It’s hard to miss this disease once we enter the hot dry months of summer. Without fail, each July/August I start to see trees turning brown and wilting suddenly, usually in patches within the canopy that can be traced back to entire individual limbs that have died. More dramatically, an entire tree will just turn brown and wilt. The pattern of wilting is very regular, and the symptoms can carry over into winter because the wilted leaves sometimes stay on the tree well after normal leaf drop in the fall.
Verticillium Wilt (VW) is easily recognizable once you know what to look for, but not easily treated. To make matters worse, it affects a large number of trees and plants, with varying expression of symptoms from tree to tree. During my years as an arborist, I have found Norway Maple, Japanese Maple, Yellowwood, Smoke tree, Redbud and Magnolia to be very common hosts of this disease. And this is a short, short list of the plants that can be affected.
The disease is caused by a fungus present within the soil, and there are so many ways it can get into the soil that my approach is usually “if there is soil present, there is probably VW present”. What do we do? It sounds like a copout, but the best answer for dealing with this issue is the best answer for dealing with most plant pests and diseases: Choose the right plant for the site conditions. This is the best way to cultivate healthy plants that can naturally cope with the more problematic neighbors within an ecosystem. But this doesn’t really help the tree that has already been planted and is making do with what it has.
If the tree has already been planted, the next best option is to do everything possible to manipulate conditions at the planting site to minimize stress factors to the tree. Ensuring proper moisture, adding organic mulch matter, amending soil both chemically (fertilization if needed) and physically (soil aeration/relief of compaction) are all good measures that can be taken. There are systemic fungicides that can be applied to suppress the development of the disease within the vascular system of the plant as well, but these applications should be secondary to the site work.
The fungus can enter the tree by wounds in roots but can enter roots even when no wounds are present. Once in the tree, the disease tends to progress upward and outward, causing foliar wilting and dieback to varying extent. I have seen trees coexist with the disease for years and years, seemingly keeping up with the disease, losing a branch here and there, but generally coping with it. On the other hand, I have seen trees decline quickly. I recall one Sugar Maple which barely had any symptoms until it wilted from head to toe and died over the weekend.
In closing, I feel that VW is an example of how trees really die. There are questions that need to be asked, such as “Was it just a super-virulent strain that entered the tree?” or “Was it a weaker strain, but the tree was just weakened by multiple other factors?”. Sure, there may be cases where everything is perfect for the tree, but the disease was like The Terminator - unstoppable. In my experience though, most trees die because something else has made them vulnerable and weak enough that a relatively low-grade disease is what finishes them off. In that case, treating the disease is a last-ditch effort that can only really work if the treatment keeps the tree alive long enough to benefit from the effects of cultural changes that should be implemented if possible.
I think there is a life lesson that can be applied here, but I will let the reader sort that out. Have fun out there! For every wilted leaf, there are 100 other green ones.
Your friendly neighborhood arborist,
ISA BCMA® OH-5129B