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Abstract

The biotrophic maize head smut fungus Sporisorium reilianum is a close
relative of the tumour-inducing maize smut fungus Ustilago maydis with a
distinct disease aetiology. 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 with U. maydis
and with 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 only a modest impact on virulence.
This indicates that the virulence of S. reilianum is determined by a
complex repertoire of different effectors which each contribute
incrementally to the aggressiveness of the pathogen.