The disease results from the combined activity of the walnut twig beetle (Pityophthorus juglandis) and a canker producing fungus, Geosmithia morbida.
Following emergence from trees the beetles subsequently tunnel into branches and trunks of walnut for production of egg galleries or overwintering shelters.
Instead, in TCD the bark remains firmly attached to the canker face making the necrotic areas very difficult to observe.
Sudden leaf wilting, ultimately involving large limbs, characterizes end stage thousand cankers disease.
Conversely, Arizona walnut (Juglans major) appears to be quite resistant to the disease, with bark beetle attacks largely limited to small diameter branches, the fungus growing to a very limited extent, and effects of the disease rarely, if ever, progressing to involve large areas of the tree.
The first published note involving black walnut die-offs that likely can be attributed to TCD occurred in the Espanola Valley of northern New Mexico in 2001.
This appears to have been preceded by unusual black walnut mortality during the mid-1990s of undetermined cause that were noted in the vicinity of Logan, Utah.
It required discovery of the previously undescribed Geosmithia fungus - and an appreciation of its role in canker production - to complete an understanding of how the disease can develop.
Wood from infected trees can still be used for commercial value, but safety measures such as removing the bark, phloem, and cambium should be followed to reduce the risk of spreading the disease with shipment.
[5] Contacting the appropriate entities about possible infections is important to stopping or slowing the spread of thousand cankers disease.
[6] The walnut twig beetle (Pityophthorus juglandis) carries the mycelium and conidia of the fungus as it burrows into the tree.
[4] As they move through the wood, the beetles deposit the fungus, which is then introduced into the phloem; cankers then develop around the galleries, quickly girdling the tree.
G. morbida does not affect the seeds, but increased tree death would lead to decreased nut production, hindering both human industry, and creating a lost food source for wildlife.
The origin of thousand cankers disease remains a subject of some uncertainty and its resolution will require further studies likely based on genetic analyses.
Optimum temperatures for development of the Geosmithia fungus are high, also suggesting a warm climate origin, a feature that is consistent with the native distribution of Arizona walnut.
Genetic analysis of Geosmithia isolates from throughout the western United States is currently considered a high research priority.
Regardless there has been a broad range extension of the walnut twig beetle within the past 15 years so that it currently is known from all western states for which it has been surveyed (AZ, CA, CO, ID, OR, UT, WA).
Whether this has occurred through natural dispersal events or by human transport of twig beetle infested walnut products (e.g., firewood, fresh cut logs, wood pieces used by woodworkers) is not known.
As the disease develops slowly, and external symptoms may not become visible for 10–20 years after the original establishment, it is possible that additional, undetected infestations are present in the eastern US.