The variation in the reaction (susceptible or resistant) of a bacterial isolate depends on the cultivar of the host plant. The host crop plant with different genetical makeup reacts differently to the same bacterial isolate to produce susceptible or resistant reaction, and this forms the basis for variation in the given bacterial species or pathovars. The isolates of the pathovars, on the basis of their reaction to different genotypes/cultivars/varieties of the host crop, are further subdivided into races.