Geosmithia morbida

Geosmithia morbida is a pathogenic fungus that, in association with the walnut twig beetle (Pityophthorus juglandis), causes thousand cankers disease. This disease primarily affects black walnut (Juglans nigra) but can also impact other walnut species, including J. hindsii, J. californica, and J. regia. An anamorphic fungus in the Bionectriaceae family, G. morbida was first described as a new species in 2010, based on specimens collected in the southern United States. The fungus infects trees by creating necrotic lesions in the cambium and bark tissues, disrupting their vascular function.

In Europe, G. morbida has been detected in several regions but remains under surveillance to prevent widespread outbreaks. The disease significantly affects the health and appearance of walnut trees, causing crown thinning, yellowing leaves, and extensive bark damage. The combined effects of beetle activity and fungal colonization pose a serious threat to walnut populations in affected areas, often resulting in branch dieback and tree mortality within three to four years.