Growing Concerns 592: Fatal wilt disease and pruning for oaks


Protecting oaks from fatal wilt disease

Dear Janet,

 We are getting a large dead oak in the front yard taken down. The tree died in the summer of 2002 and for various reasons it was not removed sooner. Anyway, since we have a fireplace we would like to keep the wood to burn. Is if there is a way to tell if the tree died from oak wilt or just the stress from building our home? Unfortunately the contractors weren't real careful around the trees and I know some of this tree's roots were lost when the septic field went in.

I've read information on line from the USDA as far as symptoms of oak wilt and such, however when the tree died I wasn't really aware of the disease. The USDA indicates that once a red oak gets the disease it defoliates rapidly. I know this tree did not drop its leaves until the following winter when all the other oaks did. I'm not sure if this is a red or white oak. We do have both on our property.

Because we have other oaks around us I don't want to jeopardize their health. What I've read indicates that wood from an oak wilt dead tree should be covered with plastic with the edges buried for 6 months to kill the disease.

Is there any way to tell if this tree died from oak wilt and am I going to have to do more than just stack the wood and wait to burn it in my fireplace?


Dear L.,

Construction is a tree killer but you're right to be concerned about oak wilt (Ceratocystis fagacearum), a fungus disease that's on the increase. It quickly kills oaks in the red oak group (black-, red-, and scarlet oaks, which have leaves with sharply pointed lobes) and causes loss of limb and sometimes death in the white oak group (including white-, swamp white- and burr oaks, which have leaves with rounded lobes).

The fungus kills by invading, proliferating within and plugging water-conducting tissues in a tree. Leaves brown, wilt, then die, usually between late May and August, and many leaves drop early. Branches die. An infected white oak may simply lose limbs, might decline and die over years, or might survive as a symptomless carrier of the disease. A red oak, even a very large one, is likely to wilt and die in just a few weeks. 

The disease moves through natural root grafts between an infected tree and nearby trees of the same species, or by spores moved by beetles from under the bark of a dying or dead oak to wounds and weak places on other oaks.

The fungus may be active in dead wood for a year, and in roots for up to 4 years. That's why it's strongly recommended that dead oaks be removed at once, the wood chipped very small, burned or covered to prevent insects getting into it, all before the next growing season when insect movement and root growth can spread the disease further. 

There is no cure for a tree infected with oak wilt so when an oak dies suddenly or has other symptoms of this disease it's wise to protect nearby oaks. That involves prompt removal of a dead tree, trenching three to four feet deep halfway between that tree and adjacent trees of the same species to break root connections, and, in some cases, treating nearby trees with fungicide injections.

Your two-year dead wood can probably be stacked as firewood but for the sake of your other oaks, call in a certified arborist. Request an assessment of the situation and specific recommendations for the remaining oaks.

Short reports

Oak pruning time begins after killing frost.

The smell of fresh cut oak attracts various beetles that may bring with them the spores of the deadly oak wilt disease. The only safe time to prune oak trees is between late November and late March. Pruning cuts made then have time to dry and seal over before disease-bearing beetles begin to fly again.

Still have potted perennials, but no time to plant them?

The root of a hardy plant is its least hardy part. In a pot with its sides exposed to the weather, lacking the insulation of soil, even the hardiest perennial can die over winter. Move pots into an unheated garage, group them on the north side of a building and bury them in a leaf pile, or dig a trench into which to sink the pots, then fill around them with leaves.


Green thumbs up

to washing your windows, for healthier houseplants and lighter spirits during short winter days.


Green thumbs down

to the funereal look of burlapped evergreens. If it has to be wrapped to survive, why have an evergreen there?


 Originally published 11/13/04