Keys to host-specific pathogenicity in Fusarium oxysporum

Fusarium oxysporum (Fo) is a soil-borne fungal species complex which can cause disease in more than 120 plant species. However, individual strains can only infect one or a few related plant species. In this thesis the main question is: what determines the ability of a strain to cause disease in one plant species but not in others? In tomato-infecting strain Fo f.sp. lycopersici (Fol), a pathogenicity chromosome has been shown to be required for virulence and can be horizontally transferred to a non-pathogenic strain. In this thesis, the region that is required for virulence is narrowed down further through a novel procedure to identify chromosome deletion strains. It is also demonstrated that smaller versions of the pathogenicity chromosome can be transferred to a non-pathogenic strain and are sufficient to turn it into a pathogen.
Fo f.sp. radicis-cucumerinum (Forc) is able to cause disease in cucumber, melon and watermelon, while Fo f.sp. melonis (Fom) can only infect melon plants. I show that mobile chromosomes in Forc and Fom determine the differences in host range between Forc and Fom. By closely comparing these pathogenicity chromosomes, combined with RNA-seq data, I identified a putative effector gene from Fom that is able to turn Forc non-pathogenic to cucumber plants. This is the first time that a single gene is demonstrated to cause a difference in host specificity between formae speciales of Fo.