How to Identify, Treat, and Prevent Tomato Diseases

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Updated 6.16.24

Tomatoes are fun to grow, but they are susceptible to many infections. Tomato diseases are caused by bacteria, fungi, and viruses. They spread through the soil, water supply, air, infected tools, animals, and gardeners.

Fortunately most are usually not fatal. The key is to identify a disease early so you can treat it before it overtakes the plant and then manage symptoms successfully. Take these preemptive steps to handling diseases.

  • Anticipate. Find out about tomato diseases common to your area. Talk with local nursery staff and ask for information from representatives at your region’s extension service. If tomatoes in your area are prone to a certain disease, plant resistant varieties.
  • Monitor. Check your plants each day for symptoms.
  • Identify. Study symptoms and treat diseases quickly so your plants can recover and flourish.

What to look for

Diseases affect four areas on the plant: leaves, stems, the crown and fruit. Look for these symptoms to indicate possible infection.

Problems on leaves

  • dark, gray, or white spots
  • yellowed or mottled foliage
  • curled leaves

Problems on stems

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

Problems at the crown

  • formations at the plant crown
  • rotting roots

Problems on fruit

  • sunken or discolored spots
  • mottled skin
  • mold
  • misshapen or undeveloped fruit

How to treat tomato diseases

Note symptoms. Diagnose your tomato plant’s problem by comparing its symptoms with descriptions of specific diseases. Treat according to recommendation for the specific disease. See list of diseases below to help with your diagnosis!

Ways to prevent tomato diseases

Take steps to keep diseases away or reduce their severity.

Rotate crops
Many bacteria, fungi, and viruses can survive years in the soil. Reduce incidence when you plant tomatoes no more than once every three years in the same spot. Avoid planting other Solanaceous crops (members of the same family as tomatoes, including potato, pepper, and eggplant) in the same area, too – they are susceptible to many of the same diseases as tomatoes.

Improve soil
Before planting, add a good amount of compost or organic matter to the home garden to improve the soil. Extra nutrients and aeration grow stronger plants which can resist infection.

Plant disease-resistant tomato varieties
Many hybrid tomato varieties are bred specifically to resist particular diseases. Plant disease-resistant varieties to have the healthiest crop possible. Tomato disease-resistant codes are listed on seed or seedling packets 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

Don’t under-water or over-water
Keep tomatoes on a regular watering schedule to help them stay healthy and strong. Avoid over-watering which can lead to consistently wet conditions – the perfect environment for many bacteria, fungi, and viruses to multiply.

Destroy infected plants
If you allow infected plants to over-winter in the garden or if you throw them on the compost pile, there’s a good chance disease will spread to other plants or multiply in the soil. Throw away or burn infected plants.

Solarize the soil
Bacteria, fungi, and viruses can live for several years in the home garden. Treat infected soil to destroy as many of them as possible. One way is to make your own solarizing tarp. The tarp spread out over the selected garden plot collects heat from the summer sun, raising soil temperature high enough to kill disease-spreading organisms. It is most effective when put into place during the hottest part of the summer.

To make and set up a solarizing tarp, first till the selected garden area. Dig a low trench around its circumference. Soak the area thoroughly with a sprinkler. Cover the area with a sheet of clear plastic 1-4 mm thick, allowing plastic edges to extend into perimeter trench. Back fill the trench to secure plastic. Leave the plastic in place for 6-8 weeks, making sure to remove debris and excess water that pools on top so that the sun’s rays can penetrate the tarp. The sun heats to soil to destroy bacteria and other organisms. Solarizing infected soil can make it usable for the next season’s crop even though it means the selected area cannot be cultivated during the peak of the current summer.

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