Understanding and Controlling Fungal Disease on Tomatoes

fungal tomato disease

Tomatoes are prime targets for soil-borne fungal diseases which lead to defoliation, reduced yields and worse. About 85% of all plant diseases are caused by fungi, tiny multi-celled organisms that have no chlorophyll.

Typically all fungi have the same life cycle. They live and eat off of a host plant – a tomato! They are microscopic and can be spread by wind, splashing water (from irrigation or rainfall) or by mechanical means. Many plant infections are caused by touching an infected plant and then touching a healthy plant with hands or with gardening tools. Fungi can also overwinter in plant debris waiting for their next tomato meal.

Fungicides can help if applied early enough and thoroughly. They won’t kill the fungus but they will prevent the spreading of the disease which is certain to occur. Be sure to follow product instructions. The best control for all fungal disease is good cultural practice.

Here are some very important cultural practices to reduce tomato disease in your garden:

  1. Remove old plant debris. Many common weeds can host fungal disease. Do not compost this material or any plants that have any signs of disease unless you are completely confident that your pile gets hot enough to kill the spores. During the growing season spores from infected plants can splash onto you new tomato plants and the disease cycle will continue.

  2. At the end of the growing season, all tomato refuse should be discarded.

  3. Rotate your tomatoes to a different location each year. Remember that nightshades (tomatoes, eggplant, peppers) and potatoes all have similar disease problems so rotate all to new locations. A minimum of 3 years is recommended to keep the soil-born fungi at bay.

  4. Alter the microclimate. Grow in full sun. Stake or cage to keep leaves off the ground. Remove any leaves that touch the ground and as you harvest your tomatoes, trim the leaves from the ground up. Fungi thrive in damp conditions so get as much air circulation as possible particularly if in a humid climates.

  5. Water only in the morning and try not to get the leaves on the plants wet. Don’t let water from the soil splash onto the plants. Soaker hoses are particularly desirable for tomatoes.

  6. Mulch thickly (6 inches) but pull mulch slightly away from the tomato stems. In addition to preventing soil-borne disease, the thick mulch will also discourage weeds and keep the soil moist.

  7. Monitor and control pests on plants because sucking or chewing inscts provide open wounds which invite fungal disease.

  8. Plant on well drained soil. Fungal disease thrives on moisture so good drainage is critical.

  9. Avoid high nitrogen fertilizers. They cause the plant to have many leaves which reduces air circulation. Tomatoes prefer calcium and phosphorus fertilizers.

  10. Plant at the right time of the year for your area. Tomatoes grow best when daytime temperatures are 70-80 degrees and nighttime temperatures are 60-70 degrees. So Southern tomato growers are planting in March and far North growers may be planting in May.

  11. Clean garden tools after use. Just one swipe with an infected tool can spread fungal disease. After removing dirt, soak your tools for at least a minute in a disinfectant solution. 1-5 chlorine bleach was always the standard recommendation although prolonged use could pit or mar your tools. Disposable bleach free disinfectant wipes are hand for smaller tools. Other options include Lysol, Pinesol, rubbing alcohol or hydrogen peroxide.

  12. Buy tomato plants that are disease resistant.

    Disease Resistance Codes:
    • V = Verticillium Wilt
    • N = Nematodes
    • F = Fusarium Wilt
    • T = Tobacco Mosaic Virus
    • A = Alternia Stem Canker
    • St = Stemphylium Gray Leaf Spot
    • TSWV = Tomato Spotted Wilt Virus
Comments from Other Gardeners
  • Nick says:

    I would add only one thing to this list. When you plant you tomatoes, space them far enough apart for good air circulation. Understand the needs of the variety you are planting because some varieties need much more space while some can be relatively compact. Wild Fred is a newer variety I grew last year. It’s an heirloom and grows on a sturdy stalk and produces a nice compact bushy plant with delicious purplish tomatoes.

  • Steven Papendick says:

    Haveing serious problems with my tomatos heirloom and hybrids alike their curling upward then have purplish stems and leafs on the back the they are yellowing and getting brown spots thennit eventually effects the whole plant then it dies frit is irregular and gets small brown spots on it some is still edible but I lost 40 out of 50 plants please help sprayed fungicides but seem to not help much asap response

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