Don’t trim oak trees to avoid deadly fungus

By: Linda Shepard | Rochester Post | Published May 25, 2016

 Oak trees spread their branches May 5 on Oak Street in Roseville. The trees are susceptible to oak wilt and should not be trimmed in spring or summer.

Oak trees spread their branches May 5 on Oak Street in Roseville. The trees are susceptible to oak wilt and should not be trimmed in spring or summer.

Photo by Linda Shepard


METRO DETROIT — Once the oak wilt fungus enters an oak tree’s system, an infected tree can be dead within a month. 

“Even the biggest trees,” said David Roberts, senior academic specialist at Michigan State University’s College of Agriculture and Natural Resources. 

Roberts has been studying the oak wilt fungus for the past 30 years. 

“It is devastating to local communities,” he said. “More devastating locally than the emerald ash borer.” 

The oak wilt fungus is spread by sap-feeding beetles that carry fungal spores to fresh cuts and wounds on healthy oak trees. It also can spread underground through interconnected root systems, Roberts said.

“Beetles can sense the sap from miles away and can be there in minutes,” Rochester Hills Forestry Operations Manager Gerry Lee said. “The beetles feed on the sap, and boom — now (the tree) is infected.”

Healthy oak trees should be pruned in only the coldest months of the year to avoid oak wilt. The sap-feeding beetles cannot chew through oak bark. Fresh wounds from pruning, cutting, construction activity or storm damage must be present for the insect to infect a healthy tree.   

Treatment for the fungus is expensive and usually ineffective.

“The best treatment is prevention,” Lee said. “The best prevention is not to wound oaks in the growing season, which is late March through early November.”

 Torn branches or roots should be cut clean and the cut surface painted.

“If you have to remove a limb, paint the wound with tree paint or latex,” Lee said. For additional protection, cover treated roots with soil. 

Oak wilt is a fungal disease that causes water-conducting vessels in oak trees to become plugged. Once the vessels are plugged, water movement within the tree stops, causing leaves to wilt and fall from the tree. Studies show that oaks are at the highest risk of oak wilt infection during spring and the first half of summer. 

Lee also warned against transporting firewood to avoid spreading the fungus. 

“Problems are amplified by firewood transported to a new location,” he said. “Buy your firewood where your campsite is. Burn it where you buy it. It is important that people know this.”

Roberts said red, black and pin oaks are highly susceptible to oak wilt. White and bur oaks are less susceptible. 

White and red oaks can be differentiated by their leaf shape. 

“Red oak leaves are pointed, with bristles at the tips of the lobes,” Lee said. “White oak leaves have rounded tips. With red oaks, by the time you see the (oak wilt) symptoms, it is probably too late.” 

Oak wilt can trigger litigation, Roberts said. 

“People are suing tree trimmers and neighbors are suing neighbors,” he said. “One person will have a tree trimmed, and (the fungus) goes underground and moves through a community or a forest.” 

Roberts said he was involved in a court case addressing the issue several years ago. The suit concluded with a settlement of more than $60,000 for containment and eradication of the problem. 

Roberts said the oak wilt fungus was first identified in the 1930s and 1940s in the United States.

“It is believed that it came here from Central America in the 1800s,” he said. 

Oak wilt disease has been detected in Michigan for many years, he said, but outbreaks in new areas are a new cause for concern.

“We see more and more of it every year,” Roberts said. “It is all over and most people don’t know about it.”