Oak wilt is caused by the fungus Ceratocystis fagacearum. Oak wilt can be spread in two ways: 1) fungal spores travelling tree to tree through grafted roots, and 2) fungal spores spread by sap beetles that fly from infected trees or wood to healthy trees. The beetles are attracted to fresh wounds in healthy trees, and these wounds offer an introduction point for the fungus.Trimming or cutting healthy oaks from the months of November through March helps to prevent fresh wounds in healthy trees when the beetles are active. Trimming away dead and dying branches during this period can help prevent oak wilt spreading from neighboring areas to your trees. If your oak trees still need trimming before this spring, be sure to do it before the end of March. April 1 through July 15 is considered the high risk period, and all trimming of oaks should be avoided. July 15 through October 31 is considered low risk, but spread is still possible. Red oaks and pin oaks are especially susceptible to oak wilt, and once infected, a healthy tree is killed within months.