Oak wilt is an aggressive, tree-killing disease of oaks. It affects oaks in a manner similar to how dutch elm disease affects elms. The fungus enters the water-conducting vessels of the sapwood through fresh wounds or through roots connecting healthy and diseased trees.
When the fungus is in the vessels, adjacent cells develop balloon-like structures that extend into the infected vessels and plug them. This disrupts the sap flow in the vessels and the foliage wilts and falls. The disease is a threat to all oaks, but trees of the red oak group (red, black, pin and scarlet oaks) are killed more rapidly than trees of the white oak group (white, bur, and swamp white oaks). Little can be done to help infected red oaks. Tactics are available to help infected white oaks and reduce the threat of infection to noninfected oaks located close to infected trees.
Symptoms in Red Oak Group
Early foliar symptoms are wilting, bronzing and shedding of the leaves at the ends of branches in the upper crown. The symptoms can spread through the crown very quickly, often within a few weeks.
Bronzing begins at the outer leaf edges and moves toward the midrib. The boundary between the green and discolored areas is often not distinct. The leaves often wilt along with the discoloration. Both discolored and entirely green leaves fall from the tree in large numbers, but a few discolored leaves usually remain on the tree. As the disease progresses, fungus mats may be produced between the bark and sapwood the year after the tree dies. The fungus mats can exert enough pressure on the overlying bark to actually raise and rupture it. A fruity odor is produced from the mat that will attract sap beetles to feed on the mat.
Symptoms in White Oak Group
Symptoms in white oaks are much more variable than in red oaks. Symptoms may develop in the upper crowns of white oaks as with red oaks, but they do not spread as quickly. Symptoms are often restricted to one or a few branches at a time. Members of the white oak group are seldom killed outright as those in the red oak group are. Leaf discoloration occurs, but the changes are often more gradual than with the red oak group. Streaking of sapwood beneath the bark of infected branches is much more common on white oaks. Spore mats are not produced on the white oaks.
It is important to remember that oak wilt is commonly confused with drought, construction stress, borers, or root problems. The following items can help distinguish one from the other.
Drought, Construction Stress, Borers, or Root Problems
- More common during last half of summer
- Regular size leaves, little wilting
- Leaves browning uniformly
- Leaves remain on the tree after discoloring
- Dying trees scattered throughout stand
- More common on stressed sites
- Often with trunk sprouts
- Signs of borer or root disease
- More common during first half of summer
- Small leaves, thin crown, wilting
- Edges and tips of leaves turning color first
- Leaves drop soon after discoloring
- Dying trees found in groups (root grafts)
- Streaking/discoloration of vascular tissues