Calling anyone who deals with oak trees including nurseries, garden centers, homeowners and tree professionals! Let this be your friendly reminder: now that April is upon us, you should hold off on pruning your oaks until November to protect trees from oak wilt.

As the pathogen that causes oak wilt (Bretziella fagacearum) is now present in the Ontario environment, this is a reminder that prevention is your best form of defense against oak wilt. All ages and size of oak trees are susceptible to oak wilt.

For both nurseries and oaks in the landscape, the greatest risk of transmission of the pathogen to healthy trees is via pruning cuts or open wounds in trees. Infection can also happen via root grafts to healthy trees in close proximity to infected trees.

Pruning cuts and wounds are attractive to nitidulid beetles (commonly called sap beetles). These small beetles are active from April – November and can carrying fungal spores of the oak wilt pathogen. Should the beetles be carrying oak wilt fungus spores (from landing on an infected tree with pressure pads) and then land on open wounds or pruning cuts on healthy trees, the spores are then able to enter the tree and it becomes infected. At that point the fungus develops in the sapwood inhibiting the transport of essential water and nutrients and results in rapid tree mortality.

Steps you can take:

• DO NOT PRUNE OAKS between April – November: Winter pruning should be the new normal
• Familiarize yourselves with the symptoms of oak wilt
• Should you encounter suspected oak wilt, you can contact a certified arborist for a second opinion and REPORT it to your local CFIA office and wait for further instructions. DO NOT try and remove the tree(s) on your own and DO NOT try to collect your own samples.

Trees that must be pruned during the growing season, for reasons of storm damage or otherwise, should be immediately treated with a tree wound dressing or latex-based paint to seal the opening. It is very important that the fresh cuts or wounds be treated immediately because the insects that carry spores of the pathogen are often attracted to these wounds within a very short time. Tree paints are normally not recommended for general use, but using these products in this situation can protect trees from oak wilt.

If you would like more information on Oak Wilt or want to refresh your understanding, there are lots of great resources online including:
The CFIA webpage: Click here
The Invasive Species webpage: Click here
And this printable poster from ISA Ontario: Click here