Denise Schreiber is the “Ask the Expert” columnist for Pennsylvania Gardener magazine, the infamous Mrs. Know It All of the Organic Gardeners on KDKA radio and author of Eat Your Roses.

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Be on the Lookout for Oak Wilt Disease
by Denise Schreiber    

Leaves of this oak affected by oak wilt look drought stressed.1

It starts off with a whimper. Perhaps you decide to prune a branch or two off of your pin oak because they are too close to the house. You found some dried green leaves laying in the yard and thought it was just drought and it’s only June. Or Mother Nature decides to show her force with a violent thunderstorm and knock some branches off your oak trees. The next year you notice that the crown of the tree has some open spots and some dead branches. The year after that most of the crown, if not all, is dead and bark is sloughing off the tree. And your prized oak now looks like it is destined for the fireplace. It might be oak wilt disease, which is caused by the fungus Ceratocystis fagacearum.

Oak leaves showing symptoms of oak wilt.3

Leaves from an oak-wilt-infested tree litter the lawn in summer.5

Once a tree is infected with oak wilt the only cure is pruning at soil height. If you missed the joke, it means cutting the tree down. It affects members of the red oak family such as pin oaks but can also affect members of the white oak family as well. There are a number of ways the tree is infected. One is root grafts where the roots of two or more trees become intermingled much like putting your fingers together and spreading them out. When damage to an infected tree occurs, it sends out a shockwave through the root system and sends the disease to the next oak tree. The only time this doesn’t happen is when the tree is completely dormant.

The fungus Ceratocystis fagacearum creates a fungal mat beneath the bark.2

If an infected tree is pruned during active growth, it gives off an odor similar to stale beer that attracts sap- and bark-feeding beetles. There are fungal mats that develop under the bark and as they enlarge, they crack the bark open and the odor attracts sap-feeding beetles called Nitidulids. They feed off the spore mats and the fungal spores cling to the insects’ bodies. As they fly off to feed on healthy oaks, they infect the new trees.

There are also oak bark beetles that can also spread the fungus. The adults breed under the bark of infected trees, lay their eggs and fly off to infect more trees. When the eggs hatch and become adults they are also covered with the oak wilt spores and spread more infection. The fungus can be spread up to a mile by the insects. There have been studies (but it has not been proven conclusively) that suggest other insects that may occasionally feed on oaks can transmit the disease through passive movement.

So how do you prevent your trees from becoming victims of oak wilt? First, think of your choice of tree. Is it the right tree for your property? Oaks are one of the majestic trees of the forest and aren’t necessarily suited for the average homeowner’s postage-stamp lot. However if you have one already growing, you should take care of the tree by properly watering and fertilizing the tree. Do not prune when the tree is growing so as not to attract the beetles during their active season.

If you suspect that your tree is infected, call a certified arborist to confirm a diagnosis. An arborist can make the recommendations of how to deal with the situation. If you have a couple of oak trees that probably have grafted root systems, the root graft must be broken by either a trencher or a vibrating plow. This is best left to experts and shouldn’t be attempted by homeowners since it requires going 2-4 feet deep to break the graft. Again this should be done during the dormant period.

Through root grafts like these oak wilt disease can be spread from one tree to the next.4

If the tree is sloughing off bark, all the bark and spore mats should be collected and put in the garbage. Do not compost them or put them through a shredder. Burn or bury them if possible. The wood can be used as firewood but it should not be transported out of the area.

The disease affects primarily oaks, but it has been known to also infect plantation-grown Chinese chestnuts and European chestnuts. It does not affect other trees such as maples.

As a matter of good horticulture practice, clean your tools after each use and in between plants with a 10 percent bleach solution. 



1. Photo by Paul A. Mistretta, USDA Forest Service, Bugwood.or
2. Photo by Fred Baker, Utah State University,
3. Photo by J. Hunt  Symptoms Photo by D. W. French, University of Minnesota
4. Photo by Ronald F. Billings Organization: Texas Forest Service
5. Photo by Steven Katovich, USDA Forest Service,



Posted: 04/21/14   RSS | Print


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