Alternaria alternata as a causal agent of rose black spot: resistance screening and structural defense mechanisms in Rosagermplasm

Black spot is one of the most destructive foliar diseases of roses worldwide, causing severe defoliation and significant economic losses, particularly under humid conditions. Although Diplocarpon rosae is the primary causal agent globally, members of the Alternaria alternata species complex have emerged as important pathogens in Chinese rose production regions. This study aimed to evaluate the pathogenicity of these isolates, assess resistance among diverse rose germplasm, and examine the potential roles of cuticular wax and lignin in disease resistance. Koch’s postulates confirmed the isolates as causal agents of black spot-like symptoms. The pathogen was recovered from three rose cultivars, identified by morphology and ITS sequencing, and pathogenicity was verified via inoculation assays. Field evaluation of 48 germplasm accessions revealed clear variation in disease response: 8 resistant, 19 moderately susceptible, and 21 highly susceptible genotypes. Detached-leaf assays supported these findings, with R. laevigata var. laevigata and R. roxburghii f. normalis developing only minimal lesions. Resistant accessions generally exhibited higher cuticular wax and lignin contents than susceptible genotypes. Cuticular wax ranged from 13.00 to 37.90 mg g⁻¹ FW, and lignin from 6.98% to 10.95% dry weight across accessions. Alternaria isolates closely related to A. alternata were associated with black spot-like symptoms in rose germplasm. Wild species such as R. laevigata var. laevigata and R. roxburghii f. normalis may serve as valuable resources for breeding disease-resistant cultivars. However, ITS-based identification requires confirmation by multilocus phylogenetic analysis, and the roles of cuticular wax and lignin in resistance remain to be validated through further functional and multi-year field studies.