I am an ISA certified Arborist. The ISA states that you should not prune any oak trees between February and June 15th because the Oak Wilt disease produces spores that float through the air and infect other trees. There is also a beetle that spreads the disease. In Texas the period of Feb.-June is very moist and warm. It's the perfect climate for the spores to produce and spread. Anytime other than that, it is perfectly fine to prune Oak trees. Winter is good because most fungus is dormant, but the summer and fall season are okay as well. The time period mentioned before of (April-July) is incorrect.
