Oak Wilt is a systemic fungal disease that is caused by bretziella fagacearum (formerly known as ceratocystis fagacearum). The fungus causes the water conducting vessels of oaks to become clogged, reducing the flow of water up the tree. While all oak species are technically susceptible, live oaks and those in the red oak group are the most vulnerable, while those in the white oak group (such as bur, post, chinquapin, or monterrey) are far more resistant. In Central Texas, the disease has been most devastating for live oaks because of their tendency to grow large interconnected root systems. Once infected, live oaks generally die in three months to a year.
Oak wilt is generally transmitted in two ways. The first, most common avenue is through root grafts between an infected and a non-infected tree. On average, the disease spreads at a rate of 75 feet per year among live oaks. Note that this can mean a transmission speed of 150 feet per year under optimal conditions. The second means of transmission is by the spread of fungal spores from the trunk or primary branches of infected trees. This is typically caused either by careless pruning practices or the nitidulid beetle, which can carry spores from the fungal mat of an infected tree to wounds on a non-infected tree.
Oak wilt presents differently in different species. In live oaks, the leaves usually develop yellowed veins that eventually turn brown, a process called veinal necrosis. Many homeowners first notice oak wilt symptoms when the leaf veins are necrotic, but the spaces between veins are still green. In many cases, symptoms also include tip burn, scorching of the leaf margins, and interveinal chlorosis. In red oaks, the foliar symptoms of oak wilt are less predictable, but generally involve the browning or yellowing of leaves from the outer margins progressing inward.
The presence of fungal mats can sometimes help to form a diagnosis for oak wilt as well. The fungal mat develops under the bark during the more temperate months of spring and fall. The pressure created by this mat can cause the bark to crack and separate from the underlying wood.
As homeowners, the easiest thing you can do is to hire reputable tree companies who follow proper pruning procedures, paint wounds, and disinfect tools. If you have reason to believe that your trees are at risk of infection from nearby oaks, fungicidal treatments have been proven to be extremely effective as a prophylactic measure against oak wilt.
In some cases, trenching has been used as a means of mechanically severing the connections between grafted roots, and halting the subterranean spread of the disease. To be effective, trenches must be a minimum of four feet deep. It is also critical that they be placed correctly in order to provide any protection for non-infected trees. If you are considering trenching, we highly recommend consulting with a professional arborist first.
There is, at present, no cure for oak wilt. However, fungicidal injection can be a highly effective treatment both prophylactically (as noted above) and in the early stages of infection. While this treatment can not "cure" an infected tree, it can substantially prolong the life of the tree and either slow or forestall the progression of symptoms. You should consult with a qualified arborist to determine whether or not your trees are good candidates for treatment.
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