Emerging Fusarium species associated with pomegranate Nectria canker: characterization, aggressiveness, and pathogen-dependent susceptibility

Pomegranate ( Punica granatum L.), a high-value fruit crop cultivated worldwide, is increasingly threatened by canker and dieback diseases that compromise orchard productivity and longevity. During surveys of symptomatic pomegranate orchards in southern Iran, 96 fungal isolates were obtained from stem cankers and vascular necrotic tissues, 27 of which were Fusarium -like. Morphological characterization and phylogenetic analyses based on the translation elongation factor 1-alpha ( tef1 ) and RNA polymerase II second-largest subunit ( rpb2 ) genes identified these isolates as members of the F. tricinctum species complex (FTSC; namely, F. iranicum ), and F. incarnatum – equiseti species complex (FIESC; namely, F. clavum ). Pathogenicity of all 27 isolates (14 F. iranicum , 13 F. clavum ) was confirmed on detached shoots, and the two most aggressive isolates ( F. iranicum S30-1 and F. clavum A13-22) were further tested on one-year-old saplings under controlled conditions; Koch’s postulates were supported by successful re-isolation of morphologically consistent isolates. Aggressiveness assessment using three pathogenicity traits revealed significant intraspecific variability among isolates. Principal component analysis (PCA) differentiated isolates into highly, moderately, and less aggressive groups. Cultivar susceptibility was evaluated using eight pathogenicity-related traits, including lesion dimensions, internal tissue colonization, vascular progression, and incubation period. Two-way ANOVA revealed significant effects of Fusarium spp. isolate, cultivar, and their interaction on all pathogenicity traits, indicating substantial variation in pathogen aggressiveness and host susceptibility. The two pathogens elicited distinct host responses, with F. iranicum isolate S30-1 consistently causing more severe symptoms than F. clavum isolate A13-22 across six commercially important cultivars. PCA of disease variables identified ‘Malas-e-Danesiyah-e-Esfahani’ and ‘Rabab-e-Neyriz’ as highly susceptible cultivars, whereas ‘Atabaki’ showed the lowest disease severity under the experimental conditions, particularly when inoculated with F. clavum isolate A13-22. The demonstrated pathogenicity under controlled conditions, variation in isolate aggressiveness, and differential cultivar responses highlight the potential importance of these emerging Fusarium species in pomegranate canker and warrant further field-based epidemiological investigation. Their occurrence in pomegranate orchards of southern Iran underscores the need for continuous surveillance, resistance-based cultivar selection, and the development of integrated disease management strategies to safeguard sustainable pomegranate production.