Local chestnut grower discovers oak wilt is killing his trees

October 2, 2023

Pete Conrad stands next to a dying chestnut tree.

Local chestnut grower discovers oak wilt is killing his trees

Story and photos by Rob Alway, Editor-in-Chief

AMBER TOWNSHIP — Pete and Connie Conrad have been growing chestnut trees commercially for the past 20 years. Currently, the 520-acre fifth generation farm includes about 15 acres of chestnuts. In 2021, Pete noticed one of the trees in his north orchard, located on the northeast corner of Stiles and Conrad roads, was dying. 

“The leaves started falling off this tree,” Pete said, standing over the stump of what he calls “ground zero.” 

A view of the orchard from the air showing the infected area, cleared, in the center.

“From time to time we have trees die,” he said. “So, I did’t really put too much thought into it at first. This particular tree, at the time, was 20 years old. The leaves started falling off then it died. In the early part of June 2022 I had four more trees that were dying around this first one. It was pretty clear that this was spreading. So, my question became, was this something spreading through the air or by insects, or something else?” 

Conrad, who holds a bachelor’s degree in agriculture from Michigan State University, reached out to various experts who began hypothesizing. 

The original speculation was that the trees had been infected by armillaria, fungus that lives on trees and woody shrubs. 

“Armillaria happens in forests and lingers for many years,” Conrad said. “It is very prominent locally with cherry trees. This land did used to be forest and it was a strong possibility that that was the cause. It’s difficult to identify armillaria immediately. We also looked at root rot.” 

Conrad had another theory. 

“I watched a video from a chestnut farmer in Ohio who was giving a tour of his farm,” Conrad said. “He was explaining that several trees there had died of oak wilt.” 

The symptoms seemed to match Conrad’s research. 

“Chestnut trees are cousins of oak trees,” Conrad said. “Could this have been the cause?” 

According to Michigan State University, oak wilt primarily affects trees in the red oak group (Lobatae). Red oaks typically die within a year of infection. White oaks can be infected but generally do not succumb to the disease. Both oaks and chestnut are members of the family Fagaceae.

Oak wilt can spread two ways. Overland spread occurs when tiny sap beetles, also known as picnic beetles (Family: Nitidulidae) pick up viable fungal spores from an oak that has recently died from oak wilt, then feed on sap oozing from a wound on a live oak. The wound must be deep enough to penetrate the sapwood.

When beetles move from a tree infected with oak wilt to a wound on a healthy tree, the spores can establish a new infection. The fungus grows into and eventually plugs up the xylem cells (sapwood) the tree uses to transport water. This causes the tree to wilt and die. Oaks infected in the spring can die within a few weeks. Trees infected in the fall may not die until the following year.

Beetles are most likely to be carrying spores in late spring and early summer.

Another way the disease spreads aboveground is through the movement of wood from oaks killed by oak wilt into new areas. Mycelial mats under the bark of a recently killed tree have a sweet odor that attracts sap beetles. When the tiny beetles visit the mat, they pick up the fungal spores that can then be introduced into a wound on a live tree. Mycelial mats can be produced on firewood or logs from trees killed by oak wilt during the previous year. Dry wood or completely debarked wood does not contain enough moisture for the oak wilt fungus to survive and is safe to move.

Conrad decided to start taking precautionary steps to stop any further infection. 

“One of the most practical ways of management is through trenching,” Conrad said. “I found that trenching to five feet deep can be effective. It’s necessary to severe the roots that are interconnected between trees.” 

Before he had a solid answer to his mystery, Conrad started with a trench that was 200 feet diameter from ground zero. 

“I felt at least I was doing something,” he said.

Then, in September of 2022, the next group of trees within the trench began dying. 

Conrad reached out to his alma mater, Michigan State, where he contacted Dr. Monique Sakalidis, an assistant professor in the Department of Plant, Soil and Microbial Sciences. Sakalidis gave Conrad instructions on how to obtain a sample, cutting a small portion of infected branch. He then hand-delivered the sample to East Lansing. 

Pete Conrad inspects a tree.

“Within 24 hours MSU determined that it was indeed oak wilt.” 

Conrad said more trees began dying within the original 200-foot trenching zone this past spring. Then, some trees outside the zone began dying. At that point, he took more measures to stop the infection. He hired a contractor who came in with a vibrating plow with a five-foot blade. This time, they trenched around 100 trees, 11 cuts north to south and 16 cuts east to west. 

“This is the best we can do,” Conrad said. “We have put those trenches far enough away and I am fairly confident that we stopped it at one of those trenches. But, we’ll find out.” 

According to the MSU case study on Conrad’s farm, if the root rupture/trenching efforts are deployed too close to affected trees, the fungus can continue to spread. There is no other management of the oak wilt. The sap beetles that vector the oak wilt are native and tiny, and many species are present in the environment. 

“It is not practical to prevent beetles from visiting wounded trees using insecticide treatments,” the MSU case study states. “The key to preventing oak wilt establishment is to avoid wounding trees from mid-April to mid-August to limit the risk of oak wilt transmission. This is when the mycelial mats are most abundant and contaminated sap beetles are most active.”

MSU also recommends trenching as a means of preventing the further spread of transmission, likely based on Conrad’s methods. 

Conrad Farms’ origins date back to the mid-1880s, making it one of the oldest, continuously operated farms in Mason County. Pete Conrad is the fifth generation of Conrad to farm in Amber Township. He and Connie, live on Conrad Road just about a mile from the original farmstead, which is now owned by his nephew. 

Conrad Farms still grows crops, but most of those commodities are contracted or leased. Conrad Farms is part of a cooperative of growers, called Chestnut Growers, Inc., that is made up of over 30 farms from throughout the Lower Peninsula. The co-op sells its chestnuts to individual grocers, distribution centers for chain grocery stores, and restaurants.

There are four main species of chestnuts: American chestnut, Japanese chestnut, Chinese chestnut and European chestnut. Conrad’s trees are hybrids, a cross between the Japanese and European varieties. Harvest takes place in the fall. 

To date, oak wilt has killed 30 trees. Conrad said he is hoping that it will stop with 40. 

“I want other growers to be aware of oak wilt and the threat it causes to chestnuts,” Conrad said. “I know that there are many people who have chestnut trees on their properties and they need to be on the lookout for the signs and symptoms. If there is any suspicion of oak wilt, any signs of the trees dying, I would highly recommend they reach out to Michigan State right away and get the trees tested.” 

As for replacing the trees, Conrad said that oak wilt can only survive on live tissue. “I will have to wait at least three or four years before I can replace the trees,” he said. “I had been working on thinning the orchard because of many of the older trees canopying too close to each other, which can cause less production. My hope was to cut out every other tree, not an entire section of trees like this.” 

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