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The fungus Bretiella fagacearum causes oak wilt. It grows through the infected tree's water conducting system, causing the tree to wilt and die. Sap feeding beetles carry oak wilt spores to fresh wounds.
Oak wilt can attack and kill healthy trees. Oak wilt kills thousands of trees each year in forests, woodlots and urban areas in the United States. It is a serious problem for red oak species. Once a red oak tree starts showing symptoms, it will lose most of its leaves and die within about a month.
White oak species demonstrate moderate disease tolerance, often living several years after syptoms start. Spore-bearing fungal mats develop under the tree's bark in the fall or spring after the infected tree dies. The sap-feeding beetles are attracted to the fungal mats and can transport oak wilt spores to fresh wounds or recently cut oak stumps. Once established in an area, oak wilt is difficult to control. It spreads to nearby oaks through interconnected (grafted) root systems, creating an expanding pocket of dead oaks.
The greatest risk of oak wilt transmission happens in spring and early summer (April through July), when oak wilt spore-carrying beetles are abundant and fungal mats are fresh.
The Iowa DNR Forestry Section, in partnership with Wisconsin DNR, developed a tool to provide localized information on the estimated emergence status of the two most important insects that transmit oak wilt in Iowa, based on a degree-day model.
Foresters, arborists or pathologists experienced with oak wilt can often diagnose the disease in the field using host species, symptoms and mortality patterns. Properly sampling suspect trees and culturing the samples in a qualified laboratory may be needed in some cases.
Watch this short video from Iowa State University on how to prepare a sample for their diagnosis lab.
Initial symptom is a subtle off-green color shift that may be visible in the upper portion of the tree. This my first appear in mid-to late spring or throughout the summer. The leaves begin to wilt quickly from the top of the crown downward and have a bronzed look. Oak wilt is usually identified in red oaks by rapid leaf discoloration and wilting. The outer ring of vessels of diseased trees will be plugged with a brown substance that is often visible in cross sections taken in the spring after the year that a red oak completely wilted.
White oaks usually die slowly, one branch at a time, over one-to-many years. Leaf wilting and death happen in a similar way to that of red oaks, but these symptoms usually progress much more slowly in white oaks. Affected leaves show a pattern of discoloration somewhat similar to that of red oaks, but the pattern is more variable. Leaves develop yellow veins that eventually turn brown. Discoloration on leaves proceed from the margins to the base. Affected leaves fall from scattered, affected branches within the tree crown. The crown progressively (over multiple years) thins out until the entire tree is dead.
Forest Health Program Leader