Sporisorium reilianum possesses a pool of effector proteins that modulate virulence on maize.

The biotrophic maize head smut fungus Sporisorium reilianum is a close relative of the tumor-inducing maize smut fungus Ustilago maydis with a distinct disease etiology. Maize infection with S. reilianum occurs at the seedling stage but spores first form in inflorescences after a long endophytic growth phase. To identify S. reilianum-specific virulence effectors we defined two gene sets by genome comparison to U. maydis and to the barley smut fungus Ustilago hordei. We tested virulence function by individual- and cluster deletion analysis of 66 genes and by using a sensitive assay for virulence evaluation that considers both disease incidence (number of plants with a particular symptom) and disease severity (number and strength of symptoms displayed on any individual plant). Multiple deletion strains of S. reilianum lacking genes of either of the two sets (sr10057, sr10059, sr10079, sr10703, sr11815, sr14797 and clusters uni5-1, uni6-1, A1A2, A1, A2) were affected in virulence on the maize cultivar 'Gaspe Flint' but each of the individual gene deletions had an only modest impact on virulence. This indicates that virulence of S. reilianum is determined by a complex repertoire of different effectors that each contribute incrementally to the aggressiveness of the pathogen. This article is protected by copyright. All rights reserved.