August 1999

Oak wilt and its look-alikes

The symptoms and signs of oak wilt and its look-alikes are listed in the following table.  The best method of distinguishing oak wilt infections from all the other types of oak problems is to take a sample and have it analyzed at the Mn Dept. of Ag Shade Tree Lab or the Disease Clinic at the University of Minnesota.

(The next section is excerpted from the brochure, ?How to collect field samples and identify the oak wilt fungus in the laboratory?.)

Sample quality is paramount to the success of laboratory testing, and results are only as reliable as the sample itself.  Improperly collected, stored, or shipped samples may result in a false negative.  False negative means the tree has oak wilt, but the presence of the fungus was not confirmed by laboratory testing.  Following the procedures outlined in this section will help to ensure sample quality and increase the likelihood of an accurate diagnosis.

1.  Select branches that are partially wilted, with symptomatic leaves progressing from the tip of branches inward to the trunk.  Be sure that branches are not totally wilted, dry, or dead.

2.  Collect symptomatic leaves from selected branches.  Be sure to package leaf samples separately from branch samples.

3.  Look for discoloration in the sapwood of partially wilted branches.  Discoloration may be evident in a cross sectional view of infected branches.

4.  Select samples from up to three symptomatic branches per single tree.  It is critical that the sapwood is moist to the touch, and the inner bark is still alive and green.  In the field, check for this by removing the outer bark on a small section of the branch sample.  Avoid collecting samples from the extreme tips of branches: this tissue wilts first and is often too dry for successful isolation.

5.  Branch samples should be at least 1 inch in diameter, cut into 6- to 8-inch lengths, and placed in large resealable plastic bags.  Small diameter branch samples are acceptable only if the sapwood is very moist, and samples have been kept cool.  Samples from each suspect tree should be labeled separately and places in individual resealable plastic bags.

6.  Keep samples cool during sampling, shipping and storage, but do not freeze.  Sample temperatures should never exceed 85 - 90 ° F.  Along with resealable plastic bags, be sure to take a cooler and ice packs (avoid ice cubes and dry ice) with you to the field.  Never leave samples unrefrigerated or exposed to direct sunlight.

7.  Provide background information about the tree(s) such as symptoms observed and when they first occurred; age of the tree; proximity to other trees and buildings; any damage caused by storms or other events.  Use data collection forms supplied by your local diagnostic laboratory whenever possible.

8.  Ship samples by overnight mail or deliver in person to the laboratory.  Samples should remain in resealable plastic bags and be shipped in a disposable ice chest with ice packs.  Mail samples early in the week to ensure arrival at the laboratory in ample time to culture the sample before the weekend, and to avoid sample delivery on the weekend.

9.  Stop collecting samples after annual leaf fall begins in the fall.  In Minnesota, the annual cut-off date for oak wilt testing is typically mid-September.  In the spring sampling can begin after the leaves have flushed.  If sampling is done during peak infection periods (late April, May, or June in Minnesota), wound dressing should be applied to all pruning cuts.

Oak wilt and its look-alikes
  Oak Wilt Anthracnose  Two-Lined Chestnut Borer Oak Decline
Host Range All red and white oaks, more common on red oaks Common on white oaks. All red and white oaks, especially associated with stress. All red and white oaks.
Causes Fungus

Ceratocysis fagacearum


Apiognomonia quercina, Discula quercina


Agrilus bilineatus

Multiple stressors including: insect defoliators (e.g. leaf skeletonizers and miners) fungi (e.g. Armillaria spp.) And abiotic (e.g. drought, flooding, fluctuating water table, construction damage)
Pattern of Tree Damage on Site Expanding pockets or isolated trees. New infection may be associated with tree wounding (construction, pruning, storm injury). General and widespread, particularly during prolonged wet periods in spring and summer. Often scattered, not necessarily in pockets. Associated with stress (defoliation, drought, construction, etc,). Scattered to widespread (depends on site factors).
Symptoms and Patterns of Damage on Tree Red Oak- fast(< 8 weeks) progression. Wilting and browning of entire crown. Heavy premature leaf fall. 

White Oak -slow (1-5 yrs) progression. Wilt and browning of individual branches. Light leaf fall. Both-Leaves brown from tip and margin inward; "water soaked" appearance of leaves. Sapwood discoloration.

Seasonal progression. Lower, inner portions of crown most heavily affected. Individual leaf spots anywhere on leaves. Spots coalesce into large, brown blotches. Leaf drop may occur if infection is severe. Slow to rapid progression of damage. Upper, outermost portion of crown affected first or individual branches. Leaf browning , without leaf fall. Dead branches from previous year's infestations visible year round. Insect tunneling - look for larval galleries beneath bark of the branches and main stem. Slow progression (2-5 yrs). Leaves may appear chlorotic, dwarfed or sparse. Foliage may be "tufted" in crown. Individual branches may be killed. Branch dieback common. Leaves not dropped.
Time of Symptoms Usually mid to late summer. Spring and early summer occasionally later summer and fall if wet conditions persist. Leaf browning in late summer/fall. Leaves are not shed early. Variable.
Impact on Host Red oaks killed quickly. White oaks die gradually, some may live. Defoliation renders trees unsightly as shade trees. Defoliation reduces vigor. If heavy damage occurs, trees may refoliate. Usually will not kill oaks in the absence of some other stress factors, especially drought, construction and defoliation. Often results in tree death.
Other Comments Overland spread by Nitidulid beetles. Local spread to neighboring trees is via root grafts. Red oak to red oak; white oak to white oak.   If stress persists, can cause tree death (1-5 years). Caused by a complex of factors, which may include: defoliation, tree age, site, drought, frost, Armillaria root disease, oak cankers, two-lined chestnut borer, etc.