Sample Collection Prep | Branch Sample Collection | Petiole Sample Collection | Shipping Samples

Brief Background of Oak Wilt Sampling

Oak wilt symptoms occur early in June and include wilting (droopy leaves and limbs), leaf yellowing and/or bronzing, and defoliation. When suspecting oak wilt, it is important to confirm the diagnosis before implementing management measures to protect other healthy oak trees in the area. Collecting the proper sample is key. Oak wilt samples must be handled with care and mailed refrigerated as quickly as possible, as the pathogen responsible for this disease cannot survive hot weather.

All samples must be sent with a complete sample submission form. Provide as much background information as possible, including but not limited to: 

• the date when the symptoms first appeared
• the pattern and distribution of the problem on an individual tree and any neighboring trees of all species
• general weather history in your area, going back several years (trends of drought, excessive rains, high winds, etc.)
• pictures are extremely helpful. See our guide for how to take pictures of plant health problems.

Oak Wilt Sampling Instructions

The accuracy of laboratory diagnostics depends on the quality of the sample. The following checklists are provided as a guideline for sample collection (branch and/or petioles) and packaging/shipment. Adhering to these guidelines will improve the reliability of diagnostic results. 

Sample Collection Prep

• Observe symptoms, look for the symptoms (wilting, yellowing, and/or bronzing).
• Collect the sample only when you can either deliver it directly to the Clinic or send it overnight (never send on a Friday!).
• Prepare a styrofoam or plastic chest and ice packs (see example below). Refrigerate the sample (from collection to delivery). The pathogen that causes oak wilt cannot survive in hot weather. If the samples are left in the truck or on the house porch for as little as a couple of hours, the chances of recovering the pathogen decrease significantly.

Branch Sample Collection

To obtain a good branch sample for oak wilt testing, a certified arborist may be required, depending on how accessible the affected branches of the tree are. Use the Find and Arborist Tool from treesaregood.org to find an arborist near you.

• Select branches where symptoms are developing. Collect 3 to 6 living branches from the symptomatic area of a tree. Branches should be at least 1 inch in diameter and 6 to 8 inches long (we need branches, not twigs). 
• With a knife, peel the bark and inspect for vascular discoloration. When discoloration is found, collect additional branches from that symptomatic limb.
• Include leaves from branches with the sample, as sending symptomatic leaves (bronzing) is a bonus, as it gives important diagnostic information.
• Place branch pieces in a ziplock bag with DRY paper towels before placing them in the cooler.

Note: Species in the red oak group are more susceptible to oak wilt than white oaks. As such, confirming the pathogen can be particularly difficult in white oaks, given the tree's ability to limit the disease progression. Bear that in mind when sending samples. 

Additionally, tree paint to cover wounds is generally not recommended, except for oaks. Any pruning wounds when collecting samples should be promptly covered with latex paint to reduce the risk of attracting sap beetles carrying the oak wilt fungus from infected trees. 

Petiole Sample Collection

Researchers at Michigan State University confirmed that the oak wilt fungus can be recovered from leaf petioles, specifically from red oaks and chestnuts. Thus, petiole sampling is a reliable method only for red oak species. Below are guidelines for collecting petiole samples for leaves, increasing the accessibility of testing without assistance from an arborist.

• Collect 15-20 leaves from within 20 feet of the base of the tree trunk, as leaves can be blown around from other trees. Collected leaves should include an array of symptom expression, including: 
  • asymptomatic, completely green leaves
  • wilting leaves with bronzing (browning) starting at leaf margins, progressing toward the base of the leaf, less than 50% brown
  • chlorotic (yellowing) leaves
• Avoid collecting leaves that are completely brown or dry
• Freshly fallen leaves are ideal
• Leaf petioles should be intact for submission
• Place leaves in a plastic ziplock bag with DRY paper towels or newspapers. Never add moisture to leaf samples.

What is a petiole? The "stem" of the leaf is called the petiole. Leaf samples that have petioles still attached are critical for laboratory diagnostics. 

Shipping Samples

• Place your completed submission form in a bag to prevent it from getting wet from the ice packs.
• Place your sample bag(s) in the styrofoam cooler, along with ice pack(s). Seal with packing tape. 
• Ship the package early in the week via overnight delivery. Do not send on a Friday. 
• Walk-ins for physical dropoff are welcome!

See the Oak Wilt Encyclopedia page for more information on this disease, including prevention and management. 

Still have questions? No problem! Contact the Plant & Insect Diagnostic Clinic via phone (515-294-0581) or email (pidc@iastate.edu).