Forest Insect and Disease Newsletter
Oak wilt risk status elevated to "high" in mid-March
This spring may have seemed like the perfect time to get some of those outdoor chores checked off our "to-do" lists, but pruning your oaks is something you will want to procrastinate on until next winter. While the unusually warm temperatures have been great for getting things done around the house, forest health specialists predicted that the insect vectors of the oak wilt pathogen would also be active much earlier than normal this year. In fact, on March 18th the oak wilt risk status in Minnesota (available at My Minnesota Woods) was officially elevated to "high" as a result of abnormally warm conditions and lack of snow cover in a significant portion of the state. This is the earliest date ever for the transition from low winter risk to high spring risk for oak wilt in Minnesota!
During high risk periods, there is an increased chance that fresh wounds on oaks could serve as infection points for oak wilt. The general recommendation is to avoid pruning your oaks during this time. Also avoid any activities that could result in injury to oaks including pruning or felling nearby trees, damaging the base of the tree with lawnmowers or weed-wackers, or major construction. If a tree becomes wounded during the high-risk period, the wound should be treated immediately with a pruning paint or water-based paint; this makes the wound unattractive to insects and reduces the risk for infection. If you are in an area where oak wilt is present (Oak Wilt in Minnesota), it is very important to follow these recommendations. Once highly susceptible trees like northern red oaks and northern pin oaks become infected, there is virtually no chance for survival.
Oak wilt is considered to be the most important diseases of oaks in the eastern U.S.A.. It is a vascular disease caused by a fungus (Ceratocystis fagacearum) that has the ability to rapidly spread through the water-conducting tissue (sapwood) of all oak species present in Minnesota. This results in a disruption of the water supply to the canopy, and highly susceptible oak species wilt and die within a few weeks to a few months. A oaks and bur oaks are slightly more resistant to the invading pathogen, and it is possible that only localized portions of the crown may be affected, although they too may only survive for a few years. Oak wilt most commonly spreads from tree to tree through root grafts between the same species; once oak wilt is in a stand, it can spread through a stand via interconnected root systems. This can be difficult and costly to control. Root graft transmission of the fungus can only be stopped by mechanically cutting interconnected root systems with a vibratory plow or trencher. The disease can be suppressed in trees that may be infected but have yet to show symptoms with bi-annual injections of a vascular fungicide. Both methods of active control are costly and difficult to implement, so the best advice is to prevent oak wilt from entering your oak forest in the first place.
The oak wilt fungus may be carried long distances to new trees by insect vectors, especially sap-feeding beetles that are attracted to fresh wounds exuding sap. In Minnesota, the most important insect vectors are two species of these sap-feeding beetles: Colopterus truncatus and Carpophilus sayi. When temperatures are sustained between 43 to 85 F, the oak wilt fungus produces sporulating mats of mycelium under the bark of recently killed oaks. These mats will rupture the bark and have a fruity odor that is attractive to sap-feeding beetles. When beetles enter the spore mat in search of food, they become coated in the sticky spores of C. fagacearum and may then vector the fungus to other feeding sites, particularly fresh wounds. A fresh wound that exposes the sapwood is all the fungus needs to enter and begin invading the tree's vascular system. Prompt removal of recently wilted oaks before the high risk period to eliminate the inoculum source and avoidance of wounds to oaks is the only way to prevent oak wilt from gaining a foothold in your oak stand.
Remember that firewood cut from oak wilt killed trees has the capability to produce spore mats and harbor oak wilt vectors. DO NOT MOVE FIREWOOD!
Warm temperatures in late winter and early spring make it likely that spore mat production has begun and that Colopterus truncatus, our principal oak wilt vector in the spring, has emerged and is actively feeding. Be sure to put the saw and pruning shears away for the year. While the risk of infection decreases as we reach mid-summer, studies have shown that sap-feeding beetles may still be carrying the oak wilt fungus well into October. The safe time to do your pruning is between November and March, unless of course we have another early spring like that in 2012. For more information on oak wilt be sure to visit the Forest Health of the Minnesota DNR's website.