Oak wilt is a disease caused by a fungus that disrupts the transport of water throughout a tree. It's a serious disease that affects all oak trees. Red oaks are most susceptible to oak wilt and can die within a few weeks to 6 months of infection. While it has been found in more than 20 states, oak wilt is not yet known in New Hampshire. The closest known outbreak is in New York. (Photo at left by Paul A. Mistretta, USDA Forest Service, Bugwood.org)
Oak wilt is spread most often between trees through the roots, although it can also be spread by beetles. When the roots of oak trees become fused together, they can share nutrients and some diseases. The oak wilt fungus Ceratocystis fagacearum grows in the water-conducting vessels of a tree and prevents the movement of water. As water movement slows, the leaves wilt and drop off, ultimately leading to death of the tree.
Control of oak wilt is possible, although it can be expensive. The best chance at controlling oak wilt is to catch it early, when it's in a few trees rather than a few acres, and to promptly remove dead and dying trees. Landowners should submit a report if they notice either of these signs of oak wilt: 1) if an oak tree drops its leaves in July, and 2) if a dying oak smells of apple cider.
How to Identify, Prevent and Control Oak Wilt - U.S. Forest Service
Oak Wilt - Ceratocystis fagacearum - U.S. Forest Service
Oak Wilt - New York Department of Environmental Conservation
Oak Wilt Prevention (video) - New York Department of Environmental Conservation
Oak Wilt - My Minnesota Woods