Oak wilt

The best sampling procedures for accurate oak wilt testing.

 

As of December 1, 2021 our lab will no longer be accepting oak samples for oak wilt testing. We will resume oak wilt testing in the spring of 2022.  Oak wilt samples are time intensive to process; we are taking a break to allow us to focus on other responsibilities including teaching and presentations at various winter trade meetings. 

 

Effective oak wilt management starts with confirmation of the disease.

Oak wilt is a fungal disease caused by Bretziella fagacearum (previously known as Ceratocystis fagacearum) and it is widespread in the state of Michigan (see Figure 1). The fungus moves through, and eventually blocks the water conducting system (xylem) in the tree leading to wilting leaves and eventually tree death.

 

Oak_wilt_distribution_map_2
Figure 1. Diagnostic results of samples submitted to MSU Plant & Pest Diagnostics from 1/1/2015 to 12/21/2020, showing the widespread distribution of oak wilt. Center for Environmental and Regulatory Information Systems (CERIS). Purdue University. "NPDN data for Oak Wilt - Bretziella fagacearum”. Retrieved from https://npdn.ceris.purdue.edu/pest_map.php.

If you suspect oak wilt, here are some pointers on how to collect and submit a good representative sample of the oak tree. Poorly collected samples and/or samples arriving in bad condition cannot be processed for testing because the test will not detect the fungus, even if the tree is in fact infected. This is what we call a false negative result.

To reduce the risk of false negative results, keep in mind these important diagnostic facts:

  • The pathogen is found in the outer sapwood of the tree because oak wilt is a vascular disease.
  • Sample quality is critical for the success of laboratory testing.
  • Effective management of oak wilt depends on accurate disease diagnosis.
  • Samples can be collected from branches (preferred) or the trunk (called bole sampling).

MSU Plant & Pest Diagnostics will accept two different types of samples for oak wilt testing. Both require live tissue:

  • Branch samples. This type of sample is preferred (see Figure 2).
  • Bole samples. This is a destructive sampling method (see Figure 4).
A sample of 3 branches submitted to the lab.
Figure 2. An adequate oak branch sample submission: Three different, live branch sections of about 9 inches long and 1 inch thick. Photo by Laura Miles.

Branch sampling guidelines.

Submitting branches for testing is highly recommended, but it can be challenging to achieve when the canopy is too high to reach.

  1. Whenever possible, select branches with symptomatic leaves. Including some leaves (preferable in a separate bag) along with the branches allow us to better assess the expression of symptoms in the oak tree. There are other diseases and environmental stressors that can resemble oak wilt.
  2. Ideally, an adequate oak wilt submission should include three (3) sections from different and recently wilted branches that are at least 6 inches long and 1-2 inches thick (see Figure 2). Smaller branch sections are acceptable for testing but pieces of less than half an inch thick will be rejected.
  3. Remove a small portion of bark and inspect the section of exposed sapwood:
    1. The sapwood should feel moist to the touch. Dead branch sections are dry and have little weight.
    2. The inner bark (vascular cambium) should still be green (green arrow in Figure 3).
    3. The presence of vascular streaking (sapwood discoloration) is desirable as it increases the chance of pathogen detection (see branch on the right side of Figure 3).
  4. Place oak branches in a plastic bag and keep cold prior to shipping or sample drop-off.
  5. Do not add wet paper towels or any form of extra moisture to the plastic bag.
  6. Fill out a Sample Submission Form.
Oak_wilt_branch_streaking_2_arrow_small
Figure 3. Inspect the sapwood by removing a small bark section. Inner bark should appear green (arrow). Vascular streaking (sapwood discoloration) is desirable, as observed in branch on the right. Photo by Laura Miles.

Bole sampling guidelines.

Bole sampling is only recommended when the canopy is too high to reach, and the health of the oak tree is already compromised. Taking a sample from the bole (trunk) wounds the oak more severely.

  1. Remove all the bark from a section of the trunk to expose the sapwood. A hand axe works well for this task.
  2. Check the sapwood for discoloration. If none is observed, remove the bark from another section of the trunk.
  3. With a chisel and mallet (or your tools of preference), collect a strip of discolored sapwood (see Figure 4). The strip should be at least 2-3 inches wide and 6-7 inches long.
  4. Place the strip of sapwood in a plastic bag and keep cold prior to shipping or sample drop-off. Do not send bark pieces to the lab.
  5. Do not add wet paper towels or any form of extra moisture to the plastic bag.
  6. Fill out a Sample Submission Form.
Oak_wilt_bole_sampling_arrow_small
Figure 4. Some discolored trunk sapwood was exposed (arrow) after removing the bark. A strip of sapwood is being collected for oak wilt testing. Photo by Julie Stachecki.

 

The Do’s and Don’ts for testing

Do collect live branches or trunk sapwood.

Check sapwood for moisture and discoloration. Plant material should feel moist to the touch, not dry. The presence of vascular streaking (sapwood discoloration) increases the chances of pathogen detection.

Don’t send dry branches or bark pieces.

Even if the tree has oak wilt, we cannot detect the pathogen from these types of plant material. Inadequate samples can produce a false negative, which is a negative result when in fact the tree has the disease.

Do place tree sample in a plastic bag.

This helps retain the naturally occurring moisture in the sample. Also, keep samples cold prior to sending them to the lab. Include some leaves for symptom reference.

Don’t ship samples on Friday.

Samples are not delivered to campus on weekends and may not arrive in the lab in good condition the following week. Remember not to add extra moisture to the plastic bag.

Do fill out a form and place it in a separate bag to keep it clean.

Make sure to write the county where the sample was collected and the type of oak it comes from (for example, red oak, white oak, black oak, bur oak, etc.).

Don’t put the form inside the same bag with the plant material.

Dirty and wet forms are hard to read. If possible, put the form in a different bag, this will prevent it from getting damp or soiled.

Inadequate oak samples

The following are examples of samples submitted to the lab that do not meet the minimum quality standards for testing. 

Dead pieces of trunk

Dead pieces of trunk.

Pieces of trunk

Pieces of tree bark.

End of a branch with asymptomatic leaves

The end of a branch (very thin, less than half an inch). Attached leaves do not show disease symptoms.

Sample fees for oak wilt testing

Please note, oak samples are charged differently depending on the place of origin.

Description

Fee per sample

In state (Michigan)

$45.00

Out of state

$65.00

Additional information

Please contact MSU Plant & Pest Diagnostics at pestid@msu.edu or 517-355-4536 for questions and further clarification on sample collection and/or shipping.

For more information on oak wilt, read: