Thousand Cankers Disease in Europe: what are the risks for European walnut cultivation?

Black walnut (Juglans nigra), a species native to North America, was introduced into Europe in the late 17th century for ornamental purposes and for the production of valuable timber. Since the early 2000s, this species has been increasingly threatened by canker disease (TCD), a disease caused by the interaction between the fungus Geosmithia morbida (GM) and its insect vector, the walnut twig beetle Pityophthorus juglandis (PJ). Native to the United States, TCD has been reported in Italy for over a decade and, more recently, in France. TCD now represents a significant threat in Europe not only to the exotic black walnut, but also to the native English walnut (Juglans regia), a very valuable species both for the quality of its wood and for the nutraceutical properties of its fruit. In fact, English walnut, although less susceptible to TCD than black walnut, is still sometimes attacked, especially where it grows in mixed plantations with black walnut. Our multi-year investigations suggest that both the pathogen and its vector may already be present in other areas of the continent although not yet detected. The management of both GM nd PJ, which are classified as quarantine organisms in the EU, requires mandatory containment measures and a proactive approach, aimed at the timely identification and eradication of existing outbreaks. For this latter purpose, in Tuscany, one of the European regions most affected by TCD, several rapid and highly specific diagnostic protocols have been developed. Management strategies implemented here include phytosanitary surveillance at vulnerable sites like ports, airports, nurseries, storage depots and wood processing facilities, and constant monitoring of the territory (walnut plantations and walnuts planted for ornamental purposes), also with the support of molecular diagnostics, for the early detection of new outbreaks.