This research was conducted to evaluate the contribution of four candidate genes — CsABA2 (CsGy5G015450), CsLRR-RPK2 (CsGy5G015660), CsPM5.2 (CsGy5G015960), and CsaMLO8 (CsGy5G026660) — to powdery mildew (PM) resistance in cucumber germplasms. For this purpose, fifty-four germplasms, consisting of thirty-three breeding lines and twenty-one landraces from fourteen diverse origins, were evaluated for PM-resistance at the adult plant stage and genotyped using molecular markers associated with the four PM-resistance-related genes. According to the phenotypic assessment, sixteen germplasms were susceptible, nineteen germplasms exhibited intermediate-resistant, and nineteen germplasms demonstrated resistant to PM. The results from phenotypic evaluation and genotyping analysis indicated that among the PM-susceptible germplasms, 50% of CsLRR-RPK2, 31% of CsABA2, 25% of CsaMLO8, and 18% of CsPM5.2 had inconsistent genotypes. Genotype inconsistencies among PM-resistant germplasms were observed in 16% of CsLRR-RPK2, 16% of CsABA2, 10% of CsaMLO8, and 10% of CsPM5.2. This study showed that the combination of CsPM5.2 and CsaMLO8 alleles is more effective and beneficial for PM-resistance breeding programs in cucumber. And also, this suggests there is a relationship between PM-resistance and the genotype of a combination of PM-resistance candidate genes. The findings of the genotyping and PM-phenotypic evaluation of the current study may improve selection accuracy in breeding for PM-resistance in cucumber.