Diaporthe species (Sordariomycetes, Diaporthales) causing walnut blight and dieback in China.

English walnut (Juglans regia L.) is widely cultivated in China due to its economic value and nutritional benefits. Walnut stem blight and dieback is one of the most severe diseases affecting walnut productivity and quality in the country. To clarify the pathogens responsible for walnut stem disease, a comprehensive nationwide survey was conducted. From 276 walnut stem blight and dieback samples collected across seven provinces in China, 292 isolates of Diaporthe spp. were obtained. Both morphological characteristics and phylogenetic analyses based on partial ITS, cal, his3, tef1-α, and tub2 loci were used for fungal identification. Seven species of Diaporthe were identified, including one novel species, D. yunnana. Diaporthe species were most abundant in subtropical southwest China, less common in the temperate north, and absent in Xinjiang. Koch's postulates confirmed that all seven Diaporthe species could cause blight and dieback on walnut branches, with pathogenicity varying significantly among the species. D. eres and D. rostrata were the most virulent, followed by D. sackstonii, D. amygdali, D. citrichinensis, and D. yunnana, while D. psoraleae-pinnatae was the least aggressive. This is the first report of D. citrichinensis, D. psoraleae-pinnatae, and D. sackstonii occurring on J. regia.