Saturday, 20 June 2009
Few tree diseases rival Verticillium for sheer tragic loss. Unfortunately it often appears first thing in spring and the initial indication of its presence is a dead or dying tree. This disease effects a number of tree species but is very commonly the demise of maples. When it comes to maples this disease isn't picky, it can wipe out a prize cut leaf weeping Japanese maple or a weedy box elder without warning. Ash, some linden, buckeye, and russian olive can also fall prey to this potentially fatal disease.
Verticillium dahliae is a soil born fungus that invades the trees vascular system. The disease enters the trees vascular system from the roots and moves upward in the xylem as spores. Toxins produced by the disease and the trees response to the fungus results in wilting symptoms. Very often a tree will be infected with this disease and recover, losing only a branch or two. However later on when when put under stress the tree may be overwhelmed and fail completely. The disease persists in the soil for up to ten years as highly stable fungal structures. Once an area is known to have verticillium it is recommended to plant resistant replacement plants. Generally burr oak, apples, birch trees, walnuts and most conifers are resistant to this disease.
The typical symptoms mimic other vascular fungal diseases like Dutch elm disease, flagging and wilting of leaves and branch die back. In fact the field diagnosis for verticilium is cutting a branch and looking for staining in the sap wood just under the bark. Positive identification in the field should be followed up with a confirmation by a laboratory. In the lab they will put chips of the diseased sample on a petri dish and grow it to confirm the presence of the disease. In elm trees the filed sampling can sometimes result in confusion, but verticillium generally isn't fatal in American elms. Dutch elm disease however is fatal 99% of the time in non resistant elms. Of 100 elm trees sampled and showing staining of the inner bark, less than 2 will have verticillium the remainder will have Dutch elm disease and will be dead by the time the lab results get back to you. I have sampled the same elms several years in a row, with the lab results coming back positive for verticillium consistently. Some of these trees are still standing. In contrast to elms in maples the first sign of verticillium is often slow stunted leafing out in the spring. When this overall infection is present the tree will most likely fail with in weeks and simply dry up and die.
There is a strong correlation with root disturbance and the onset of verticillium infection. It is not recommended to dig tunnel or cut roots near maple trees. Incidence of this disease is low in natural stands. Once a tree has verticillium it is impossible to eliminate it from the tree or soil. Fungicides are ineffective in treating this disease. Maintaining the vigor of the plant is the best plan of action. Fertilizing with a low nitrogen high potassium fertilizer with added beneficial bacteria and root stimulants is of benefit. Generally this should be done as a soil drench and not as a deep root injection. Root injection may result in additional root injury and should be avoided on trees susceptible to verticillium. Trees under moisture stress are more susceptible to verticillium. Wood chips from these trees should be composted and allowed to heat up before using them in landscapes. If you think your tree may have a problem contact your certified arborist.