Range-wide population decline of a foundational riparian species tree is linked to an endemic fungal pathogen in the western United States

Screwbean Mesquite (Stromocarpa pubescens), an ecologically and culturally important riparian tree in the North American desert southwest, has experienced a precipitous decline over the past two decades with several documented die-off events resulting in local extinctions. The decline has received little attention, however. To better understand causes of the die-offs, we surveyed 29 populations across the species' range for patterns of decline and found that 16 populations contained trees exhibiting identical disease symptoms (gummosis, cankers). Diseased populations were clustered in the western portion of the range where local extinction has occurred. The fungus Neoscytalidium dimidiatum was isolated and identified molecularly in 100 % of diseased tissue samples taken from 24 trees at six sites. Pathogenicity tests were performed by inoculating 10 naive trees with isolated N. dimidiatum. Inoculations produced identical disease symptoms and the fungus was reisolated from all infections. Branch canker symptoms were present at all but two sites experiencing active decline and sites with branch canker had experienced roughly 30 % more recent mortality than disease-free sites. More intensive monitoring of 244 trees at six sites throughout 2022 revealed that trees with branch canker symptoms showed more than four times new dieback (i.e. tissue death) through the growing season than non-diseased trees and ended the 2022 growing season with 29 % more dieback than non-diseased trees. Neoscytalidium dimidiatum is endemic to the region but this is the first documented example of widespread and aggressive infection by the species across the range of a native tree species.