Oak Wilt - Prevention & Management

What is Oak Wilt?


Oak wilt is a fungal disease that can quickly kill an oak tree.

It is caused by the fungus Ceratocystis fagacearum. Symptoms vary by tree species but generally consist of leaf discoloration, wilt, defoliation, and death. The fungus is spread from diseased to healthy trees by insect vectors or via connections between tree roots. Management of the disease consists mainly of preventing infection by avoiding tree wounds, removing diseased trees and digging trenches that disrupt root connections. Fungicide treatments are available and are mostly preventive as well. Oak wilt is an important disease of oak for timber production and of oak trees in urban areas. (Wikipedia)

How to Identify Oak Wilt?

                                               Foliar Symptoms on Live Oak
Live-Oak-Wilt-Leaf-Details-169x300Leaves on diseased live oaks often develop chlorotic (yellow) veins that eventually turn necrotic (brown), a symptom called veinal necrosis. Initially, the leaf vein is a darker green then the rest of the leaf. This early symptom is called vein banding and is seen before veinal necrosis develops in leaves on the tree. Defoliation may be rapid, and dead leaves with brown veins often can be found under the tree for months after defoliation. Leaves may exhibit other patterns of chlorosis and necrosis, such as interveinal chlorosis, marginal scorch, or tip burn, but these symptoms are less reliable than veinal necrosis for diagnosing oak wilt in live oaks.

Red-Oak-Wilt-Leaf-Details1-208x300Foliar Symptoms on Red Oak
Foliar symptoms of oak wilt on red oaks are less distinct. In early spring, young leaves simply wilt, turning pale green and brown, usually remaining attached for a period of time. Mature leaves develop dark green water soaking symptoms or turn pale green or bronze, starting at the leaf margins and progressing inward. This can begin on one branch and quickly engulf the entire tree. Red oaks generally die within 4-6 weeks.

Oak-Wilt-Disease-Details-21-300x220 - CopyFungal mats are reliable indicators for diagnosis of oak wilt. These specialized spore-producing structures most often form in the spring on red oaks that developed advanced symptoms of oak wilt the previous late summer or fall. Red oak infections in late spring and summer usually do not give rise to fungal mats due to high temperatures and low soil moisture conditions. Fungal mats can be found by looking for inconspicuous narrow cracks in the bark of dying red oaks leading to hollow areas between the bark and wood. They often have a distinctive odor similar to fermenting fruit. Fungal mats can be exposed for inspection by chopping away the loose bark.

How Does Oak Wilt Spread?
Oak wilt spreads to other oak trees in two ways – long distances with the aid of certain beetles or locally through common or grafted roots. Sap-feeding (nitidulid) beetles are believed to be responsible for much of the long distance spread of oak wilt. During the spring, the oak wilt fungus forms special spore-producing structures called fungal mats on red oaks.  Nitidulid beetles are small (about 1/8-inch long) and are attracted to oak wilt fungal mats because the mats have a sweet, “fruity” smell. Mats form underneath the bark of diseased red oaks and are not known to occur on live oak trees. The fungal mats apply pressure under the bark causing a tiny crack to form.These mats can be found on the trunk and major branches of red oaks. When a nitidulid beetle feeds on an oak wilt fungal mat, spores of the oak wilt fungus will cling to the body of the beetle. Nitidulid beetles also feed on tree sap associated with fresh wounds. If a beetle contaminated with oak wilt spores lands on a fresh wound on a healthy oak, then that tree can become infected. Tree wounds can be made by man or nature, but nitidulid beetles are attracted to both. Once established, the fungus moves from one tree to the next through common or grafted roots.

How to Manage Oak Wilt?

There are three primary approaches used for oak wilt management in Texas. Successful control usually depends on an integrated program incorporating measures from all three approaches. The first approach attempts to prevent the formation of new oak wilt infection centers by eliminating diseased red oaks, handling firewood properly, and painting wounds on healthy oaks. The second approach involves trenching or other measures to disrupt root connections responsible for root transmission of the pathogen. Finally, injections of the fungicide propiconazole (AlamoTM) into individual, high-value trees help reduce crown loss and may extend the life of the tree. These measures will not cure oak wilt, but can significantly reduce tree losses.