Exotic and Invasive Pests
Thousand Cankers Disease and the Walnut Twig Beetle in California
Thousand cankers disease (TCD), caused by the fungus Geosmithia morbida, is killing walnut trees in California and threatens wildland and landscape trees as well as commercial walnuts. The fungus is spread by the feeding and reproductive activities of a tiny bark beetle, the walnut twig beetle (Figure 1), Pityophthorus juglandis. The fungus enters the tree through the feeding or reproductive activities of the beetle, and colonizes and kills the phloem and cambium of the branches and main stem. As the beetles and pathogen spread, small cankers form and coalesce, girdling branches. Thousand cankers disease gets its name from the large number of dark cankers that rapidly develop on affected branches.
The walnut twig beetle (WTB) is believed to be native to Arizona, California, Mexico, and New Mexico, where it attacks various indigenous western black walnut tree species. Although previously not considered a pest, WTB’s apparent new association with the Geosmithia fungus, now makes it a pest of significant concern.
TCD was first recorded in northern California in Davis in June 2008; however, it was probably in the state for several years prior to 2008. The disease has been confirmed from Los Angeles County in the south up to Sutter County in the north. TCD is also known to be present in Colorado, Idaho, Oregon, Utah, and Washington.
The disease is only known to occur in walnuts. WTB is believed to attack all species of walnut; however, TCD has primarily killed black walnut trees. It has been most frequently observed on the two California native species of black walnut, Juglans californica and J. hindsii. Eastern black walnut, J. nigra, and hybrids of that species or hybrids of California native walnuts such as the Paradox hybrid (J. hindsii x J. regia), which are important rootstocks for commercial walnuts, have also shown symptoms of the disease. Thus far, English walnut, J. regia, the primary commercial species, has exhibited TCD symptoms only in rare cases.
From a distance, trees affected by TCD will show flagging and branch dieback (Figure 2). Close examination of the bark surface of tree branches will show pinhole-sized WTB entrance or emergence holes (Figure 3). Attacked branches are usually 1.5 cm or greater in diameter. Dark wet cankers are often found next to beetle holes. Upon removal of the surface bark of the canker, several beetle feeding or reproductive galleries as well as areas of necrotic phloem tissue may be observed (Figure 4). As the beetles and pathogen spread, new cankers form and coalesce, girdling branches. TCD gets its name from the large number of dark cankers that rapidly develop on affected branches (Figure 5). As the upper branches die, the crown of the tree dies and the tree attempts to re-sprout branches from the trunk (Figure 6). At this stage WTB may colonize and inoculate the fungus into the main stem of declining trees. Removal of bark from affected trees will show WTB galleries, which are 1 to 2 inches long and etched against the grain on the wood surface (Figure 7).
WTB is believed to have 2 to 3 generations a year in California. Adults emerge for an initial flight period in April and May followed by a longer second generation flight period in mid-July to mid-September. After flying, male beetles initiate brood galleries on branches often near leaf scars or lenticels. Males produce a pheromone and attract 2 to 3 females, which attract additional beetles to the tree. Females deposit eggs in galleries (tunnels) that are directed against the grain and constructed in the phloem and xylem (wood) surfaces. The gallery imprint is left on the wood surface. Small white C-shaped larvae hatch and create feeding mines that extend from the egg galleries. These mines are contained in the phloem and filled with dark brown to black-colored boring dust. Larvae complete development in the mines and subsequently pupate within a single pupal cell. Adults emerge and either remain at the original tree or fly to other trees to mate and reproduce. WTB does not appear to be attracted to stressed or injured branches or trees. Beetles are believed to inoculate the Geosmithia sp. fungus into the phloem during construction of feeding or reproductive galleries. The fungal pathogen colonizes and kills the phloem. Dead tissue is limited to the phloem and cambium and the fungus does not penetrate woody tissues. Secondary saprophytic fungi may opportunistically colonize the wood beneath cankers.
No pesticides or control methods are currently available to save trees infected with thousand cankers disease. To prevent spread, infected trees should be removed and material destroyed by grinding or burning immediately to insure that beetles are destroyed. Please report any possible detections on English walnut to your agricultural commissioner’s office or to your local University of California Cooperative Extension office.
Andrew D. Graves, Department of Plant Pathology, University of California, Davis, CA.
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