Oak wilt is a very complex disease. How new infection centers are established is not fully understood even by researchers and expert arborists. In an attempt to suppress the disease over the years, various agencies across the U.S. have made overly simplistic blanket statements advising people to avoid pruning during specific months of the year. Such statements are problematic because they do not advise people of the following facts:
- Calendar-based dates cannot reflect actual temperatures and humidity, which play crucial roles in insect activity and fungal spore development. For instance, in Central Texas there are often many consecutive days in February and sometimes even March, in which it is far too cold for insects to fly. Likewise, in June it is often too hot for the fungal spores to remain viable.
- Although there is a recognized increase in susceptibility in the spring, removing branches during the spring that are at constant risk of being wounded, such as low branches over streets or those rubbing against other branches or buildings, can reduce the risk of oak wilt by eliminating these open wound sources, provided they are removed properly.
- There are many other sources of open wounds, some of them most prevalent in the spring, such as hail and wind storms. The amount of wounding that occurs with these natural events is far more extensive than most people imagine. Other significant sources of wounding includes squirrels, birds and insects, all of which cause very high levels of open wounds.
Competent arborists have known and adjusted for these facts for years when pruning, but agency recommendations have remained generic. That is why Mr. LeBlanc recently led a group of commercial arborists and Texas Forest Service and Extension Service representatives in creating a pruning recommendation for the general public that clarifies some of these oak wilt factors. Here is that statement:
Pruning Guidelines for Prevention of Oak Wilt in Texas
Oak wilt, caused by the fungus Ceratocystis fagacearum, is the most destructive disease affecting live oaks and red oaks in Central Texas. Most of the tree mortality results from tree- to-tree spread of the pathogen through interconnected or grafted root systems, once an oak wilt center becomes established. New infection centers begin when beetles carry oak wilt fungal spores from infected red oaks to fresh, open wounds on healthy oaks. Wounds include any damage caused by wind, hail, vehicles, construction, squirrels, birds or pruning. Research has shown that both oak wilt fungal mats on infected red oaks and insects that carry oak wilt spores are most prevalent in the spring. Below is a brief description of how you can reduce the risk of fungal spread when pruning.
- Always paint fresh wounds on oaks, including pruning cuts and stumps, with wound dressing or latex paint immediately after pruning or live tree removal at all times of the year.
- Clean all pruning tools with 10% bleach solution or Lysol TM between sites and/or trees.
- If possible avoid pruning or wounding of oaks during the spring (currently defined as February 1 through June 30). Reasons to prune in the spring include:
- To accommodate public safety concerns such as hazardous limbs, traffic visibility or emergency utility line clearance.
- To repair damaged limbs (from storms or other anomalies)
- To remove limbs rubbing on a building or rubbing on other branches, and to raise low limbs over a street.
- On sites where construction schedules take precedence, pruning any live tissue should only be done to accommodate required clearance.
- Dead branch removal where live tissue is not exposed.
- Pruning for other reasons (general tree health, non-safety related clearance or thinning, etc.) should be conducted
before February 1 or after June 30.
- Debris from diseased red oaks should be immediately chipped, burned or buried.
Regardless of the reasons or time of year, proper pruning techniques should be used. These techniques include making proper pruning cuts and avoiding injurious practices such as topping or excessive crown thinning. If you are uncertain about any of this information, you should consult with a Texas Oak Wilt Certified arborist, ISA Certified Arborist, or an oak wilt specialist from a city, county or state government agency such as the Texas Forest Service or Texas AgriLife Extension Service.
References (available at http://www.TexasOakWilt.org):
Appel, D.N., and R.F. Billings (eds.). 1995. Oak wilt perspectives: Proceedings of the Nation Oak Wilt Symposium, June 22-25, 1992. Austin, TX. Information Development, Houston, TX. 217 p.
Billings, R.F., and D.N. Appel (eds.). 2009. Proceedings of the National Oak Wilt Symposium. June 4-7, 2007, Austin, TX. Texas Forest Service Publication166. 267p.
Prepared January 12th, 2011 in cooperation between Texas Forest Service, Texas AgriLife Extension Service and International Society of Arboriculture Texas Chapter.