M. Grabowski, UMN Extension Educator
|Bur oak blight, M. Grabowski, UMN Extension|
This summer many bur oak trees are suffering significant leaf browning and death due to bur oak blight. Frequent rain events at the time new leaves were reaching their mature size created highly favorable conditions for infection by the fungus that causes bur oak blight. For property owners with affected bur oak trees, now is the time to submit a sample for diagnosis. If disease is severe, contact an arborist now to schedule treatment for bur oak blight in spring of 2017.
Bur oak blight, often referred to as BOB, is a plant disease caused by the fungus Tubakia iowensis. The BOB fungus survives the winter on infected leaves that remain attached within the tree canopy. In wet spring weather, the fungus releases spores that start infections on new leaves. Although the infections occur in spring, the most obvious symptoms do not appear until the end of July or early August. Initially, dark discolored lines can be seen forming along major leaf veins. As the disease progresses the discoloration expands into brown wedge shaped areas. Leaves may remain partially brown and green or may turn completely brown and withered. When autumn leaf drop occurs, many leaves infected with bur oak blight remain attached to the tree.
|Wedge shaped brown areas and completely|
brown leaves are both symptoms of bur oak
blight. M. Grabowski, UMN Extension
Bur oak blight is a slow moving disease. The leaves in the lower canopy are usually first to be infected. Some of these leaves remain attached until the following growing season and will allow the disease to move up to higher branches. Each year, the disease infects a greater portion of the tree canopy. This slow progression of disease eventually stresses the tree and allows secondary pests and pathogens, like two lined chestnut borer and Armillaria root rot to infect and further injure the tree. The combination of these infections can lead to tree decline and death.
Many pests and pathogens can cause leaf browning in oak trees. Oak wilt, another fungal disease of oak trees, can cause similar symptoms to bur oak blight. It is important to identify exactly what pest or pathogen is causing the problem, so the appropriate management strategy can be applied. Property owners with bur oak trees in poor condition should submit a sample to the UMN Plant Disease Clinic for diagnosis.
|Browning begins on major leaf veins and expands into large|
wedge shaped areas. M. Grabowski, UMN Extension
Once bur oak blight has been confirmed, a management plan can be created. Trees can tolerate BOB for multiple years and do not require treatment if less than 50-60% of the leaves in the canopy are infected. Trees with low levels of infection should be examined each year in August to determine if severity has increased to the point that intervention is needed.
In high-value, landscape trees, once 50-60% or more of the canopy is infected with BOB, treatment of bur oak blight with a fungicidal injection is recommended to prevent tree decline. A trunk injection of propiconazole fungicide in spring when new leaves have just reached their mature size has been shown to significantly reduce bur oak blight in most cases. This treatment must be applied by an arborist and timing of the treatment is critical. Property owners wishing to have their trees treated in 2017 should contact an arborist now to schedule the treatment.
Bur oaks are sensitive to propiconazole fungicides. As a result, the fungicide must be applied at the rate of 10 ml per inch of trunk diameter at breast height. This is half the maximum rate labeled for oak wilt. Applying the fungicide at a lower rate will help to reduce phytotoxicity, but even at low rates, gardeners may notice some leaf browning and other leaf damage the year the application is made. Unfortunately no other fungicides have proven effective in protecting bur oaks from BOB. Once an application has been made, disease should be significantly reduced and the tree will not need to be treated again until bur oak blight once again affects 50-60% of the canopy.