Morphological characterization and phytotoxicity divergence of Elsinoë arachidis causing peanut scab in China

Peanut scab, caused by Elsinoë arachidis, is a major disease in peanut-growing regions of China. To clarify the phenotypic characteristics and toxigenic divergence of the pathogen, 70 strains from major peanut-growing regions were subjected to morphological characterization, phylogenetic analysis, elsinochrome (ESC) quantification, and phytotoxicity assessment. Based on colony color, the strains were divided into five morphological groups (Group A-E), with the dark red pigmented Group D being dominant (50%). Growth rates varied among strains, with the coefficient of variation within each morphological group ranging from 10% to 25%, but no distinct pattern was observed across groups. All isolates were identified as E. arachidis and resolved within a species-level clade based on ITS and TEF1-α phylogenetic analyses. ESC quantification revealed a significant correlation between colony color and toxin accumulation, with coral red and dark red strains (Groups A and D) accumulating higher ESC levels. Phytotoxicity assays demonstrated a significant positive correlation between lesion area and ESC accumulation (r = 0.921, P < 0.01). This study systematically characterized the intraspecific phenotypic differentiation of E. arachidis in China for the first time, establishing a correlation between pathogenic intensity, colony color, and ESC synthesis capacity. Furthermore, colony color was proposed as a visual phenotypic indicator for the rapid identification of highly virulent strains, and ESC accumulation was identified as the key metabolic process underlying phytotoxicity differentiation. These findings enhanced the understanding of population differentiation in E. arachidis and provide a theoretical basis for elucidating disease outbreak mechanisms and developing targeted control strategies.