Systemic Tissue Colonization and Pathogenic Differentiation in Cucurbit-Associated Fusarium oxysporum from Taiwan

Background Fusarium oxysporum causes destructive wilt diseases in cucurbit crops and exhibits strong host specificity among different formae speciales . However, cross-pathogenicity has been reported among cucurbit-associated isolates, whereas its relationship with systemic tissue colonization across different biological contexts remains poorly understood. This study investigated the relationships among pathogenicity, systemic tissue colonization, and molecular characteristics of cucurbit-associated F. oxysporum isolates across different host species, environmental conditions, and propagation systems. Results Representative cucurbit-associated F. oxysporum isolates exhibited distinct pathogenicity and systemic tissue colonization patterns, with only limited cross-pathogenicity observed among host species. Across the evaluated host species, environmental conditions, and propagation systems, disease development was generally associated with systemic colonization beyond the cotyledonary node into the epicotyl, whereas weakly pathogenic or non-pathogenic interactions were predominantly associated with colonization confined to the hypocotyl. Temperature, host developmental stage, and grafting status influenced disease expression but did not substantially alter the overall patterns of systemic tissue colonization associated with pathogenic differentiation. Integrated analyses consistently grouped the isolates according to their pathogenicity and tissue colonization characteristics. Furthermore, phylogenetic analysis based on the SIX6 gene was largely congruent with the biologically defined groupings and provided greater discriminatory power than the conserved IGS and EF-1α loci. Conclusion This study demonstrates that the ability to achieve systemic tissue colonization beyond the cotyledonary node is more closely associated with pathogenic differentiation among cucurbit-associated F. oxysporum isolates than with host origin alone. The integration of pathogenicity, tissue colonization, and SIX6 sequence analysis provides complementary biological and molecular evidence for characterizing pathogenic differentiation among cucurbit-associated F. oxysporum isolates and may facilitate resistance screening, rootstock evaluation, and disease management in cucurbit production systems.