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Forest Health Matters: Thousand Cankers Disease of Walnut
Forest Health Matters: Thousand Cankers Disease of Walnut
David Shaw, Paul Oester, with Jay Pscheidt, OSU Extension Plant Pathologist.
A new disease syndrome has emerged in the western USA on black walnut (Juglans nigra) called, “thousand cankers disease”. This disease is caused by a newly described fungus called Geosmithia morbida. The fungus is vectored by the walnut twig beetle (Pityophthorus juglandis) a scolytid bark beetle. The disease causes mortality of both black walnut and Hinds’s walnut (Juglans hindsii), but the cultivated English walnut (Juglans regia) and many Juglans hybrids are more resistant. It is called, “thousand canker disease” because cankers develop around every location where beetles attack trees. The beetle doesn’t just attack twigs, as was thought to be typical in its’ native range, but attacks larger limbs and trunks, therefore causing thousands of cankers.
The beetle is native to the Southwest USA and northern Mexico, where it occurs on twigs and small branches of the Arizona walnut (Juglans major). Apparently, the twig beetle has moved into the urban and rural areas of the west where black walnut was planted, and at some point picked up this new fungus. The disease was first noticed in Boulder, Colorado after all the black walnut trees in town died over a 2 to 3 year period. The disease has since been detected around the west, and is currently known in California, Oregon (throughout the state), Washington (eastern), Idaho, Utah and Colorado (Figure 1). It is feared that the disease is a major threat to the forests of the eastern USA where black walnut is native, and several close relatives also may be threatened. Unfortunately, the disease was detected this year (July 2010) in Tennessee, and our worst fears may become realized. The unrestricted movement of firewood and wood products is a direct threat to native forests, and this now appears to be another case study.
In Oregon, Jay Pscheidt, OSU Extension Plant Pathologist Specialist, led the investigations into thousand canker disease on Oregon’s black walnut and closely associated hybrid walnuts. He summarized his observations in the 2010 Plant Disease Management Handbook, available on-line or in print, where we’ve captured the following accounts. His page is: http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=1505
Small diameter black walnut (Juglans nigra) trees in Colorado died within one to two years after first symptoms were observed. Decline and death seems to occur much more slowly in the PNW on large diameter black walnut hybrids (J. nigra X J. hinsii). Young, small diameter trees found growing near declining trees have remained symptomless. Western black walnut species may host both insect and fungus, show minor symptoms or even appear to recover in succeeding years. Preliminary testing found J. nigra to be much more susceptible than J. hinsii or other Juglans species. This may account for the different disease progression in different areas. Several municipalities and homeowners have spent thousands of dollars to prune out dead wood only to find more dead limbs later in the year. Even with limb removal tree owners are eventually faced with extensive mortality of huge trees.
Symptoms: The dominate symptom is that branches fail to leaf out in the spring. Branches may leaf out very weakly in the spring or, more rarely, die anytime during the summer. Generally dead leaves do not remain on the declining branches. Dieback always appears to start in the upper crown and progresses to lower branches in succeeding years. Some trees appear to have top dieback appearing almost concurrently with limb dieback at various places throughout the crown. Trees may die within 2 to 5 years once symptoms are noticed. This problem has been observed mostly in mature trees of large diameter (Several feet!).
To find cankers, carefully remove the bark from symptomatic limbs. When peeling the bark, do not cut too deeply as beetle galleries and fungi are found in the living bark (cork cambium and not the wood cambium). Individual cankers may at first be only a few millimeters in diameter, but ultimately can be 3 cm or greater (1” +) and have an elongate oval shape. Typically the appearance of a shallow tunnel produced by the walnut twig beetle will be present near the center of the canker. Many small, dark, dead areas (cankers) can be found under the bark of heavily infected trees. The cumulative effect of innumerable cankers produces limb girdling that cause dieback.
No witches’ brooms have been observed on declining trees. Sometimes epicormic shoots (shoots arising from a dormant dormant bud on the stem or branch) will be seen but if present they are not abundant, except in The Dalles where epicormic shoots are very numerous. Declining trees do not consistently have visible trunk cankers, heart rots, butt rots, storm damage, commercial pruning or herbicide use in common.
Cultural control: Little is known.
- In a single case, a set of 2 trees in each of 2 locations (for a total of 4 trees) that were topped or pruned hard due to their location under power lines have remained healthy while nearby trees that were not pruned declined or died. This indicates that pruning hard (lower on the branch or trunk) may help trees that exhibit less extensive dieback. This limited evidence suggests that removing limbs below the margin between necrotic and healthy cambium may reduce dieback rates
- If trees continue to dieback the following year then tree owners might consider selling their trees for lumber to recoup pruning and tree removal costs.
- Do not sell or transport firewood or logs with intact bark from infected trees to areas without the problem.
Research on insecticides and/or fungicides applied to the trunk, crown or soil or injected into the tree has not been done to date. Most experts agree that these applications would have minimal if any effect on disease progression. Anecdotally, infested black walnut trees in Colorado that received repeated commercial insecticide treatments have continued to decline and die.
It is not known if treatments to control powder post beetle on lumber will also control walnut twig beetle. Yes, there are several entomologists looking into developing data on insecticides.
For more information:
California Thousand Canker Disease Field Indentification Guide: http://www.walnutcouncil.org/Seybold_Walnut%20Twig%20Beetle%20and%20Thousand%20Cankers%20Disease%20ID%20Guide-Final.pdf
Thousand Cankers Black Walnut Disease Website (Missouri Dept Conservation): http://www.thousandcankerdisease.com/
Thousand Cankers Pest Alert, Colorado State University: http://www.ext.colostate.edu/pubs/insect/0812_alert.pdf