No abstract

Thousand cankers disease is caused by the coalescence of numerous Geosmithia morbida cankers on branches and stems of Juglans species, leading to branch dieback and eventual tree death. The fungus sporulates in galleries of the walnut twig beetle (Pityophthorus juglandis), allowing for acquisition of pathogen propagules and its subsequent transmission to other branches or trees following adult emergence. Recently, G. morbida has been isolated from Xylosandrus crassiusculus and Xyleborinus saxesenii collected in Ohio and Stenomimus pallidus collected in Indiana. These beetles are known to colonize diseased Juglans nigra in these states. In this study, an operational trap survey for ambrosia beetles, bark beetles, and other weevils was conducted in four eastern states, and captured beetles were assayed to detect G. morbida using both culture and PCR-based methods. A new primer pair (GmF3/GmR13), based on the β-tubulin region, was designed for G. morbida DNA detection. The pathogen was detected on 18 insect species using molecular methods, and live cultures were isolated from two species. This is the first report of the pathogen in Illinois and Minnesota.

Butternut trees (Juglans cinerea) clonally propagated via grafting from source trees with putative resistance to butternut canker were inoculated monthly with two isolates of Sirococcus clavigignenti-juglandacearum in a field planting in Minnesota. Significant differences in resulting canker length were found among (i) month of inoculation, (ii) accession, and (iii) isolate. Results support the suggestion that canker-free butternut occasionally found growing among butternut severely affected by butternut canker potentially have disease resistance and may have value for genetic conservation and restoration of the species. Developing an effective resistance screening technique will require additional information on the natural infection process and testing under various climatic conditions across the range of butternut.

The fungus Sirococcus clavigignenti-juglandacearum causes butternut canker in North America and is not known to kill or seriously affect tree species other than butternut (Juglans cinerea). However, this putative exotic pathogen has been found on branches of black walnut (J. nigra) and heartnut (J. ailantifolia var. cordiformis), raising questions regarding the potential host range of the pathogen and its spread to new areas on tree hosts other than butternut. Results of artificial inoculations of several hardwood species indicated that the pathogen, although not causing lethal cankers, has the ability to colonize and survive in the wood of several genera, including Quercus, Carya, Corylus, Prunus, and Castanea. Several commercially important Persian walnut (J. regia) cultivars tested were moderately or highly susceptible, underscoring the importance of preventing the movement of the pathogen into commercial Juglans plantations in California, where its ability to cause disease and damage trees is unknown. Evidence for the presence of variation in aggressiveness among isolates of S. clavigignenti-juglandacearum was detected based on differences in canker lengths resulting from inoculations with two isolates of the pathogen. In a field test of several previously named butternut cultivars, all were found to be highly susceptible to the disease, calling into question the value of these selections for nut production or future restoration uses.

Butternut is a relatively uncommon hardwood tree native to eastern North America. The species’ abundance has declined over the past 50 years, primarily because of an invasive pathogen (Ophiognomonia clavigignenti-juglandacearum [Oc-j]) and loss of suitable habitat for regeneration. Although genetic diversity of butternut is highest along the southern range edge, genetic diversity rangewide is fairly high, except in small and isolated populations. Although there is little evidence for even moderate resistance in native butternut, hybrids with Japanese walnut, a closely related species, display enough tolerance to infection to persist on the landscape and bear abundant nut crops year after year. Cryostorage of native embryogenic axes has yielded promising initial results as a strategy for gene conservation, but additional action is needed to conserve the remaining native gene pool. We describe a strategy for canker-resistance breeding in butternut using naturally occurring hybrids, hybrids in research orchards, and sources of native trees from as many regions as possible. Forest managers are encouraged to find surviving trees and collect seed for planting in suitable habitat to develop actionable knowledge that will enable the restoration of butternut with enough resistance to be self-sustaining on the landscape.