This site is for people who like plants -- growers, enthusiasts, aesthetes, novices and professionals, those who appreciate wild things and those who appreciate the cultivated. I garden in Chelsea, and I've been visiting people's yards for 20+ years in the course of my work. My goal is to make this blog a community project, so if you share my interests, please consider becoming a participant and contributing content -- Guerin. Info: guerinw@gmail.com

Wednesday, March 16, 2011

Walnut trees threatened by a thousand cankers

With 60 degree weather in the forecast, we hope to move onto happier flowering topics soon . . but in the meantime, here's one more heads-up about threats on the horizon. This one is called 'thousand cankers disease.'  It affects our common and valuable black walnut (Juglans nigra). It's been known from out west for about eight years, but was identified on August 5 of last year in Knoxville, Tennessee. This is the first time the disease has been found within the native range of black walnut.  Read on . . .

Here's a likely scenario:
An unassuming walnut twig beetle long made its living colonizing a species of walnut restricted to New Mexico, Arizona, and the state of Chihuahua. Then came development and the introduction of the eastern black walnut species into these western communities. The twig beetle finds it manages quite well on this new host, thank you very much, and its range extends up to Idaho and California. Along come a fungus, Geosmithia morbida, which hitches a ride on the beetle. It's not a particularly aggressive fungus -- it doesn't spread within the tree like the fungus that causes Dutch elm disease; instead it just kills a localized area of cambium under the bark.

The wee beetle, however, is doing fine. Striking repeatedly into the bark (35 times per square inch and more!), it leaves a bit of fungus behind each time.  The dead areas in the tree coalesce, the vascular system is disrupted, and the walnut tree is dead within three years. 

Cut to August 5, 2010 -- the beetle and the fungus and the disease are found in Tennessee, within the native range of black walnut. Entomologists, pathologists, and Extension agents are scared. They recommend that people DO NOT send suspected samples, just forward the address and they'll take a look on site. So far no controls have been developed to combat this disease.


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