Tissue type and cultivar-by-isolate interactions govern walnut (Juglans regia) resistance to Diaporthe eres in...

Diaporthe eres is increasingly reported as a causal agent of canker, shoot dieback, and fruit rot of walnut (Juglans regia L.) in diverse production regions. In this study, multi-locus phylogenetic analysis using ITS, tef1, and tub2 gene sequences confirmed that two isolates, ED 5/4 (Edirne) and BUR 7/4 (Bursa), belong to D. eres sensu stricto, showing 99-100% sequence identity with ex-type strain AR5193 in BLASTn analysis and clustering with high bootstrap support in Maximum Likelihood phylogenetic trees. Pathogenicity assays conducted on green fruits, current-season shoots, and one-year-old dormant branches of seven cultivars revealed strong and statistically significant cultivar × isolate interactions, demonstrating that resistance responses were highly isolate-specific. Isolate BUR 7/4 displayed higher virulence overall than ED 5/4. Lesion development differed markedly among tissue types, with current-season shoots consistently exhibiting substantially greater necrosis than one-year-old dormant branches, indicating a pronounced influence of tissue ontogeny on host susceptibility. Cultivar performance varied among tissues: 'Chandler' showed the greatest resistance in woody tissues, whereas 'Fernor' and 'Orman 77' were consistently susceptible. Fruit assays demonstrated variable husk susceptibility but uniformly severe, cultivar-independent kernel decay, indicating the absence of internal resistance once the husk barrier was breached. D. eres was re-isolated from all symptomatic tissues while controls remained healthy, fulfilling Koch's postulates. These results establish D. eres as an important contributor to walnut canker and fruit rot in Türkiye and emphasize the roles of tissue-specific susceptibility, isolate diversity, and cultivar-dependent responses in shaping disease outcomes and informing resistance-based management strategies.