Seeing the grounds crew here at Rose Villa take out several Japanese Maples got me to thinking about Verticilium wilt. We had talked about this soil disease during the Master Gardener class I took this last winter. And I have to report that the instructor did not have many encouraging words to say about this disease.
Verticilium wilt is a fungus disease that lives in the soil and enters the plant through the root system. Japanese maples are particularly susceptible. Often the first sign anything is wrong is when one main branch starts to wilt, for all the world it looks like lack of water is the problem and in fact that is true. The fungus has invaded the xylem cells in the cambian layer and in reaction to this the plant produces toxins called tyloses or gums that close off the invaded cells in an attempt to isolate the fungus. This closing off of xylem cells prevents water from being transported to the leaves. Generally only one branch at a time is affected, although once the fungus gets into the tree’s root system the entire tree will die. Depending on the individual tree it may take many years for the entire tree to die or the tree may die quite quickly. I’ve seen it go both ways.
Verticilium wilt fungus can live in the soil without a host for up to 20 years so you can see that it can be a very serious problem. So what can be done? Controlling the disease with a fungicide spray on the leaves is not effective and in order to effectively use a soil fumigant you will have to remove the tree. And even then the fungus would simply move in from the surrounding area very quickly.
The only real solution is “one cut pruning”, which means to cut the affected tree down to the ground and plant something else that is resistant to Verticilium wilt, there are many trees to chose from; apple, birch, dogwood, honey locust, larch, oak, pine, spruce and walnut to name a few.
Verticilium wilt does not just attack trees; ornamental shrubs and even vegetables are also affected. Dow n in the vegetable garden here at Rose Villa last season I saw many examples of plants in the Solanum or nightshade family being attacked by Verticilium wilt. The nightshade family includes tomatoes, potatoes and eggplant. Virtually all the potato patches were showing signs of infection by harvest time and maybe half the tomato plants. The eggplants, what few there were of them were the healthiest of the Soanum family throughout the growing season.
The instructor at the Master Gardener class commented that he knew of no potato variety that was resistant to Verticilium wilt; but that there were a number of tomato varieties that were resistant. He said to look for the letter V. (or F. for Fusarium wilt, a related soil-borne disease) on the plant tag or the seed packet. That would indicate a plant that was resistant to the disease.