Pathogenicity Assessment of Gnomoniopsis castanea on Nut and Bark Tissues of Chestnut Cultivars in Türkiye

Gnomoniopsis castanea , an emerging fungal pathogen, has recently been associated with severe nut rot and branch cankers in chestnut ( Castanea spp.) orchards worldwide. This study aimed to evaluate the pathogenicity of four molecularly identified G. castanea isolates (Gc_11, Gc_19, Gc_30, and Gc_60) on the nut and branch tissues of four chestnut cultivars (‘Marigoule’, ‘Bouche de Bétizac’, ‘Akyüz’, and ‘Macit 55’) grown in Türkiye. Inoculation assays on nuts revealed that all cultivars were susceptible to the pathogen, with ‘Marigoule’ showing the highest disease severity index (DSI), followed by ‘Bouche de Bétizac’, while ‘Macit 55’ and ‘Akyüz’ were less affected. Branch inoculation assays using a detached shoot method also confirmed pathogenicity on woody tissues. Notably, ‘Akyüz’ displayed the lowest lesion development, suggesting a degree of resistance. Differences in virulence were observed among isolates, especially in branch tissues. The pathogen was successfully re-isolated from symptomatic samples, fulfilling Koch’s postulates. These findings provide the first comparative pathogenicity evaluation of G. castanea isolates on widely cultivated chestnut varieties in Türkiye. The results highlight the urgent need for integrated management strategies and resistant cultivar selection to mitigate the increasing threat of G. castanea in chestnut production.