According to David L. Roberts Ph.D. a Senior Academic Specialist at Michigan State University there are many positive reasons for pruning oak trees during the winter while they are dormant. Dormant pruning enables an arborist to "correct disease problems which would be more risky during the summer season" (Roberts, 2002). Additionally "bacterial populations in cankered branches are lower and dormant, and hence, less likely to be transmitted via our pruning utensils" (Roberts, 2002). Oak Wilt can be become a major concern for trees that have been pruned in the summer and have open wounds which will attract insects that often carry the deadly fungal spores.
Oak Wilt Management Through Winter Pruning:
While many professional plant health care providers are aware of Oak Wilt and its potentially lethal effects on oaks in our landscapes, some are either unfamiliar with or ignore the potential for oak wilt. Pruning during the summer months can actually promote disease transmission and the development of disease epicenters which can then devastate stands of oaks. The mechanism of disease transmission is that insect vectors (primarily sap beetles, including picnic beetles) are attracted to wound sites, and they carry fungal spores from infected trees or dead logs to recently pruned trees.
When I first proposed that we need to cease pruning during the warm season, my comments were met with mixed reviews and even some controversial remarks. The fact is, we need to make sure our industry is providing services aimed at improving the health of plants and not aimed at their destruction. Luckily, some companies have instituted a winter pruning program, sometimes advertised through various ingenious avenues, aimed at oak wilt management as well as management of other diseases. I think these efforts are very worthwhile, and I have even been thanked on occasion with comments about how winter pruning has actually expanded their businesses.
What is the best time for pruning oaks? From my perspective the safest and most appropriate time to prune oaks is dormantly, from late October to early March. Obviously, this time will vary from year to year depending on weather conditions. During early March in 2000, experts in Minnesota reported picnic beetle activity on March 1st. During most years, we would not expect much if any picnic beetle activity during the first week of March or perhaps even through the entire month of March. It is wise to keep in mind that during freezing or near freezing conditions, transmission of oak wilt is almost impossible. Unusually warm climatic periods during dormancy may be conducive for oak wilt spread.
I have often been asked why my recommendations are different from some others’ recommendations which suggest only the cessation of pruning during April, May and June. Many of the oak trees being pruned are large, at least 100 years old and are in very prominent locations. They are simply irreplaceable. Although I am certain that the likelihood of oak wilt transmission declines sharply after the three primary months, most scientists admit that we do not understand all of the mechanisms or vectors of oak wilt. No one will admit that the transmission of oak wilt is impossible, for example, during the month of August. In a recently new USDA bulletin, the authors suggested that oak bark beetles may also transmit oak wilt. I believe it is smart to avoid any possibility of oak wilt by limiting our pruning period.
Sanitization and Pruning:
The oak wilt fungus could potentially be transmitted via pruning utensils because of the transfer of fungal material residing in saw dust or chips. This is especially true during the warm season when fungal growth is at its maximum. However, there is probably little chance of spreading oak wilt during the dormant season but I'd still recommend sanitizing equipment between trees. There is probably no need to sanitize utensils between cuts on the same tree because the tree is either infected or its not, and the transfer of the fungus between branches of the same tree will probably not greatly impact a tree already infected.
The Benefits of Pruning Oaks During the Dormant Season:
The occurrence of dead wood is a common reason to prune oaks. Unfortunately, I think in most instances it is not possible to prune dead wood without the exposure of live tissue - the reason why dead wood should be done dormantly and not during the spring and summer months on oaks. Proper pruning of trees can make them more storm resistant as well, whether the storm threat is from ice damage or from high winds. Pruning is often regarded as a proactive, health maintenance insurance plan to guard against potentially serious problems, analogous to the ever more popular and wise preventative care in human health plans.
Retrieved from: http://treedoctor.anr.msu.edu/pruning/
David L. Roberts, Ph.D.
Senior Academic Specialist
Michigan State University
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