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The most widespread and damaging fungal disease of the oat plant is crown rust. Resistance to the crown rust pathogen, <i>Puccinia coronata</i> Cda. f. sp. <i>avenae</i> (<i>Pca</i>), at the seedling stage of <i>Avena sterilis</i> accessions from the Polish national genebank was characterised by five North American and Polish pathotypes of <i>Pca</i> of diverse pathogenicity. <i>Pca</i> pathogenicity was determined on a series of 34 differential lines carrying known seedling resistance genes. Seventy-five percent of studied accessions showed a heterogeneous infection pattern, 17% behaved as homogenous susceptibles, and 7% of tested genotypes could be unambiguously described as resistant. This study proved that <i>A. sterilis</i> accessions preserved in a genebank as complex populations could be a very valuable source of resistance to crown rust. The complexity of analysed populations was ascertained by a detailed variance analysis of transformed resistance/susceptibility data. We demonstrate here that hidden sources of resistance may be discovered in accessions with general susceptibility.