Methods for identifying and preventing verticillium wilt in home gardens.
A persistent, soil-borne, fungal disease, verticillium is found on many fruits and garden vegetables, especially tomatoes, potatoes, peppers and eggplant. Verticillium also attacks trees including ash, oaks, and maples as well as cherry, peach, and apricot trees. Berry canes, including raspberries and blackberries are particularly susceptible to infection, as are strawberries.
Verticillium wilt even shows up in rhubarb and mint. All in all, the six known species of the verticillium fungus combine to harm some 350 different plants.
In garden plants, first signs of verticillium are usually seen in wilting, yellowing, and curling leaves. Discolored streaks are often seen in strawberry stems and runners, and in berry canes. These plants eventually die. Plum apricot, and peach trees will show symptoms but live on, producing smaller and less numerous fruits
Verticillium wilt, much like its cousin fusarium wilt, lives in the soil. It enters plants through the roots and is drawn up to stems, leaves, and fruit through water uptake. At the same time, it robs plant of moisture. The fungus is most active in humid climates. Cool nights and moist conditions, the kind that favors peas, tend to encourage it. It’s an annual problem in the Pacific Northwest.
The fungus inhibits circulation in the host plants, clogging its vascular system, and interfering with the upward flow of water. Symptoms are easy to spot. Typically, leaves at the ends of branches turn yellow, wilt and fall before the entire limb or branch dies. Often one branch or one side of the plant is affected. New growth dies off in summer months.
Verticillium wilt overwinters in garden debris and fallen fruit, and lives in the soil (see Fall Garden Cleanup). The disease is transmitted and moved during cultivation, transplanting, or by flowing water and soil moisture. Verticillium is also transmitted from plant to plant by grafting and budding.
Verticillium is patient. Without a host, it can exist for several years in a form that arborists call “survival structures.” Once a host-plant is introduced, the mycelium multiply exponentially over a number of seasons. The infection spreads and intensifies as dead materials, say from a diseased maple or magnolia tree, fall to the surrounding ground and reinfect the soil.
Note: Verticillium fungi can survive for 10 or more years in the soil without a host plant.
- Choose resistant varieties when available.
- Remove stricken growth and sterilize pruning clippers (one part bleach to 4 parts water) between cuts. It is best to remove the entire plant and solarize the soil* before planting again in the same location.
- Verticillium is slow to claim peppers and tomatoes. Some growers, through judicious pruning of affected parts of the plants and heavy watering, attempt to force tomatoes and peppers to produce a crop, even if the results are small and less numerous. This practice can increase the chances of future infections. It’s best to discourage the disease’s spread by removing all traces of the disease as soon as it appears.
- Don’t throw wilted leaves or streaked stems — or any part of an infected plant — into the compost pile. These should go in the trash, carefully bagged so as not to spread or leave any trace. If you can burn them, do.
- Crop rotation will have limited success, as so many crops are susceptible to the disease.
- Don’t plant potatoes, eggplant, or other solacenous vegetables where any of them have grown the previous year. If you’ve had trouble with wilt, don’t plant the same ground with those plants for at least four years.
- Vegetable crops reported as resistant to verticillium include, lettuce, peas, beans and corn. Asparagus, like all grasses, is resistant. It’s a good choice for long rotation as it occupies the same space for several seasons.
- Use balanced, slow-release organic fertilizers — high nitrogen fertilizers should be avoided.
- Don’t cultivate soil deeply around susceptible plants. The process can bring mycelium to shallow rooted vegetables including lettuce and other greens.
- Refrain from using herbicides, including OMRI approved treatments, around susceptible plants. Dead and dying weeds encourage the growth of the fungus.
- Weekly sulfur applications applied once the symptoms appear may have some effect against this soil borne disease, but prevention practices will be more effective in the long run.
- Neem oil, which naturally contains sulfides, has been reported to be effective in preventing the fungus from spreading.
- Treating trees for verticillium wilt is often a matter of maintenance. Traditionally, soil fumigants were used to control verticillium mycelium. These have been shown to have little effect and, of course, they’re often toxic to other living things. Carefully removing infected limbs and keeping the tree well-watered can help. Raking up leaves and other debris immediately is important.
Tip: Actinovate Lawn & Garden contains Streptomyces lydicus, a naturally occurring soil bacterium that is found in healthy soils. When applied as a soil drench or foliar spray, it establishes itself on plants’ roots and leaves and provides protection against verticillium and other root and foliar diseases. Actinovate is OMRI Listed for use in organic gardens.
* To solarize your soil, you must leave a clear plastic tarp on the soil surface for 4-6 weeks during the hottest part of the year. Soil solarization will reduce or eliminate many soil inhabiting pests, including nematodes, fungi, insects, weeds and weed seeds.