Thousand Cankers Disease Complex: A Forest Health Issue that Threatens Juglans Species across the U.S.

Abstract

Thousand Cankers Disease (TCD) is a disease complex wherein the fungus (Geosmithia morbida) is vectored by the walnut twig beetle (WTB, Pityophthorus juglandis). The disease causes mortality primarily of eastern black walnut (Juglans nigra), although other walnut and wingnut (Pterocarya) species are also susceptible. Black walnut is native to the Eastern and Midwestern U.S. but is widely planted in western states. Total standing volume in both urban and forested settings is approximately 96 million cubic meters, and is valued at $539 billion. Although native to the Southwestern U.S., the range of WTB has expanded considerably. The spread of G. morbida coincides with that of WTB. TCD was introduced into Tennessee in 2010, and has spread to seven eastern states. Trees infected with TCD exhibit drought-like symptoms, making field detection difficult without molecular and/or morphological methods. The recently sequenced G. morbida genome will provide valuable research tools focused on understanding gene interactions between organisms involved in TCD and mechanisms of pathogenicity. With no chemical treatments available, quarantine and sanitation are preeminent options for slowing the spread of TCD, although biological control agents have been discovered. High levels of black walnut mortality due to TCD will have far-reaching implications for both eastern and western states.

Eastern Black Walnut (Juglans nigra L.) Originating From Native Range Varies in Their Response to Inoculation With Geosmithia morbida

“…High genetic diversity and complexity of the pathogen suggests that G. morbida is native and has co-evolved with some Juglans spp. in the past (Daniels et al, 2016). Our results support that the higher resistance of western populations reflects co-evolution of J. nigra and G. morbida which would further support that G. morbida is likely a native organism.…”

“…Chemical controls (i.e., insecticides and fungicides) may be effective against TCD (Mayfield et al, 2019) but are impractical in the forest and chemical controls are not viable management options for nut production due to pesticides restrictions (Nix et al, 2013;OFR, 2016). Semiochemical treatments may be effective in an orchard-type setting (Audley et al, 2020) but management is largely focused on quarantine efforts (Daniels et al, 2016) and techniques to sanitize felled trees to prevent the transport of infected logs (Sitz, 2013;Mayfield et al, 2014;Audley et al, 2015;Audley et al, 2016;Elisa et al, 2019;Seabright et al, 2019). Exploration of biological control agents is in the initial stages and may be an option to augment other control tactics in the future (Lambdin et al, 2015;Castrillo et al, 2017;Mayfield et al, 2019).…”

A novel molecular toolkit for rapid detection of the pathogen and primary vector of thousand cankers disease

“…However, only 17 out of 40 branch section samples from Tennessee trees were assessed as positive. These findings likely reflect the difference in disease incidence that has been reported between regions [ 15 ]. The low detection of TCD in branch sections from Tennessee is not necessarily due to the inability of the protocol to provide detection of G .…”

“…In Tennessee alone, the value of J . nigra is estimated at $1.37 billion in urban areas and $1.47 billion in forest areas, underscoring its economic importance in the region [ 15 – 17 ]. Currently, TCD has been reported in seven eastern U.S. states: Indiana, Maryland, North Carolina, Ohio, Pennsylvania, Tennessee, and Virginia [ 1 , 12 , 18 , 19 ].…”

“…To prevent the spread of TCD within the USA, several county and state quarantine measures are in place [ 22 ]. However, concerns remain about the efficacy of these measures due to difficulties in early detection, accurate diagnosis, and the capacity to find both organisms in the field [ 15 , 23 ]. To confirm the presence of the disease in newly affected areas, some plant pest regulators require detection of both organisms in the field.…”