Question: My tomato plants are wilting and the problem seems to be getting worse. Do you think it is a root rot or a wilt disease? H.W.
Answer: Root rot is a possibility after all the rain we’ve had. Gently dig in the soil to see if the roots look healthy.
Since the introduction of verticillium and fusarium resistant tomato varieties, wilt diseases in tomatoes are fairly rare.
The symptoms of verticillium and fusarium wilts are very similar. Older leaves turn yellow, wilt and may fall off. The wilt fungi plug the vascular system of the plant causing stunting and leaf loss. Any fruits will be small. If you slit the stem line of an infected plant near the soil surface, you should see tan streaking between the center pith and the outer stem surface.
Although caused by different fungi, both wilt diseases are soil-borne and survive in the soil for years. The tomato fusarium fungus only infects tomatoes but verticillium can infect over 200 different plants. Laboratory testing is required for definitive diagnosis. For more information check University of Wisconsin Plant Disease Diagnostic Clinic (http://pddc.wisc.edu) website. Remove and destroy all parts of infected plants. In future grow resistant varieties that are designated VF or VFN.
In my experience the most common cause of tomato wilt is black walnut toxicity. Tomatoes are extremely sensitive to juglone, a toxin produced by black walnut trees. Symptoms are identical but stems lack vascular streaking. Tomatoes anywhere near the roots, leaves or nuts of a black walnut will be affected.
Tomatoes are extremely sensitive to juglone, the chemical produced by black walnut trees. All parts of black walnut trees including nuts, shells, leaves and roots contain juglone. Because the root system of trees can extend two to three times the height of the tree, you may have walnut roots near your tomatoes without having a large tree close by. Even the roots of young sapling trees will create growing problems in highly sensitive plants like tomatoes.
Some gardeners try to rid the soil of nuts and roots but it can take months to years for soil microorganisms to break down plant juglone. Take care not to use black walnut leaves as mulch or in compost. Next year plant tomatoes in another part of your yard, in containers, or in raised beds.
Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email Larson at firstname.lastname@example.org or call her at 262-857-1942.