NationalNational PestPest AlertAlert

Thousand Cankers Disease of Walnut Distribution and Transmission these -vectored diseases only require one introduction of the fungus into the tree (TCD) is caused by a fungus ( and then the fungus spreads through the morbida) that is vectored by the walnut twig ( whole tree becoming systemic and even able juglandis). It is unknown if G. morbida is native to North America. to spread through root grafts. TCD requires The is native to North America and was first re- multiple introductions by the beetle. corded feeding on walnut ( major) in the southwest- ern U.S., but it is not harmful to Arizona walnut. The walnut twig Host beetle has expanded its host range to other of walnut includ- ing black walnut, which is not native to western states. TCD was TCD affects many walnut trees (Juglans Windham Alan courtesy confined to western spp.) and species vary in their susceptibil- Figure 2. Microscopic states (California, ity to TCD, but black walnut () view of the conidiophores Oregon, Washing- is very susceptible. Black walnut is an and conidia of ton, Idaho, , important tree in the forest ecosystem and . , Colo- landscape, and is highly valued for lumber, rado and Arizona) veneer and nut production. until 2010 when the disease was found Symptoms in the native range of black walnut. It A black walnut tree can be infected with is unknown how TCD for many years before showing

widespread the symptoms, but once branch dieback appears Windham Mark courtesy disease will become the tree rapidly declines and dies within a Figure 3. The yellowish- or the long-term few years. The first symptom to appear is brown walnut twig beetle courtesy Alan Windham Alan courtesy effects on black flagging leaves (leaves wilting and yellow- is less than 2 mm in Figure 1. Walnut trees killed by Thousand walnut. The ing mid-summer) followed by thinning of length with 4-6 broken Cankers Disease in Knox County, TN. carry the fungal the canopy from twig and branch dieback. grooves near its head spores (conidia) on their bodies and transfer the spores to trees Eventually the whole tree dies as thousands (asperities), yellow hairs when they tunnel into the branches or trunk. of cankers girdle branches and the trunk. (setae) on its head and Symptoms of TCD include: a sharp angle to its Biology • Flagging leaves (Fig. 7) posterior (steep • Wilting of foliage (Fig. 8) declivity of The fungus invades the phloem tissue located just beneath the bark • Branch dieback elytra). and kills it. This darkened, dead (necrotic) tissue is called a canker. • Thinning canopy (Fig. 9). Cankers caused by G. morbida are small, but repeated feeding and • Epicormic shoots (water spouts) (Fig. 9) egg laying by the walnut twig beetles lead to the introduction of • Cankers under the bark (Figs. 5 and 6) the fungus into multiple areas on the same tree. Numerous can- • Dying tree (Fig. 1) kers girdle and kill branches and/or the whole tree by halting the normal flow of nutrients. Essentially the tree is killed by thousands of cankers. This is very different from Dutch elm disease or oak wilt; courtesy Mark Windham Mark courtesy Figure 4. Walnut twig beetle feeding tunnels and emergence holes.

Management of TCD in the trees and is not seed-borne. Do not move walnut wood across state lines with- The symptoms of TCD on black walnut out checking for state quarantines that have

are similar to other common diseases of been instituted to prevent infected walnut Windham Mark courtesy walnut as well as decline from other causes, from moving into the state. Figure 7. Flagging (yellowing) of foliage particularly environmental stresses. It is im- If you have a walnut tree exhibiting signs of is an early symptom of thousand cankers portant to evaluate the whole tree and make disease. a proper diagnosis. There are no manage- decline it is important to contact your state ment options for the disease or the beetle at extension service. Do not mail samples to this time and future control options will be a diagnostic laboratory without contacting limited due to the tree producing an edible them first to receive packaging instructions nut. to prevent beetle emergence during ship- ment. Preventing the Spread of TCD TCD can be spread by moving walnut wood Contributing Authors: Laura Jesse, for woodworking or as firewood. Nuts are & Insect Diagnostic Clinic, Iowa State not a concern since TCD is not systemic University; Mark Windham, Department of Windham Mark courtesy Entomology and Plant Pathology, Univer- Figure 8. Check wilted branches for walnut sity of Tennessee; Alan Windham, Depart- twig beetles. ment of Entomology and Plant Pathology, University of Tennessee; Jesse Randall, This publication was produced and distrib- Department of Natural Resources Ecology uted in cooperation with the USDA NIFA Management, Iowa State University; Mark Integrated Pest Management Program, the Shour, Department of Entomology, Iowa North Central IPM Center and the Land State University; Donald Lewis, Department Grant Universities. For more information of Entomology, Iowa State University; Mark about the development of this pest alert or Gleason, Department of Plant Pathology, to obtain copies, contact Susan Ratcliffe at Iowa State University [email protected] or by phone at 217- courtesy Mark Windham Mark courtesy 333-9656. Figure 5. Canker damage to cambium and phloem tissues due to of Geosmithia morbida on black walnut tree. courtesy Alan Windham Alan courtesy Figure 9. Thousand cankers disease can lead to shrinkage of canopy (dead courtesy Mark Windham Mark courtesy limbs) and a profusion of water spouts Figure 6. Numerous cankers can be found close to each other in the (epicormic shoots). same limb.