avoid the headache of disease in the first place.

To prevent fungal diseases in tomatoes, plant tomatoes with appropriate spacing so that they don’t crowd each other and trap heat and moisture on the leaves. Watering the plants' roots with a soaker hose will keep water in the soil rather than catching on the leaves from above. Keep branches off of the ground, where they would sit in moisture and breed disease.

Bacteria can enter from cuts and damage to the plant and fruits, so be careful when you support tomato plants. Tying them up increases the odds of a cut in the plant, so use cages to support your tomato plants. Alternatively, use soft fabric strips or pantyhose if you must support them with ties.

Finally, when you’re planting, start with a good foundation. Test soil to make sure it is healthy and has a good pH level. Organic soil amendments can go on in the fall and winter to ready it for spring planting.

With the soil ready, choose plants that are disease-resistant—the acronym "VFNT" on a tomato seed or plant label indicates its resistance to verticillium wilt, fusarium wilt, nematodes, and tobacco mosaic virus. Make sure you rotate the crops out by family—potatoes, tomatoes, peppers, and eggplants are all in the same family and will share diseases.

An ounce of prevention is, in this case, worth pounds of fungicides and rotten tomatoes! Start well at the beginning of the season, and you’ll harvest plenty of delicious, organic tomatoes later on.

Article Sources
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  1. Tomato Blight. Purdue University Department of Agriculture

  2. Tomato Leaf Spot Diseases. University of Minnesota Extension

  3. Verticulum Wilt. University of California Davis

  4. Quick Guide to Insects and Diseases of Tomatoes. University of Minnesota Extension