Tuesday, August 3, 2010

Update on Thousand Cankers Disease

The following is an excerpt from an email sent to several listservs by Whitney Cranshaw regarding a new confirmation of Thousand Cankers Disease in Tennessee.
"Thousand cankers disease of walnut has been confirmed from Tennessee.  This was first identified about 10 days ago with a sample we received from Knoxville, but a formal announcement has been pending per the Tennessee Department of Agriculture's wish to delay announcement for further confirmations.  As this has been done, and I see an AP report on the internet, I think that we can now let it be generally known. 

The known infestation is in and around Knoxville.  As this is a fairly recent find, the extent of the infestation has not been delimited.  But the extent of the infestation suggests that this disease (i.e., the walnut twig beetle and its associated fungus, Geosmithia morbida) has been there for a decade or more.

This is a disaster of tremendous proportion.  It had been my deepest hope - clearly a naive hope - that this problem would stay bottled up in the western states where black walnut, Juglans nigra, is planted but not native.  I had long ago given up that black walnut would survive in the West and that is a sad situation, but not tragic.  Now that it is irrevocably established in the center of the native distribution of Juglans nigra, there are no geographic/ecological barriers to prevent its ultimate spread throughout the US.  Furthermore, the fact that the disease appears to be progressing as a lethal tree killer in Tennessee as it has been doing for 10-20 years in the Rocky Mountain States answers the question as to whether this is a regional problem.

It also suggests that there may well be many other infestations in the Midwest that have gone undetected.  This is a very difficult disease to detect in early stages.  Apparently in Tennessee, as in our area, what attracted attention were plantings that showed symptoms of apparent drought stress.  But it is not drought stress nor related to drought.  For some help with this situation we have a web site:  http://www.colostate.edu/Depts/bspm/extension%20and%20outreach/thousand%20cankers.html.  There are sheets on diagnosis, Q & A, and a fact sheet (that needs a bit of updating).  Also available are links to pictures and powerpoint talks, including the version of "Nightmare on Walnut Street" that I presented at the ISA meeting last week in Chicago.

I am assuming that there will now be a scramble to have state quarantines become a reality.  As I understand it, following Missouri's lead, that Nebraska, Kansas, Michigan and Indiana have or are in the process of enacting state quarantines that restrict movement of certain Juglans material that originates from TCD-affected states.  And this disease is a deal breaker.  It is relatively slow to develop, at least compared to DED or EAB, but its progress will be inexorable.  My guesstimate from watching it in urban settings is that about 30 years after this is introduced into a city, all the black walnuts will be dead.  That is based on the disease taking 10-20 years to show symptoms after the initial point infestation and 10-15 years for it to progress across a city once the first symptomatic tree is detected.  How this disease will progress where there are native stands affecting the epidemiology will undoubtably change things in ways we will all have the unfortunate chance to see in the upcoming years. 

However, containment/slow the spread is still something we need to throw ourselves into.  The longer you can delay the introduction of the disease (by movement of fungus contaminated walnut twig beetles), the longer your black walnuts can survive.  Perhaps with dedicated effort we can push back the ultimate effects of this disaster for a generation or two in many areas, giving us valuable time to develop means of managing it and finding resistant cultivars."

Posted by Whitney Cranshaw, 8.2.2010

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