Putative effector genes distinguish two pathogenicity groups of Fusarium oxysporum f. sp. spinaciae.

Fusarium wilt of spinach, caused by Fusarium oxysporum f. sp. spinaciae (Fos), is an important disease during warm conditions in production regions with acid soils, yet little is known about what confers pathogenicity to spinach in Fos genetically. To identify candidate fungal genes that contribute to spinach Fusarium wilt, each of 69 geographically diverse F. oxysporum isolates was tested for pathogenicity on each of three spinach inbreds. Thirty-nine isolates identified as Fos caused quantitative differences in disease severity among the inbreds that revealed two distinct pathogenicity groups of Fos. Putative effector gene profiles, predicted from whole-genome sequences generated for nine Fos isolates and five non-pathogenic, spinach-associated F. oxysporum (NPS) isolates, distinguished the Fos isolates from the NPS isolates, and separated the Fos isolates into two groups. Five of the putative effector genes appeared to be unique to Fos as they were not found in 222 other publicly available genome assemblies of F. oxysporum, implicating potential involvement of these genes in pathogenicity to spinach. In addition, two combinations of the 14 known Secreted in Xylem (SIX) genes that have been affiliated with host pathogenicity in other formae speciales of F. oxysporum were identified in genome assemblies of the nine Fos isolates: either SIX8 and SIX9, or SIX4, SIX8, and SIX14. Characterization of these putative effector genes should aid in understanding mechanisms of pathogenicity in Fos, developing molecular tools for rapid detection and quantification of Fos, and breeding for resistance to Fusarium wilt in spinach.