So often new phytopathogens emerge and appear primarily in acute form and then take a chronic form; such populations, however, in general have a limited appearance because of the lack of suitable environmental conditions. The emergence of new pathogens needs to be explored in the light of their evolutionary adaptation. This new volume focuses on the study of quantitative aspects of host-phytopathogen linkages that result in the emergence of aggressive phytopathogens. The book examines the evolution and adaptation of phytopathogens from several cropping systems.