By Bev Johnson

Master Gardener

Bunkey is discouraged. It seems as though every tree in his yard is subject to some killer disease. Maples will have a problem if we have a dry summer since we had an “open” winter, little or no snow. Maple’s roots are quite close to the surface of the soil, so they suffer from freeze drying if the soil is dry. They start dying from the top down.

Ashes have Ash borer; Elms have Elm borer, and our oaks are at risk from the Two Lined Chestnut borer. Oak wilt has been found in Otter Tail County but does not appear to be widespread. Oak wilt can kill red and pin oaks in a few short weeks, while Bur and white oaks take a little longer.

Oak wilt is a fungus that causes infected trees to plug their own water transport system in an attempt to limit the infection. The tree then wilts and dies. Any insect that visits an infected tree can spread the infection. In the spring, an insect will land on the spore pad produced by the infected tree, then fly to another oak that has an open wound, thus infecting that tree too. Spring wood has larger openings in the wood than later in the season, so the fungus has easy entry into the tree. The infection can spread as far as 20 to 30 feet in a year through connecting roots. Trees are noted to be able to communicate through their roots.

In red and pin oaks, wilting starts at the top of the tree. The leaves turn a dull green then bronze starting from the tip to the base of the leaf which then falls to the ground. Bur oaks do it a bit differently. They have wilting branches here and there throughout the tree. They are a tough tree. Some of them can live for several years with the infection and some can even recover. Unfortunately, pin and reds aren’t so lucky.

If Bunkey suspects he has Oak Wilt and not drought stress, construction injury or Two Lined Chestnut borer, the Plant Disease Clinic at the University of Minnesota can give him a definitive diagnosis. Wilting branches infected with Oak wilt will have brown vascular discoloration. Collect samples with this discoloration, 6 to 10 inches long and at least ½ an inch in diameter from a wilting branch. Keep the samples cool until you can send them to the clinic at 495 Borlaug Hall. 1991 Upper Bradford Circle, St. Paul MN 55108-6030. There is a charge.

If your oaks are dying, there is only one solution, cut them down, preferably before March 15 next year. Live, but diseased trees are producing spores that can easily infect surrounding uninfected oaks. The logs need to dry quickly so cut them into short lengths. If you are going to use them for firewood, cover the woodpile with 4-mil clear plastic until next spring. Spores rarely form on white or bur oaks so that wood pile doesn’t need to be covered if you are using it for firewood.

Healthy oaks should be pruned in late fall or winter. Birches and Maples when they are fully leafed out. Apple trees should be pruned in February to early March. Unfortunately, we usually have a good cover of snow then. Bunkey really looks forward to hauling a 6-foot ladder through the snow drifts to prune apple trees. He lost 2 hand pruners one year. He later found one of them with the lawnmower. Oh well, he needed new mower blades and the pruners had gotten pretty dull.