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 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 can 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. Dozens of tomato varieties are available today with new hybrids being introduced every year. Many of these are developed with resistance to certain tomato diseases. Whether you are purchasing plants or starting your own from seed, when it's time to put them in the garden, choose the strongest, healthiest plants available.

To prevent fungal diseases in tomatoes, give them plenty of growing space. Crowding your plants will trap heat and moisture on the leaves and vines and that can result in problems. Watering the plants' roots with a soaker hose will keep water in the soil rather than catching on the leaves from above. Overhead watering also can cause soil born bacteria to splash up onto your vines. Keep them off of the ground, where they would sit in moisture and breed disease.

Your tomato plants will benefit from suckering in the early stages of growth. This is the process of pinching out the new vines that start up in the "V" where two older vines meet. Vines, in general, grow prolifically and by reducing the number of vines growing will strenghthen the entire plant and encourage good fruit set. It is important to not remove any sucker that appears just below a flower. Doing so can cause the plant to "top out" and stop growing.

There may very well be as many staking systems out there as there are tomato growers. A good support system is essential to a good tomato harvest. Separation of the main vines of the plant gives the entire plant good air circulation. Use care when attaching vines to stakes, fencing, and other types of supports. Soft materials work best as ties and the knot side should always lie against the support, not the actual vine. This will prevent accidental damage to the plant which can allow bacteria in.

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. Fertilizers can be added to the soil in the planting hole as you set your tomatoes out. Formulas created specifically for tomatoes are available and can work to establish a strong root system. Always read labels for correct amounts and application. Keep the base of your plants weed free with careful hoeing or mulch.

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. Dig a fairly deep hole for each plant and remove the seed leaves and any lower leaves before placing the lower third of the plant in the hole. Avoid planting tomatoes where other nightshade plants, potatoes, eggplant, and peppers, were grown the previous year. Companion plant calendula which will attract insects away from your tomatoes.

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.

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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