How to grow tomatoes when your soil harbors Fusarium or Verticillium wilt fungus

There is no tomato as delicious as a freshly picked, vine-ripened tomato from the backyard. But the pleasure of growing your own tomatoes may be mitigated when your tomato plants are infected with Fusarium or Verticillium wilt.

There are many diseases of tomatoes, such as early blight, leaf spot and anthracnose. But the wilt diseases, Fusarium and Verticillium wilt may be the most devastating. Both wilts have similar symptoms. At first the lower leaves wilt, turn yellow and die. In many cases, a single shoot or one side of the tomato plant is affected before the whole plant wilts. Symptoms on one side only, indicate Fusarium wilt. To distinguish between the two wilts, the stems of the plants should be sliced open. In plants affected by Fusarium wilt, dark chocolate-brown streaks run lengthwise through the stem. Verticillium wilt causes much lighter discoloration of the stem. Fusarium wilt generally occurs in midsummer when soil temperatures are high, whereas Verticillium wilt, usually, does not kill plants, but stunts and weakens them and reduces the size of the tomatoes. Fusarium wilt is more likely to kill the whole plant.

Fusarium and Verticillium wilt are caused by soil-borne fungi. The Verticillium fungus likes cool soils and attacks over 200 plant species, whereas the Fusarium fungus is more specific for tomatoes and likes soil temperatures of 78-90 degrees Fahrenheit. Both fungi invade the roots through wounds caused by cultivation, secondary root formation or soil nematode feeding. Once in the plant tissues, the fungus produces toxins and invades the xylum (water-conducting tissue). The spores of the fungus lodge in the vascular tissue and produce a new infection. The tomato plant responds to the invasion of the fungus by producing substances that shut down infected vascular tissue and reduce the flow of water and nutrients from the roots upwards. Although the purpose is to limit the infection by the fungus, this response by the tomato plant is devastating. It prevents water and nutrients to reach the upper part of the plant and so the plant starves. Although it is very difficult to rid your soil of Fusarium and Verticillium wilts, there are measures you can take.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• FV resistant tomato hybrids
• Fertilizer with high potassium content and low nitrogen (only as nitrate)
• pH meter (or send soil sample out for pH test)
• Solarization materials
• containers or raised planting beds

1. Diagnose the problem. If your tomato plants wilt a short time after watering or rainfall, you probably have Fusarium or Verticillium wilt. But make sure you don't plant tomatoes near Black Walnut or Butternut trees, because their roots release a toxin that will also cause your tomato plants to wilt.

2. Plant tomato plants resistant to Fusarium and Verticillium wilt. These hybrid tomatoes are available at garden centers and through garden catalogs. A "V" after the hybrid's name means it is resistant to Verticillium wilt and an "F" means resistance to Fusarium.

3. Use fertilizer low in nitrogen. Nitrogen should be in the nitrate form, not as ammonia nitrogen. High soil nitrogen and low potassium make your plants more susceptible to the fungus. Test your soil or have a sample tested at your extension center for nitrogen, potassium and pH. Adjust your soil to a pH of 6.5 to 7.0 to retard disease development.

4. Pick up leaf litter and remove diseased plants. You can avoid spreading the fungus by cleaning your tools.

5. Solarize your soil to kill the fungus. This will only work if you have hot and sunny summers. It involves covering an empty and moistened garden bed with heavy transparent plastic sheeting and then "baking" the soil for 4-6 weeks during the hottest part of the summer. You should then be ready to plant next season.

6. Grow tomato plants in containers or raised beds with uncontaminated soil. This is a good solution if all else fails or you want to grow tomatoes that are susceptible to Fusarium and Verticillium wilt.

Tips & Warnings

• When you solarize your soil, make sure all edges of the plastic are covered with soil so that the soil does not dry out.
• Solarization may also kill some beneficial organisms in the soil.