Originally, it’s been stated to not trim April 15th-July 15th. Although the chances of oak contracting oak wilt decrease slightly after mid-summer, other factors, such as insects, come into play. If insects such as the picnic beetle are present, there is a potential the disease will likely spread.
We will not trim oaks until a hard frost. One of our estimators visited a property on the lakeshore this summer to look at an oak dead from oak wilt. Apparently, a couple of low branches were trimmed off a nearby oak in the previous year in September. Immediately following the trimming, tree paint was painted over the wounds. A few weeks later, the oak died from oak wilt. It was removed the following Fall and, in the Spring, a nearby oak had also died due to oak wilt. Even though this homeowner waited until September to trim the oak, and even with tree paint applied to the cuts immediately following pruning, this oak still succumbed to oak wilt.
Because this disease is complicated, expensive to contain, and spreads easily, it is simply not worth the risk to trim an oak before a hard frost.