Oak Wilt

Oak Wilt is a tree disease caused by a non-native fungal pathogen which can afflict White and Red Oaks in the Eastern United States. The disease is more severe in Red Oak species, such as the Northern Red Oak and Northern Pin Oak. Red Oaks will die rather quickly from Oak Wilt, while White Oaks tend to contain the spread of the disease to infected branches.

Oak Wilt is prevalent in the southern Wisconsin counties of Wisconsin, but has not yet spread to some northern areas of the state. This article will introduce you to the signs and symptoms of Oak Wilt (Ceratocystis fagacearum), as well as management strategies we offer concerned homeowners.

Oak Wilt Symptoms

One of the first symptoms homeowners notice is the "flagging" of branches in your beloved Oak tree. These are branches with dried and dead leaves occurring in the upper canopy of trees in full sun. These symptoms may occur following storm damage in the summer, because breaks in the tree bark make the tree vulnerable to disease. If you notice summer storm damage on an oak tree, calling an arborist immediately is a great pro-active step to avoiding Oak Wilt.

Another symptom of Oak Wilt is the brown discoloration of leaves starting from the tip of the leaf and progressing towards the midrib, though this may be irregular. This symptom can be accompanied by uncharacteristic leaf drop. As mentioned earlier, this disease progresses rather quickly in Red Oaks, leading to rapid death, while White Oaks may only show wilting leaves the first year with symptoms progressing more slowly over the next few years.

Symptoms are useful for leading to a disease diagnosis in plants. Symptoms area plants' reaction to a disease, while the signs discussed next are the causation. Be aware that symptoms can be shared across many tree diseases, which is a common cause of misdiagnosis. The best way to determine if a tree has oak wilt is to take a branch sample from the affected portion of the tree and ask an arborist or other tree professional to evaluate.

Signs

The Oak Wilt Fungus can spread through intersections of Oak roots, which can make its spread difficult to control. The fungal mats in the trees' cambium produce volatile compounds that attract the vector for this disease. (A vector is the transmission factor that causes new infections of Oak Wilt in healthy trees by entering wounds.) Two species of sap beetles are the culprits for overland Oak Wilt transmission. Signs of Oak Wilt would be either the fungal pressure pads found under the tree bark or presence of the beetle.

Management Strategies

Oaks are particularly susceptible to Oak Wilt infection in the spring and summer, while the beetles are infecting fresh wounds. For this reason, you should only prune Oak trees between November (first hard frost is a good measure) and March if possible. Any wounding from storms occurring between the spring or summer months should still be pruned as soon as possible to prevent infection, and should also be treated with a pruning sealant.

If you have an existing Oak infected with Oak Wilt that needs to be removed, you should:

1. Schedule a removal in the winter months to prevent further spread to neighboring trees.
2. Some homeowners may also consider trenching around infected trees to prevent root spread of the fungus. This measure is not a guarantee because you do not know for sure if all the root grafts between trees will be severed, but is a good alternative to the fungal treatment.
3. Another more effective preventative measure is to treat your trees with a systemic injection of propiconazole, a fungicide that can protect high value trees for up to two years. This treatment can slow the infection of White Oaks with less than 30% crown decay symptoms, but is not recommended for infected Red Oak trees.

We want to help protect your Oak trees and maintain the health of our urban canopy. Contact Heartwood if you need help managing or caring for your oaks ,or if you just have a question.