Forest health experts say unseasonably warm weather is raising concerns that oak wilt, a serious and almost always fatal fungal disease of red oaks, will likely appear sooner than normal. Therefore, I encourage you to stop pruning oaks from now through the end of July.
Typically, the high risk period for oak wilt transmission is April through July. However, due to unusually warm spring that we have had this year, the risk of oak wilt is likely to already exist.
You should take special care to avoid wounding oaks from now through July. In fact, any action that might provide an opening into the tree, such as carving initials into the tree or attaching a birdfeeder or clothes line, could provide an opportunity for the oak wilt fungus to invade and establish itself in the tree. If an oak tree needs to be pruned from now through July, you should consider using wound dressing or paint on the cut surface as soon as the wound is created.
The way oak wilt works is really small sap beetles transport fungal spores by landing on fungal mats found beneath the cracked bark of trees that died the previous year. The spores are then transmitted from a beetle’s body onto the fresh wound of a healthy oak tree while the beetle is feeding at the pruned or damaged site.
The risk of spreading oak wilt is low after July, however, I always advise to be cautious and avoid pruning or wounding oaks until November just to be safe.
The common beetles that transmit oak wilt disease are not capable of boring into a tree.
Oak wilt also can spread from a diseased tree to a healthy tree through a connected root system.
This ability to spread through root grafts means that even if only a single oak is wounded and subsequently infected with oak wilt, a new oak wilt “pocket” may develop in a location where oak wilt did not previously exist and will radiate to other oaks through the connected root systems. If no management steps are taken, the pocket could continue to expand year after year. Once oak wilt establishes itself in an area, control of the disease is both difficult and costly. The prevention of oak wilt is the best approach. To prevent oak wilt you need to contact your local tree service. If you are in the Chicagoland area Trees "R" Us, Inc. will help you prevent your oak trees from becoming contaminated with oak wilt and help you treat your infected trees.
Builders and developers should also be very careful as many oak wilt infections and deaths have occurred through inadvertent damage to roots, trunks, or branches during the construction process.
Oak wilt can be found in all counties of Illinois and 18 other states. The estimated range of oak wilt runs from southern Michigan through central Pennsylvania along the Appalachian Mountains south to Georgia, westward to the Great Plains and including much of Texas, then northward into Minnesota.
Every year, the disease kills many oaks in Illinois by interfering with the tree’s water and nutrient-conducting systems, essentially starving the tree. Leaves begin to wilt, and the tree may eventually die. Trees in the red oak group, such as northern red and northern pin oak, are especially vulnerable, and once wilting symptoms become visible, the tree loses most of its leaves and dies very quickly, often within weeks. Trees in the white oak group – those with rounded or lobed leaves – are more resistant to oak wilt, and the disease progresses much more slowly, often one branch at a time. White oaks could live with oak wilt for many years, and some trees may recover from the disease.
On another note, pruning deciduous trees in general should be avoided in the spring, as this is the time when tree buds and leaves are growing and food reserves are low. The best time to prune any deciduous tree is winter, followed by mid-summer -- after leaves have completed their growth.