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Forestry Extension: Thousand Cankers Disease

Thousand Cankers Disease (TCD)

A fungal infection called “Thousand Cankers Disease” (TCD) has ravaged black walnut trees in a handful of western states over the last decade. In July of 2010, foresters discovered infected trees in Knoxville, Tennessee.

Unlike the western states, Tennessee is ‘home’ to the eastern black walnut tree, supporting extensive natural populations that could fuel an uncontrolled TCD outbreak in the eastern U.S.

According to the USDA Forest Service, Missouri has more black walnut trees than any other state within the species’ natural range. And according to Missouri Department of Agriculture 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.

Signs

The walnut twig beetle (Pithyophthorous juglandis) is very small; about the same size as the letter 'i' on a Liberty dime.

By itself, the beetle does very little damage. But, an aggressive fungus (Geosmithia. sp.) is carried on the backs of these insects. To make matters worse, these beetles are not intent on drilling just one hole; they make several holes. And every time the beetle does, it deposits fungal spores in the wound. The fungus then kills a patch of tissue about the size of a quarter under the bark. As the cankers fuse together nutrients can no longer move in the tree and it dies

The sheer number of beetle attacks and the resulting cankers led those who discovered the problem to call it the "thousand canker disease".

Pathways

Human activities, such as selling walnut logs and lumber with intact bark as hobby wood on the Internet or moving it as firewood, are the primary pathways the disease is thought to spread. It will only take one board sold on the Internet or one log transported as firewood from an infected area to an uninfected area to spread the disease.

Symptoms

The earliest disease symptoms are thought to appear in mid-July and can then be found throughout the rest of the growing season. Symptoms may include one or more of the following:

Yellowing, browning, wilting of leaves anywhere in the tree crown (Fig. 1)
Dead leaves often remain attached to twigs
Limbs die back, usually from the top downward (Fig. 2)
Sprouts may grow from below dead branches (Fig. 3)

Fig.1

Fig.2

Fig.3

TCD should not be confused with walnut anthracnose

Walnut anthracnose is a very common foliar disease on walnut. But, there are significant symptom differences between the two diseases.

Anthracnose-infected leaves appear in late-summer as opposed to TCD-infected leaves showing up in mid-summer
Anthracnose-infected leaves lose their leaflets from the bottom up while TCD-infected leaves do not lose any leaflets. The entire leaf wilts, turns brown and remains attached to the twig.
Anthracnose-infected leaves are yellow and have black spots all over the leaflets. The compound leaves also lose their leaflets from the bottom up while TCD-infected leaves have no black spots.

State Quarantine

All walnut plants and plant parts as well as all hardwood firewood from TCD-infected states are now prohibited from entering Missouri. This includes nursery stock, budwood, scionwood, green lumber, and other material living, dead, cut, or fallen including stumps, roots, branches, and composted and uncomposted chips.

Exceptions are nuts, nutmeats, hulls and processed lumber. Processed lumber means 100 percent bark-free, kiln-dried with squared edges.

To Report a Suspect Tree

If you suspect that your black walnut has TCD, contact either your county MU Extension center or the local Missouri Department of Conservation office.

For the Latest Information

Currently, there is no dedicated Missouri website for this disease. But, we have included TCD on the 'Other Exotic Pests' page at www.eab.missouri.edu.