Powdery mildew, caused by Blumeria graminis DC. f. sp. avenae Em. Marchal, is currently one of the most serious foliar diseases of common oats worldwide, leading to significant grain yield losses. So far, 13 resistance genes to powdery mildew have been identified and characterized in oats, but only a few of these genes are effective against the existing pathogen races. Therefore, there is a need to identify and characterize new and effective sources of resistance. Avena sterilis L. is identified as the most promising source of resistance genes against fungal diseases. However, identifying effective sources of resistance requires more extensive research using a larger group of differentiated isolates of B. graminis f. sp. avenae. This study aimed to characterize the effectiveness of resistance identified in A. sterilis genotypes. The research focused on 28 A. sterilis genotypes, from the IPK Genebank collection, which were known for their high level of resistance in screening tests. The effectiveness of resistance was evaluated in host–pathogen tests using 55 different B. graminis f. sp. avenae isolates collected from various regions of Europe. These isolates were known for their different infection patterns to known powdery mildew resistance genes. In the initial stage of the investigation, the 28 previously selected A. sterilis genotypes were tested against 5 different B. graminis f. sp. avenae isolates, resulting in the selection of seven resistant genotypes. Further tests using 50 different pathogen isolates revealed that only two of these genotypes showed very high effectiveness. These studies indicate that effective sources of resistance should be tested with a variety of pathogen isolates. Tests based on only 2–3 isolates are suitable only for preliminary screening.