March is the last month foresters recommend trimming oaks. Spring and early summer pruning causes oak trees to be vulnerable to oak wilt, an often fatal fungal disease. The most susceptible oaks are the Red Oak species — Northern Red (Quercus rubra), Northern Pin (Quercus ellipsoidalis) and Black Oaks (Quercus velutina) — and they don’t have to be unhealthy to be affected. Trees in the White Oak group are more resistant to oak wilt. The disease progresses more slowly, so White Oak (Quercus alba), Swamp White Oak (Quercus bicolor) and Bur Oak (Quercus macrocarpa) trees can live for a long time with the disease. White Oak has even been know to recover from the disease.
The first sign of Oak Wilt is the early dropping of leaves. After they begin to drop, it can take only a month for the tree to die. You’ll know the falling leaves are due to Oak wilt because the leaves will still be partially-green.
What is Oak Wilt?
“Oak wilt is caused by the fungus Ceratocystis fagacearum. The fungus grows through the infected tree’s water conducting system, causing the tree to wilt and die. Oak wilt is introduced to an area by sap-feeding beetles that carry oak wilt spores to fresh wounds. Spore-bearing fungal mats develop under the bark in the fall or spring following the death of the infected tree (Figure 3). The sap feeding beetles are attracted to the fungal mats and can transport oak wilt spores to fresh wounds or to recently cut oak stumps. Once in an area, the disease spreads to nearby oak trees through interconnected (grafted) root systems, creating an expanding pocket of dead oak trees.” (WDNR)
What if an Wound Appears during Summer Months?
If an oak tree is damaged during the summer months, paint the fresh wound immediately with wound sealer to prevent sap feeding beetles from introducing new spores. And, if you’re building on a landscape where oak trees are present, be sure to include in your contract with the builder or the developer the consequences of damage — even inadvertently — to roots, trunks or branches during the construction process. You might also consider adding damage to pine trees and birch trees as well, since these trees are very susceptible to insect damage as well.
When to prune?
The old rule of thumb used to be not to prune during the months that have no r’s in them, i.e., May, June July, August. Current thinking has changed that rule to not pruning April 15 through July 15. WDNR recommends pruning only during the months of November through March. However, there is an if — if you prune when temperatures are above 50 degrees, be sure to use a wound sealer immediately. Please note, however, that tree wound paint can actually slow the natural closing of a wound, so try not to get into the situation of having to use it.
Besides not pruning oaks, consider not pruning any deciduous trees in the spring. This is the time when buds and leaves are growing and food reserves are low making them susceptible to insects and disease. The best time to prune any deciduous tree is after the leaves have completed their growth — mid-summer to winter.
The WDNR has an excellent brochure on how to prune trees in Wisconsin here.
Should you have an oak tree succumb to oak wilt, be sure to clean saws and other equipment used to cut up and move the wood. Also, don’t give the wood away or transport it to another area. If you can’t burn it yourself, drop it at the local landfill. And, if you need to cut down an entire tree, be sure to treat the stump with wound dressing.