What is Fusarium forma specialis?

The forma specialis concept has been a cornerstone in Fusarium phytopathology for 85 years, classifying pathogens based on host specificity. However, its validity as a natural and practical framework has been increasingly questioned. In this study, we critically re-evaluate the forma specialis concept through an extensive survey of Fusarium wilt diseases across 37 crop hosts from 23 provinces of China. Through multi-locus phylogenetic analyses, morphological assessments, and pathogenicity tests on 659 strains isolated from 171 diseased samples, we identified 46 Fusarium species, including seven newly described taxa, and uncovered extensive cross-host pathogenicity, with up to 21 species associated with the same wilt disease. In addition, 57% of samples exhibited multiple species co-infections, revealing significant inconsistencies with the forma specialis framework. These findings challenge the long-standing paradigm of host specificity in Fusarium pathogens and advocate for a shift of perspective to a "pathobiome" framework, where disease dynamics are largely driven by community-level interactions rather than single-pathogen relationships. Applying these conceptual advances to Fusarium wilt research could fundamentally transform our comprehension of host-pathogen relationships and facilitate the development of more ecologically sustainable disease management approaches.