Identification and pathogenicity analysis of leaf brown spot of Juglans regia in China.

English walnut (Juglans regia), has high economic and ecological value. As an important tree species for eliminating poverty, it is planted in many Provinces of China. In 2021, new pathogenic fungi were observed in English walnut in Guangyuan City, Sichuan Province, China. The initial symptom of leaf infection is that the leaves are covered with small black spots, which gradually expand into larger brown spots. Most of the spots appeared at the edges of the leaves, and yellow whorls were observed at the junction between the spots and the healthy leaves. The pathogenic fungi were isoalted form collecting disease samples and purified by single-spore culturing. In vitro and field experiments showed that the pathogen could cause brown spots on walnut leaves. The inoculation experiment showed that the symptoms in the field experiment were the same as those observed on the spot; however, slight differences were observed in the in vitro experiment. Ten isolates were obtained from walnut leaves with brown spot symptoms, and these were further characterized based on morphology and DNA sequencing. ITS (internal transcribed spacer), LSU (large sub-unit rDNA), rpb2 (second largest subunit of RNA polymerase) and tub2 (beta-tubulin) gene regions were used to construct phylogenetic trees and determine the evolutionary relationships among the collected strains. The isolate was identified as Nothophoma quercina by morphological and polygene analyses. As far as we are aware, the brown spots on walnut leaves caused by N. quercina is the first report of its kind.