Cultivation of resistant varieties is an environmentally friendly and inexpensive method of crop protection. Numerous alleles of specific disease resistance occur in cereals and other crops, and knowledge of their presence in individual varieties has wide utilization in research and practice. Postulation based on phenotyping host—pathogen interactions and the gene-for-gene model is a common way of identifying these genes. The same technique and design of tests are used for postulating virulence when pathogen populations are studied. Powdery mildews caused by different formae speciales of Blumeria graminis (Bg) are important cereal diseases. In this contribution, experimental methods are described that use a model organism Bg f. sp. hordei, which can be employed for other cereal mildews and possibly rusts. It includes illustrations and a summary of our long-term practical experience. It also critically evaluates the benefits of leaf segment tests compared with screening whole plants.