(News article for June 10, 2019)
During the late spring and summer, we often get calls about wilting tomatoes. There are a quite a few possible causes of this but, other than lack or excess of water, the diseases southern bacterial wilt and southern blight are probably the most common reasons that tomato plants wilt in our area.
Southern bacterial wilt affects tomatoes and other plants in the same family, such as potatoes and eggplants. Bacteria and the slime they produce clog the water-conducting tissue of the plant and cause it to wilt. A high level of soil moisture and hot temperatures favor this disease, so our conditions are often conducive for it. If a plant has bacterial wilt and you cut through the stem at the base, you may be able to see a brown color on the inside of the stem. While a lab test can confirm the presence of the disease, we can often be fairly sure that bacterial wilt is the cause of plant wilting by doing a bacterial streaming test. This involves cutting the stem at the base and suspending the stem in water. If the plant has bacterial wilt, a cloudy substance can often be seen streaming from the stem within several minutes.
Another disease that often causes wilting of tomato plants is southern blight. When this disease is present, white fungal growth and mustard seed-sized fungal structures called sclerotia can often be seen at the base of the stem, near the soil line. As with bacterial wilt, tomato is not the only host of this disease. Southern blight can affect many other types vegetable plants, including peppers, green beans, and watermelons.
In the case of both bacterial wilt and southern blight, unfortunately, there isn’t much that can be done for the current crop of tomatoes once you have the disease. For the current season, make sure that you don’t spread soil from around the base of the diseased plants to the soil around healthy plants. Tools, shoes, and hands can spread infested soil, so be sure to clean these before working with healthy plants.
In the future, rotate where you plant tomatoes and other plants susceptible to the disease(s) that you’ve had. The pathogens that cause bacterial wilt and southern blight can survive in the soil for multiple years, and the longer you wait before replanting in a particular area, the less likely plants are to be infected. Solarizing soil using clear plastic is one option for reducing the pathogen population in the soil. Soil solarization must be done properly to be effective at killing disease-causing organisms. It’s more involved than just laying clear plastic on the surface of the soil. If you want more information on how to do this, let me know.
Since a high level of soil moisture is conducive to these diseases, raised beds may also help you avoid problems with them. Finally, if you haven’t had your soil tested to determine whether the pH (acidity or alkalinity) needs to be adjusted for optimal vegetable growth, management of these diseases is another reason to do so. Low soil pH favors both of these diseases.
Those who have grown tomatoes for a long time are probably aware that leaf spot diseases like early blight and bacterial spot are common here. To be clear, this article is about reasons that the whole plant wilts rather than the various fungal and bacterial leaf spots that can cause leaves to die and, over time, give plants a wilted appearance. That’s a topic for another week!
Fungal growth (mycelia and sclerotia) of the southern blight pathogen, Sclerotium rolfsii, at the base of a tomato plant. (Photo by Mary Helen Ferguson)