ABSTRACT
Modification in capability of a phytopathogen to sustain in a new area is necessarily essential. This is determined by the adaptation of phytopathogen to the abiotic condition at first, and then to the biotic adjustments leading evolution. Indeed, the process of evolution is started after genetical adaptation with its host. This chapter explores the role of variables influencing virulence, impact of population size on evolution, and host evolution participating in adaptation and evolution of phytopathogen. The virulence evolution in a phytopathogen is greatly dealt; whereby pathogen fitness, host existence, and survival capability are covered. The chapter also highlights the difficulties during evolution. In short, every change is liable to diminutive units of adaptation, which ultimately leads to evolution of a pathosystem. Population size of a phytopathogen is a major factor that determines the degree of evolution.
