Oak Wilt Identification

Symptoms

Oak wilt causes leaves of infected oaks to wilt and fall off the tree. In fact, a good indicator of oak wilt is a carpet of leaves under an oak in mid-summer. Red oaks infected with oak wilt lose about 90 percent of their leaves in two months; bur and white oaks quickly lose leaves from a few branches at a time during the summer. Rapid leaf loss distinguishes oak wilt from two-lined chestnut borer infestation.

Leaves at the edge of the canopy are the first to wilt. The wilt then progresses toward the inner canopy. This pattern of disease distinguishes oak wilt from common leaf diseases such as anthracnose and bur oak blight, which in general are not a concern.

About half of infected red oaks produce fungal mats and pressure pads between the wood and the bark. These pressure pads have a fruity or wine-like odor, are oval and gray when fresh, and often reach two inches in length. Many times the pressure pads cause the bark to crack. The fungal mats produce the oak wilt spores.

Branches with wilting leaves sometimes have brown streaks in the wood beneath the bark. In cross-section, these streaks appear as a ring of dark spots. This symptom is not always visible, and dead branches cannot be used for this discolored characteristic because they discolor from other causes.

As the disease spreads through connected roots, a "mortality pocket" forms, with dead oaks in the center and dying oaks to the outside.

Identification - Laboratory Diagnosis

Oak wilt is not easy to identify without lab tests, particularly on white and bur oaks. Oak wilt in red oaks can also be challenging to identify without seeing disease progression over time. To confirm oak wilt, you can submit wilting branch samples for analysis to a laboratory. For accurate identification, your sample should be:

  • an actively wilting branch
  • one-half inch in diameter
  • kept cool until shipping
  • shipped overnight

Information on submitting samples can be found on the University of Minnesota Plant Disease Clinic website and on the University of Wisconsin Plant Disease Diagnostics Clinic website.

Fallen leaf showing oak wilt symptoms.Red Oak showing canopy die back.

Fallen leaf showing oak wilt symptoms.Fallen leaf showing oak wilt symptoms.

Crack from oak wilt pressure pads under the barkCrack from oak wilt pressure pads under the bark

Oak wilt pressure pads.Oak wilt pressure pads.

showing branch size and brown streakingBrown streaking in the sapwood of oak branches infected with oak wilt.