Oak Wilt

Overview

Oak wilt is a serious and often lethal disease caused by the fungus Bretziella fagacearum (formerly Ceratocystis fagacearum), which invades the water-conducting tissues of oak trees (Genus Quercus). The disease is most severe in species of the red oak group (black, pin, red, scarlet, shingle), as it can kill trees in several weeks to months (e.g. 4 – 6 weeks). Species in the white oak group (bur, chinkapin, post, swamp white, white) are also susceptible, but may take one to several years to succumb to the disease. Sap feeding beetles (family Nitiduliade) and possibly oak bark beetles (family Scolytidae) are the primary vectors. Oak wilt has been confirmed in at least 22 states from Minnesota east to Pennsylvania, south to South Carolina and Tennessee, west to Texas, and north through Kansas and South Dakota. There is no cure for oak wilt. Control of the disease consists of preventing it from spreading to healthy trees.
 

Symptoms and Diagnosis

Trees infected with oak wilt in late spring typically begin showing symptoms in mid to late summer. In red oak species, wilt symptoms are first noticed at or near the top of an infected tree, with sections of branches turning red-brown (flagging). Leaves turn a dull green, bronze or tan, which progresses from the leaf tips and edges and moves inward, towards the base of the leaf. Leaves may also droop and curl, appearing wilted. Infected trees may drop leaves after symptoms appear, with severe defoliation occurring by late summer. A cut branch from an infected tree may show a brown or black discoloration in the outer annual sapwood ring. In the fall or next spring after defoliation or tree death, red oak trees may produce spore-bearing fungal mats beneath the bark. The fungal mats emit an odor that smells of wine or fermenting fruit, which attracts beetles. The fungus also produces “pressure pads”, which cause the bark to crack vertically, creating an access point for beetles to pick up fungal spores.

Oak wilt disease progression in white oaks is similar to red oaks. However, since white oak species are more tolerant of the disease, symptoms appear more gradually, and trees may only have a single limb or scattered limbs with symptoms over a period of one to many years. Fungal mat formation is less common or apparent in white oaks.  

Laboratory testing is required to positively diagnose oak wilt. Disease symptoms can resemble or be mistaken for other oak diseases such as bacterial leaf scorch, oak anthracnose, or oak decline.

Life Cycle

The fungus spreads through the water-conducting vessels of the sapwood (xylem). The tree’s response to the presence of the fungus results in the disruption of sap flow, and the affected areas wilt. Oak wilt can spread to healthy trees through natural grafts with roots of adjacent oaks of the same species (local spread). Root grafts join the vascular systems of trees together, forming a network through which the disease can spread. Trees connected by grafts are often killed in groups or “disease centers”. The disease can also be spread by sap-feeding nitidulid beetles which are attracted to the fungal mats of infected trees (overland spread). Beetles pick up spores on their bodies, which can then be transferred to healthy trees via open wounds or fresh pruning cuts. Beetles are typically active from April to July, though additional flights may occur through late summer.

Integrated Pest Management Strategies

1. There is no cure for the disease. If oak wilt is suspected, a laboratory test is needed to make a positive diagnosis. Contact a certified arborist or a University Extension office about sampling procedures and fees associated with laboratory testing. 

2. Avoid pruning our wounding oak trees from April ~ July. During this time of year, beetles are active and attracted to sap flowing from wounds or pruning cuts. Those that are carrying oak wilt spores can transfer the disease through these openings to healthy trees. Avoid pruning oak trees or any activity that causes wounding during this time. The safest time to prune oaks is in winter when trees are dormant, before mid-March. 

3. Apply a wound dressing to pruning cuts. If pruning must be done during the growing season, such as removing storm-damaged limbs, immediately apply a wound dressing to the cut. Applying a wound dressing is generally not recommended for most trees, as it disrupts the tree's natural ability to seal wounds. However, in the case of oak trees, the wound dressing reduces beetles' attracting to sap from a fresh cut, limiting potential transmission of the fungus. Beetles have been known to find wounds within ten minutes of pruning. 

4. Sever root grafts. Destroying root grafts by mechanical means to a depth of about 4-5 feet can interrupt the disease cycle and slow the spread of the fungus from diseased to healthy oak trees. This is done by trenching or using a vibratory plow. Trenching must be done before a diseased or dead tree is removed, as there is a risk of infected sap moving into a healthy tree through the root grafts. Chemical options are available for areas where trenching is not possible (sidewalks, driveways, or areas with buried pipes/lines/cables). Always consult with a certified arborist, forester or oak wilt specialist before trenching. Root grafts typically do not occur between red oak and white oak group species. 

5. Remove infected trees. After trenching, infected oaks should be removed promptly to prevent fungal mats from developing and serving as a source of inoculum for new infections. Wood, including the stump, should be destroyed by chipping, burning, burying or removing the outer bark to prevent favorable conditions for the fungus to develop. Infected wood intended for use as firewood should not be moved during peak beetle activity (April ~ July). 

6. Limit planting of oaks in areas with oak wilt. In areas where oak wilt has been confirmed, consider planting other native tree species to reduce disease pressures. 

Organic Strategies

Strategies 1, 2, 3, 5 and 6 are strictly organic approaches.