My mind and memory were put to the test recently when Robert's dad asked me about oak wilt. As I mentally researched, I realized it had been years since the topic had come up in conversation.
In fact, the last time I recall discussing oak wilt in any depth was in the late-70s. I remember then how a major outbreak of oak wilt followed on the devastating path of Dutch Elm disease, which had wreaked havoc along city streets, parks and pathways across Minnesota.
At that time I was pursuing a career in horticulture and landscape design and tree diseases were hot talk in the field. The presence of these two deadly afflictions changed the way we looked at pruning and planting.
I recall more about Dutch Elm because there was such a concerted effort to cut down infected trees and to destroy the bark, where the vector bark beetles lived and overwintered. The felling of the elm's graceful arching arms that framed and shaded so many lovely boulevards, such as St. Paul's Summit and Lexington Avenues, was indeed a horrific sight.
Oak wilt occurred more in the wild and, therefore, received less notice and media attention. Clearly, it was not on the tip of the tongue of the average Minnesota resident. So when I was queried about a recent diagnosis of oak wilt, I really had to think.
Other than knowing it was a fungal infection and that red oaks were more susceptible than white oaks, I realized I needed to brush up on oak wilt. In checking one of my old texts on tree diseases, I was surprised to find it yielded such little information.
My research continued and produced the following data from two sources, TreeHelp.com and Forest Insect & Disease Leaflet 29 from the U.S. Department of Agriculture Forest Service. The material was so well organized and presented much of what follows is verbatim.
What Causes Oak Wilt?
Oak wilt is caused by a fungus (Ceratocystis fagacearum) that clogs the vascular system of oak trees, thereby preventing the flow of water and nutrients. Once infected, the entire tree literally wilts and dies and thus the name oak wilt.
What Trees are at Risk?
No species of oak is known to be immune to this vascular disease. Infections have been found in 16 native oak species, including most of those of commercial importance. Species of red oak get the disease more frequently and succumb more readily than white oak.
Inoculation experiments have demonstrated that more than 35 native and exotic oaks are susceptible, as well as American and European chestnuts, species of chinquapin (chinkapin), tanoak, as well as several varieties of apple.
In red oaks, oak wilt is almost always lethal and death can occur in as little as one month. There is currently no known cure and the best way of dealing with oak wilt is to isolate and then destroy the affected trees. This disease has been discovered in 21 eastern U.S. states with the heaviest damage occurring in the states surrounding the Great Lakes. However, oak wilt has been reported as far south as Texas.
How is Oak Wilt Disease Spread?
Oak wilt is spread largely in two ways--insect transmission and root transmission.
Insect transmission is generally by means of the sap (Nitidulid) beetle. These beetles are attracted to the damaged parts of trees where sap may be present and to the fungal mats created by the oak wilt fungus. The beetles transfer fungal spores attached to their bodies as they move from tree to tree. This is of particular concern as they move from the spore mats of infected trees to freshly damaged areas of healthy trees.
Root transmission is by the underground root systems that often connect one oak tree with another. The fungus simply migrates from oak to oak by travelling through the vascular system of the roots in the same way that it spreads within the tree itself.
Why Are Red Oaks More Susceptible To Oak Wilt?
Red oaks are more susceptible than members of the white oak family because they do not have the ability to produce the 'tyloses' or vascular plugs that white oaks create to contain damage due to breakage or disease.
It must be noted, however, that in most cases, the white oak's natural defenses only slow, not stop, the spread of oak wilt, allowing the white oak to survive, perhaps, for years instead of months. And it should be remembered that, even in cases where a white oak survives an attack of oak wilt, the tree might remain a host for the fungus and give it a base from which to spread.
How is Oak Wilt Detected?
Leaf damage is the earliest indication of oak wilt. Leaves of infected trees begin to lose their green color, dulling and then browning or yellowing from the outer edges inward. The leaves may appear to be water-soaked and wilting and may begin to curl around the mid-rib. As the disease advances, leaves begin to drop, sometimes while still green, starting from the ends of the branches.
The speed of the progression of the disease depends upon the species of tree infected. Red oaks can die from oak wilt in as short a time as a month. Texas live oaks may survive as long as six months.
White oaks may survive as long as several years after infection. In a small number of instances, Texas live oaks and white oaks appear to have survived oak wilt infection.
In red oaks, a 'fungal mat' may appear when large masses of fungal tissue break the bark. These mats give off a distinctive odor, which attracts different species of insects.
Unfortunately, particularly in the case of red oaks, by the time symptoms are noticed, it is already too late to save the tree.
Next week I'll tie up the loose ends of the subject and conclude with information on how to diminish the spread of oak wilt.
Thanks Grandpa for teasing my brain!
(Sources: TreeHelp.com; Forest Insect & Disease Leaflet 29, U.S. Department of Agriculture Forest Service.)
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