This passage refers to a group of disease-causing agents that exhibit varying levels of pathogenicity depending on the specific host species, varieties, or cultivars they infect. This group, referred to as a “population of a pathogen,” is genetically diverse and may not exhibit the same level of pathogenicity across all host types. The term “population of a pathogen” was introduced by E.R. Wallace as a more precise alternative to the concept of a “physiological race,” which is based on incomplete information about pathogenicity. To establish the boundaries of a physiological race, it would be necessary to determine the pathogenicity of the pathogen to every variety and cultivar of the host species, which is in practice not feasible. Instead, the concept of a “population of a pathogen” acknowledges the limitations of our knowledge and describes subsets of the pathogen that are pathogenic to a limited range of host varieties or cultivars.