Wednesday, July 9, 2014

Say Goodbye to Our Black Walnut Trees

Photo from Google Street View
  • Post by Dan Hilburn
Next time you drive out Center Street in Salem take a look at the black walnut trees on the State Hospital campus. Those big, beautiful trees are doomed. If you look up, you can see the tops are dying. Within a decade, they’ll all be dead and gone. The same fate awaits other black walnuts in Oregon. The killer is a fungus that causes thousand cankers disease (Geosmithia morbida) and it’s vector, a tiny insect called walnut twig beetle (Pityophthorus juglandis). This is an invasive species story with a twist – both the fungus and the beetle are believed to be native to North America, though not to Oregon.

Until recently walnut twig beetle was known only from the Southwest where it lived in harmony and obscurity with its host, Arizona walnut.

Until recently, the walnut twig beetle lived in obscurity with its host, the Arizona walnut. 
The beetle expanded its diet to include non-native walnuts.
The killer fungus came along too.

Unexplained die-offs of black walnuts in the mid-West in the early 2000’s were initially blamed on drought and/or the walnut twig beetle. Eventually researchers noticed large numbers of dark cankers under the bark of dead trees. And that led to the 2008 discovery of the associated fungus. The researchers named the fungus thousand cankers disease.

The walnut twig beetle, a type of bark beetle native to the southwestern United States and parts of Mexico, is the only confirmed vector of the pathogen. Apparently, it adjusted to non-native black walnut street trees and expanded its range throughout the West. The fungus hitchhiked with the beetle.

No one has yet figured out a control or management strategy that works. Severe pruning and burning of infested branches may slow the disease, but eventually, even big, healthy black walnuts succumb. Neighborhoods with black walnut street trees are going to look bare when they are removed.

It is sad that we’re going to lose some street trees in Oregon, but the real tragedy will be in the East where black walnut is native and treasured for it’s high-value wood. Several states have enacted quarantines to lessen the risk of human-aided introduction. The biggest threat is movement of infested logs and firewood.

The biggest threat is people moving infested logs and firewood.
If the logs or firewood are put on a truck and moved to a mill, buyer or campsite, the beetles can emerge in a new place and Thousand Cankers Disease jumps ahead like a spot fire started by a spark.

When trees die, people naturally want to sell the trunks and cut up the rest for their fireplaces. If the logs or firewood are put on a truck and moved to a mill or a buyer somewhere down the road, the beetles can emerge in a new place and thousand cankers disease jumps ahead like a spot fire started by a spark. Since it takes several years from infection to the first symptoms, by the time trees start dying no one will remember the shipment of infested wood that inoculated the neighborhood.

For us in the West, its a story of a native beetle/fungus with an expanded range attacking non-native street trees. The take home lesson is that firewood should be bought and burned locally. Don’t take it with you when you go camping out-of-state. The same principle applies to logs. When mills and kilns are local, pests and diseases are less likely to hitchhike to new areas.

I used to have a black walnut overhanging my driveway. The falling nuts were hard on my car and messy to clean up so I wasn’t sorry when the power company cut it down. Black walnuts are the sort of tree you enjoy if it is across the street and belongs to someone else, like the State Hospital -- I’ll still be sad to see them go.

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