Oak Wilt - Disease Cycle

Disease Cycle

The oak wilt fungus spreads in two basic ways: transfer of spores from diseased to healthy trees by insect vectors and movement of the fungus from a diseased tree to a healthy three through root grafts. After a tree has wilted and died, if conditions are right, spores are produced on fungal mats that form under the bark of the tree. These mats produce asexual spores called endoconidia, which are barrel shaped spores produced in chains. If compatible mating types are present, these mats will also produce sexual spores called ascospores in fruiting structures called perithecia. Certain species of sap beetles are attracted to the fruity smell of these mats. They will visit the mats to feed and breed, thereby picking up the fungal spores. These same beetles are also attracted to the bleeding sap of wounded oak trees and can then deposit the spores picked up from the fungal mats. This method of transmission is important for introducing the fungus into a new area it could not have reached by transmission via root grafts.

Transmission via root graft is the most common means of spread, as trees within as much as 15 m (50 feet) of a diseased tree can be infected. Oak wilt usually moves from diseased trees to healthy trees through roots that have become interconnected (root grafts). In this case, spores (also endoconidia) that have been produced inside the tree travel through the vascular tissue. Most root grafts form between oaks of the same species; red oak roots graft more commonly than do white oak roots, and grafts between red and white oaks are very rare. Although possible, it is rare for oak wilt to jump between oaks of different species via root grafting – different species do not graft often, and so contaminate each other less frequently.