Tom Butzler is a gardening columnist in the Lock Haven Express and also blogs in Gardening in the Keystone State. He is currently a horticulture educator with Penn State Cooperative Extension.

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Thousand Cankers Disease Arrives in Pennsylvania
by Tom Butzler    


Black walnuts are showing advanced symptoms of thousand cankers disease in this picture taken September 18, 2009. The tree died the following June
[1]


Canker development around a walnut twig beetle gallery in an English walnut.  [2]

Plant pathologists are usually not the most imaginative bunch when naming plant diseases. For instance, the rose disease caused by the fungus Diplocarpon rosae, causes a black spot on the foliage. This disease was given the name “black spot.” Another example is the fungal organism that causes a leaf spot on strawberry. In this instance, it was given the colorful name “common leaf spot.”

No doubt then, that the imagination was running in full tilt as plant scientists gave the name “thousand cankers” to a new disease on black walnut. It appears to be an appropriate name as one small canker will not kill a black walnut but thousands will.

The fungus, Geosmithia morbid, was unknown in the United States until black walnut trees started to die off in the Western U.S. Upon further investigation, scientists found numerous cankers underneath the bark that interfered with nutrient flows throughout the plant. Once the plant’s “plumbing system” was compromised, the tree died within several years.

But the discovery process yielded another interesting aspect to this disease story. Many of our common Pennsylvania fungal diseases, such as powdery mildew, botrytis and rusts use air currents to move from plant to plant. This fungus has a two-pronged approach: get into new areas and invade healthy trees via insects and man.


Exit wounds made by adult walnut twig beetles [3]

Thousand Cankers Disease (TCD) was discovered in New Mexico around 2001 and spread up the West Coast within a couple of years. This was accomplished with the fungus hitching a ride on the body of the walnut twig beetle, which is a native to the Western U.S. When the walnut twig beetle tunnels into a walnut twig, spores of G. morbid are introduced underneath the tree bark and start the infection process.

This is a very small insect and not known for flying long distances, so it appeared to be confined to the west of the Mississippi. However, a startling discovery was made in Tennessee last year when TCD was positively identified. It is highly unlikely that a walnut twig beetle flew in from the western U.S. with the fungus. Man probably had a role in this big leap into the East by transporting infected walnut wood or walnut wood products.

Tennessee placed quarantine on movement of all hardwood from that area and encouraged residents to not transport firewood within the state. Another big geographic jump was made when the disease was positively identified in Bucks County, Pennsylvania, this August. Again, it is highly likely that man played a role in moving this disease complex into the Commonwealth.

Black walnuts are not utilized as an ornamental tree but they are still valued by Pennsylvanians. According to the Pennsylvania Department of Agriculture, black walnut trees, which make up less than half of 1 percent of hardwood trees in Pennsylvania, produce high-valued lumber used in woodworking and furniture-making. The nuts of the trees are consumed by humans and wildlife.

There are several symptoms that might help a homeowner identify the problem. Initially, leaves start to yellow and the upper canopy thins out. As years pass and the disease progresses, larger limbs die off. At this point, you could peel some of the bark back and see cankers that extend throughout the tree and darken the wood. TCD only affects black walnut, so if your other trees are showing similar symptoms, check with a local horticulturist to determine the cause of thinning.

Since this disease began in the West, scientists have had a head start in researching possible control methods. Unfortunately, the prognosis is pretty grim. According to Colorado State University, “Effective controls for Thousand Cankers Disease have not yet been identified.” Like Tennessee, Pennsylvania has put quarantine in place to limit the spread of this disease outside of Bucks County.


The Black walnut
(Juglans nigra) native range, according to a map from the USGS.

 

 

Photo Credits:

[1] Photo credit:  Ned Tisserat, Colorado State University, Bugwood.org
[2] Photo credit: Ned Tisserat, Colorado State University, Bugwood.org
[3] Photo credit:  Whitney Cranshaw, Colorado State University, Bugwood.org

 

Posted: 10/03/11   RSS | Print

 

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