Don’t prune oak trees until November to protect against invasive disease

Leaf discoloration caused by oak wilt. Many factors cause discoloured oak leaves, including drought.

Residents in northern Ontario are being asked not to prune any oak trees for the next several months to prevent the spread of a devastating tree disease that has made its way to Canada.

While its origin is unknown, oak wilt disease was detected in Mexico and the southern United States in the 1940s.

It arrived in Canada in 2023 on a residential property in Niagara Falls, followed by confirmed detections in Niagara-on-the-Lake and the Township of Springwater, according to the Invasive Species Centre in Sault Ste. Marie.

“Ontario residents are urged to refrain from pruning oak trees during the no-pruning season, which runs from now until November,” the centre said in a news release Tuesday.

“Pruning or otherwise damaging oak trees during this time makes them especially vulnerable to oak wilt disease.”

The disease is caused by a fungus that disrupts the movement of water and nutrients through oak trees.

“The leaves of infected oaks can start to wilt and drop prematurely, eventually resulting in complete defoliation,” the centre said.

“All oaks are susceptible to oak wilt, but red oaks are particularly vulnerable and can die in as little as two to six weeks following infection.”

Oak wilt has also been confirmed in 24 U.S. states, including those close to Canadian borders such as Michigan, Ohio, Pennsylvania and New York.

Fresh wounds attract the disease

“The loss of oak trees can be felt in both urban and natural spaces as oaks are found in many backyards, parks, woodlots, and forests,” Colin Cassin, executive director of the Invasive Species Centre, said in the news release.

“To prevent the spread, we encourage people to avoid pruning any oak trees until November, which is when the sap beetles that transmit oak wilt are less active. Fresh wounds attract these beetles, increasing the risk of infection.”

If pruning must occur from April to November, or if a tree is injured, apply a thin layer of wound paint or shellac to the wound immediately.

Other steps to protect oak trees include not transporting firewood and monitoring the health of oak trees. Canadians are encouraged to report any suspected signs of oak wilt to the Canadian Food Inspection Agency.

Signs of the disease include wilting leaves starting from the top of the tree, discoloured leaves beginning on the edges and moving inward, white, grey or black fungus mats just under the bark that smell like fruit and vertical bark cracks in the trunks.

More information on the disease is available at the invasive species centre’s website.