Picture this: It’s July. The heirloom tomato plants you purchased at the Piedmont Master Gardeners’ annual plant sale have been chugging along since you planted them in mid-May, after the soil warmed up and the danger of frost was over. You’ve staked them and pruned them to two main stems. You’ve mulched them with grass clippings and watered them every morning, taking care not to splash soil onto the leaves. And then one day you head out to the garden and your plants are yellow and as limp as if they’d never been watered.
Your tomatoes have succumbed to a wilt or a root rot. The symptoms are similar: Plants are limp and wilted as though they’ve been deprived of water, no matter how moist the soil is. Leaves may be yellowed. Plants may perk up overnight and wilt again in the heat of the day. Eventually, though, the wilt will become permanent.
The underlying causes differ. Wilts clog up the plant’s vascular system – specifically the xylem, which brings water and nutrients up from the roots. Root rots, on the other hand, destroy the plant’s root system, preventing the plant from taking up water and nutrients in the first place.
Different pathogens cause wilts and root rots. Wilts can result from fungal infection, most commonly Verticillium or Fusarium. They can also result from bacterial pathogens like Ralstonia solanacearum. Tomatoes are particularly susceptible to wilts, as are other nightshades like potatoes, peppers, and eggplants. Cucurbits like cucumbers, muskmelons, pumpkins, and watermelons are also prone to wilts. Many other vegetables, including brassicas, alliums, beans, peas, beets, and asparagus are moderately susceptible to wilts. The characteristic sign of a wilt is discoloration in the plant’s vasculature, easily seen if you cut across the stem (see picture).
Root rots are caused by both fungi and related pathogens termed oomycetes, or water molds. The fungi include Rhizoctonia and Fusarium (a different variety than the one that causes vascular wilt). Water molds include Phytophthora and Pythium. Tomatoes, peppers, and eggplants are especially susceptible to root rots, as are many fruit trees. Root rots can be identified by pulling up an infected plant and washing the soil off the roots. Healthy roots are a light yellow-white color, whereas rotten root tissue is brown or black (see picture). Pythium affects the outer layers of the roots; a gentle pull will slide the damaged outer layer away from the healthy core. Phytophthora rot extends all the way through the root. Rhizoctonia and Fusarium cause a reddish-brown dry rot. Root rots may extend up into the lower stem and are sometimes termed crown-and-root rots.
Prevention
Keep root rots and wilts out of your garden. The pathogens are soil-borne and can survive in soil for many years. Once they are established, it’s virtually impossible to remove them. Plant only healthy plants in your garden. If in doubt, remove the pot and examine the roots – it’s OK to do this in a store! Wash soil off of tools before using them in your garden, especially if you’ve used them in a different location. For extra protection, sanitize tools with a 10% bleach solution after washing off the soil.
If a plant is affected by a wilt or root rot, remove it immediately. Once a plant is infected there’s no way to cure a wilt or root rot. Don’t compost it. Instead, dispose of it by burning (if it’s legal in your community) or by burying it far away from your garden. If you like, send the infected plant to Virginia Tech’s Plant Disease Clinic for professional diagnosis.
Fortunately, there are several strategies for preventing wilt and root rot pathogens from infecting garden plants:
- Don’t over-water: soil-borne pathogens thrive in wet soil. Most vegetables need about 1 inch of water per week. This works out to around 2.5 quarts per square foot, or a little less than 5 gallons in a 3-foot diameter circle.
- Promote well-drained soil. Add organic matter, like well-aged compost, to your garden. Use no-till practices to preserve the organic matter already present in the soil.
- Plant resistant varieties. In particular, there are many tomato varieties that are resistant to certain soil-borne pathogens. Check the plant’s label, or the seed catalog, for V, indicating Verticillium resistance, or F, for Fusarium. You might see one or more numbers after the F. Fusarium has 3 strains, or races, and different plant varieties have different levels of resistance to the various races. It might take some experimentation, a.k.a. trial and error, to figure out which pathogen, and which race, might be in your soil.
- Graft susceptible plants onto resistant rootstocks. Again, this is mostly applicable to tomatoes. Rootstock varieties are resistant to soil-dwelling pathogens but produce tasteless tomatoes. It’s quite feasible for home gardeners to graft tomatoes, and the only specialized equipment you’ll need are grafting clips. Purdue offers a fantastic written guide and video explaining the process.
Tomato grafting. Image (cropped): Cary Rivard. CC BY-SA 3.0 - Tidy up your garden at the end of the season. Scrupulously remove all plant debris from the garden, even if the plants didn’t suffer from wilts or root rots. Pull up all stakes, cages, etc., hose them off, and store in a dry place.
- Rotate crops. Don’t grow plants susceptible to these problems – especially nightshades – where other susceptible crops were grown the previous year. Skip two years if possible.
Treatment
There are a few methods for killing off soil-borne pathogens, or at least reducing levels enough to help keep your vegetables healthy. Their main downside is that these methods are labor-intensive, and success is not guaranteed. The prevention strategies discussed above are more likely to keep wilts and root rots at bay. Nevertheless, here are a couple of techniques to consider in combination with preventative measures:
- Solarization involves laying out a clear (usually) plastic sheet for several weeks. This traps solar radiation and heats the soil enough to kill microbes – as well as insects and weed seeds. One obvious downside is that the heat will also kill beneficial bacterial and fungi. (Earthworms are mobile enough to find cooler soil.) Proponents argue that since beneficial soil microbes are present at much higher levels than pathogens, they can re-establish their populations very quickly.
- Phosphorous acid fungicides are effective against oomycetes. Typically they’re applied as soil drenches, and require multiple applications. These fungicides are a form of phosphorus – note the extra “o” in “phosphorous” – that has no fertilizer effect and doesn’t cause algal blooms. Fruit trees and perennials like asparagus might benefit from fungicides because they don’t lend themselves to the sanitation and rotation methods mentioned above. (Always use personal protective equipment when applying fungicides, and follow all directions on the product label.)
Closing thoughts
As if it isn’t hard enough to keep your vegetables safe from soil-borne diseases, many landscape plants are also susceptible to wilts and root rots caused by the very same pathogens. Most notoriously, sudden oak death is caused by a type of Phytophthora. My rhododendrons are also struggling with Phytophthora, and they may or may not be responding to the phosphorous acid soil drenches I’ve been applying recently. Symptoms and prevention strategies are the same as for garden plants.
References and further reading
Featured image (cropped): Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org. CC BY-NC 3.0
Are phosphorous and phosphoric acids equal phosphorous sources for plant growth? University of Florida
Drying up root and crown rot pathogens Clemson Cooperative Extension
Fusarium crown and root rot University of Minnesota Extension
Fusarium wilt University of California Agriculture and Natural Resources
Fusarium wilt University of Minnesota Extension
Growing VFN resistant tomatoes (PDF) UC Master Gardeners of Tulare and King Counties
Plant disease: Phytophthora root and stem rot Nebraska Institute of Agriculture and Natural Resources
Phytophthora root and crown rot in the garden University of California Agriculture and Natural Resources
Pythium root rot of garden plants University of Illinois Extension
Root rots: can you tell the difference? Michigan State University Extension
Root rots in the garden University of Wisconsin
Root rot of herbaceous plants University of Illinois Extension
Soil solarization for gardens and landscapes University of California Agriculture and Natural Resources
Tomato wilt problems (document SP-370C; PDF) University of Tennessee
Verticillium wilt of vegetables University of Wisconsin
