Oak wilt kills hundreds of thousands of oaks (Quercus spp. and cvs. Zones 3–11) in the Midwest and beyond every year, but what is it? Oak wilt is caused by a fungus (Bretziella fagacearum, formerly Ceratocystis fagacearum) This lethal fungal disease is a threat to oaks in residential settings and forests. Oak wilt fungus grows through the infected oak’s water conducting system (xylem) and clogs the vascular system, leading to noticeable symptoms and in many cases, immediate decline and death. Thus far, oak wilt has been identified in more than 20 U.S. states, and for the first time in Ontario in May 2023.