By Brian Hudelson, Plant Disease Diagnostic Clinic
I have recently received a slew of questions about wilted tomatoes in home gardens. Here are the top five reasons that tomatoes can wilt based on samples that I have received in my clinic over the years.
Walnut toxicity
One of the lessons that I have learned after doing plant disease diagnostics for over 20 years is that when a home gardener consults me about wilting tomatoes, the first question I should ask is, “Do you have a walnut tree near your vegetable garden?” More times than not, the answer is “Yes” and the walnut tree is the cause of the problem. Black walnuts produce toxins (exuded by roots and produced in leaves and fruits) that adversely affect a wide range of plants.
Tomatoes are particularly sensitive and are often die from the exposure. Anytime that tomatoes are grown in the root zone of a walnut tree (which extends three to five times the height of the tree from the trunk), problems can arise. Cutting down walnut trees will not solve the problem in the short term, because roots from the cut tree can continue to exude toxins for 15 to 20 years. Often the best recourse when walnut trees are present in a landscape is to grow tomatoes in raised beds or in pots to keep tomato roots as far above walnut roots as possible.
Drought stress
Lack of rain has been a potential cause for wilting in tomatoes and virtually every other plant. Most established plants require about one inch of water per week. When rain is insufficient (as it has been in much of Wisconsin this year), it’s important to apply supplemental water to plants with a soaker or drip hose. Proper watering can not only prevent wilting in tomatoes, but it can also help improve calcium uptake and reduce problems with blossom end rot. Using an inch or two of a high quality mulch (my favorites are shredded oak bark mulch and red cedar mulch) around plants can help retain moisture and lessen wilting issues. Mulching around tomatoes also helps reduce movement of spores (produced in bits of old tomato debris in the soil) of the fungi that cause Septoria leaf spot and early blight.
Bacterial canker
The bacterium that causes this disease (Clavibacter michiganensis subsp. michiganensis – THERE’S a mouthful) is seedborne, so gardeners typically introduce this pathogen into their gardens on contaminated tomato seeds or transplants. Plants initially look healthy, but the bacterium eventualy colonizes, discolors and disrupts the water-conducting (vascular) tissue inside the plant, leading to wilting. Infections can lead to long, somewhat subtle cracks in stems and ultimately less subtle open wounds (i.e., cankers) in stems near the soil line. Another telltale symptom of the disease can be ghostly-white spots with a darker center (called bird’s-eye spots) on tomato fruits. Removal and destruction of infected plants, and rotation away from susceptible vegetables (e.g., tomatoes and peppers) for several years in the affected area of a garden are typical management strategies.
Verticillium wilt
Many gardeners are familiar with this disease in the context of the death and destruction it brings to woody trees and shrubs. However, Verticillium, the cause of Verticillium wilt, is an equal opportunity destroyer and can kill a wide range of herbaceous plants as well, including popular vegetables such as solanacoues crops (e.g., tomato, potato, eggplant, pepper) and vine crops (e.g., cucumber, squash, pumpkin). This fungus is routinely found in the soil and can build up over time if susceptible vegetable crops are grown over and over again in an area where the fungus is located.
Verticillium infects through the roots and colonizes and plugs a tomato’s (or other plant’s) water-conducting tissue, leading to wilting. Discoloration of a tomato plant’s vascular tissue is a typical symptom of this disease, but stem cracks and cankers are not. Rotation can be useful as a control strategy for Verticillium wilt, although it is less effective than for bacterial canker because of the wider host range for Verticillium (including many weeds). For tomatoes, use of resistant varieties can also be useful. To identify resistant varieties, look for a “V” after the variety name on a tomato seed packet or in the variety description in your favorite seed catalog.
Fusarium wilt
This disease is very similar to Verticillium wilt except for the fungus involved. For Fusarium wilt, Fusarium oxysporum f. sp. lycopersici is the culprit. Fusarium oxysporum is a large fungal species with many special forms (that’s what “f. sp.” stands for), each one adapted to infect a specific host plant or a very small range of host plants (e.g., vine crops). Fusarium oxysporum f. sp. lycopersici is specific to tomatoes and will not infect other vegetable crops. If you ever encounter this disease, rotation away from tomatoes in the affected area for several years should work well as a management strategy. In addition, you can use resistant tomato varieties. Look for one or more ‘F’s” after the variety name.
Need help or more information?
As you can imagine, figuring out the exact reason your tomatoes are wilting can be challenging, particularly if there is disease involved. For help with proper diagnosis of tomato wilts (and other plant problems in general), contact the PDDC at pddc@wisc.edu or (608) 262-2863.
To find out more about the clinic and its activities, check out the PDDC website. To keep up-to-date about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu and ask to be added to the PDDC’s listserv (UWPDDCLearn).
