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Growing Good Food at Home

How to Spot, Treat, and Prevent Tomato Diseases

Tomato plant afflicted with late blight

Tomato plant diseases are caused by bacteria, fungi, and viruses. The diseases can spread through the soil, water, air, infected tools, animals, and even gardeners themselves.

Spot diseases on your tomato plants sooner so you can treat plants before they are destroyed by disease and can successfully manage symptoms. Here is a three-step approach to keeping vigilant about the presence of tomato plant diseases:

  • Research the Risks. Find out about tomato diseases that are prevalent in your geographic area. When possible, buy disease-resistant tomato plant varieties to avoid diseases in the first place—many hybrid tomato varieties have been specifically designed to resist certain diseases.
  • Examine Plants Daily. Check your tomato plants every day to be sure no disease symptoms are present or emerging.
  • Spot Diseases Early. Catch and treat disease quickly so your plants can recover and thrive.

Tomato diseases can affect the leaves, stems, crowns, and fruit. Here is what to look for to spot possible infection:

Tomato leaves—possible disease symptoms:

  • dark, gray, or white spots
  • yellowed or spotted/blotched foliage
  • curling leaves

Tomato stems—possible disease symptoms:

  • softness or mushiness
  • dark, gray, or discolored streaks
  • mold or mildew
  • stunted growth

Tomato crowns—possible disease symptoms:

  • malformations at the plant crown
  • rotting roots

Tomato fruit—possible disease symptoms:

  • sunken or discolored spots
  • spotted/blotched skin
  • mold or mildew
  • misshapen or undeveloped fruit

Make your own natural disease and pest control spray with benign materials. Mix one tablespoon of baking soda, ½ teaspoon of a mild dish detergent, and 2 ½ tablespoons of olive oil in a gallon of water to make a solution that will repel all kinds of bugs, as well as a fungicide for blight and mildew on the tomato plant leaves. Shake it well in your bottle before spraying, and repeat every week for it to be continuously effective.

We’re believers in not using toxic materials in the garden—they can hurt the plants, hurt the soil, damage the environment, and harm you.

Preventive measures to avoid tomato plant diseases

Try some of these techniques for avoiding tomato diseases in the first place:

Rotate crops regularly
Many bacteria, fungi, and viruses live in the soil for years and are just waiting to prey on your tomato plants! Minimize the likelihood of these diseases when you plant tomatoes by planting no more than once every three years in the same location.

Avoid planting other crops such as potatoes, peppers, or eggplants nearby, too. As members of the same plant family as tomatoes, these crops are susceptible to the same diseases as tomatoes and having them nearby could facilitate rapid spread of any developing disease.

Improve your soil composition
Before planting your tomatoes, add a good amount of compost or organic matter to improve the soil. Extra nutrients and good aeration help grow stronger plants that will resist disease and infection.

Plant disease-resistant tomato varieties
Many hybrid tomato varieties have been developed specifically to resist particular tomato diseases. You can plant disease-resistant tomato varieties to always have the healthiest plants and harvest. Tomato disease resistant codes are listed on seed packets or seedling containers in capital letters. They include:

    • V = Verticillium Wilt
      F = Fusarium Wilt
      N = Nematodes
      A = Alternaria
      T = Tobacco Mosaic Virus
      St = Stemphylium (Gray Leaf Spot)
      TSWV = Tomato Spotted Wilt Virus

Water your garden properly
Don’t underwater or overwater your tomato plants. By keeping a regular watering schedule, you’ll keep your plants vibrant and healthy. Overwatering and watering directly on the plants—instead of the preferred watering method, directly at the stem on the soil—leads to consistently wet conditions, which allows bacteria, fungi, and viruses to thrive and multiply.

Destroy infected plants
Throw away or burn infected plants. Don’t keep infected plants over the winter in your garden, and don’t throw them on your compost pile. Disease-ridden plants, even when dead, will spread the disease to other plants or even your soil.

Have you had problems with diseases attacking your tomato plants? What types of problems do you regularly face with your tomatoes? Please tell us how you treat and prevent diseases for destroying your tomato crop.

  • Rhonda D.

    I noticed some of my tomato leaves in the inner part of the leaf is turning a pale yellow
    Am I watering to much? Does it have a disease? What can I do to fix the problem?
    Please help

    • Norann O.

      Rhonda – That is a great question, but one that has a few answers. To check the moisture level of your soil, stick your finger several inches deep in the soil near your tomatoes. If it feels dry then its time to water. If it’s moist then skip it.

      Tomatoes like quite a bit of fertilizer and if your soil’s nitrogen level is low you often see older leaves turn yellow. If the newer/younger leaves are turning yellow then your soil may be low in iron or other nutrients. But without a soil test, it’s hard to know for sure. I would start with applying a balanced fertilizer (I like 10-10-10) and see if things improve.


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