Green Horizons

The lighter shaded portion of the country in this graphic notes walnut’s native range.

In the last issue of GH we talked about a serious new pest, Thousand Cankers Disease (TCD), threatening eastern black walnut planted as ornamental trees in the western United States. And we hoped it would stay out West… at least for awhile. Unfortunately, this past August, foresters found both the walnut twig beetle and the fungus responsible for TCD in a Knoxville, Tenn., suburb. Since the initial find, the disease has been found in four counties around the greater Knoxville area.

Here’s the kicker. Based on the time it takes from the initial attack to when the tree shows any visible symptoms, our tiny little "friend" probably made its way east some eight to 10 years ago!

Of all the eastern states comprising black walnut’s natural range, Missouri has the largest number of walnut trees growing in forest conditions; more than 55 million trees. That is double the number found growing the next two closest states of Kentucky and Ohio. According to Missouri Department of Conservation estimates, the disease could cost the state more than $850 million over a 20-year period due to losses in the wood products industry and nut production as well as costs associated with the removal and replacement of urban trees.

The Missouri Department of Agriculture (as well as departments of agriculture in Indiana, Kansas, Michigan, Nebraska, North Carolina and Oklahoma) has enacted a statewide quarantine that bans the import of "all plants and plant parts of the genus Juglans including but not limited to nursery stock, budwood, scionwood, green lumber, and other material living, dead, cut, or fallen, including logs, stumps, roots, branches, and composted and uncomposted chips" from the western states of Arizona, California, Colorado, Idaho, Nevada, New Mexico, Oregon, Washington, Utah, and any other areas of the United States as determined by the state entomologist to have Thousand Cankers Disease of Walnut. This last clause catches Tennessee, as well as any states where TCD might be found in the future.

This quarantine also prohibits moving any hardwood firewood from these states into Missouri. Sound familiar? It should. Similar restrictions apply to transporting hardwood firewood from states infested with another pest, the emerald ash borer (EAB).

Specific exceptions to the state quarantine are nuts, nut meats, hulls, processed lumber (that is 100 percent bark-free, kiln-dried with squared edges), and finished wood products without bark, including walnut furniture, instruments, and gun stocks.

Finding TCD this late in the growing season is a mixed blessing. Since the obvious symptoms of wilting, yellowing or collapsed brown leaves still attached to the branches are best observed in June and July, we have some time to put together a good monitoring program and educate natural resource professionals and the general public. However, it gives the disease more time, too.

Remember, TCD has not been detected in Missouri yet, but we do need to be aware of this serious threat to our most valuable hardwood species. More information on the disease is available from the Missouri Department of Agriculture at http://mda.mo.gov/plants/pests/thousandcankers.php.