Oak wilt

Oak wilt is a fungal disease which can quickly kill an oak tree. The tree reacts to the presence of the fungus by plugging its own cambial tissue in an attempt to block the fungus from spreading further. As the area around cambium (the vascular tissue) is crucial for delivering nutrients and water to the rest of the plant, this plug prevents them from travelling up the trunk of the tree, eventually killing it.

The fungus originated in eastern Russia, and was brought into North America by human trade. It is very similar to Dutch elm disease. It affects much of the eastern and central US, from Virginia to Minnesota to Arkansas, with pockets of infection down to Texas. It is particularly common in the Midwest.

Infections and Symptoms
Oak wilt spreads in two basic ways. A transmission via root graft is the most common source of infection, as trees within as much as 15 m (50 feet) of an infected tree can be infected. The second method of infection is via sap beetles. These beetles are attracted to the bleeding sap of the oak tree, as well as the fungus in an infected or dead tree, and so can transfer the disease to healthy but injured trees. This is less common as trees are rarely infected this way unless injured, but it is the only way to jump barriers (rivers, for example) and infect trees in new areas.

Oak wilt is identifiable by the rapid pattern of wilting starting from the top of the tree and progressively dying down to the bottom, and on specific leaves, wilting from the edges to the base. Oak with oak wilt stand out with their dead crown compared to a green canopy in the summer, so much so that oak wilt infections can be spotted from the air. A new infection via beetles instead of root grafts can kill a tree somewhat more slowly, if a branch is infected instead of the trunk.

Effects on Species
Oak wilt affects all oak species, but has somewhat different effects on different groups. Red Oaks such as the Northern red oak are particularly susceptible, and when infected, generally die over the course of a single summer. White oaks are more resistant, and can live for several years after infection, losing a few branches each season (from the top down). White oaks in particular are resistant. Some types of white oak, such as bur oak are more susceptible, although still not as much as red oaks.

Although possible, it is rare for oak wilt to jump between oaks of different species via root grafting. Different species do not graft often, and so contaminate each other less frequently.

Management
Although similar to Dutch Elm Disease, Oak wilt is more controllable. Prevention is key, as there is no permanent cure. To prevent beetle transmission, oaks should never be pruned in the spring months. Late fall and early winter are preferable. Also, care should be taken to prevent injury during this period, particularly during construction. If a tree is injured through a storm or accident in the spring, tree paint to cover the wound is advised, although it is not a good idea in general. Reducing the source of infection is also helpful. Dead oaks should be checked for fungal mats in the spring, and if present, all wood should be chipped, burned, or covered in plastic. Logs from wilted trees should never be moved to unaffected areas, even for firewood.

If a tree is discovered to be infected, a trench (or better yet, two at different distances) should be dug between it and any other trees of its species to prevent root graft disease transmission. This will sever any root grafts that could carry the disease to neighboring trees. Injections of propaconazole can help to prevent transmission as well. Injections every other year can also be used to treat a white oak, if it is not yet greatly affected, although this is expensive. An infected tree should not be immediately cut down, as this can force the oak wilt down into the roots to affect other trees, and can also injure nearby trees and make them more susceptible to beetle transmission. The tree should be cut down the following fall, and debarked, chipped, burned, or covered in plastic by the next spring to prevent contamination by fungal mats.