Epidemiology is the study of the reasons behind these outbreaks so that they may me stopped completely or at least reduced in frequency and duration. In human medicine epidemiologists study things like diet, living conditions, social behavior as well as the weather, movement of infected individuals, dispersal of new strains of pathogens etc. Plant pathologists study the same type of things but concentrate much more on weather and dispersal. All epidemics have a set of common elements represented by the disease triangle -susceptible host, virulent pathogen, and conducive environment. 1. susceptible host.
Genetic uniformity makes it worse.
Type of crop has influence also. Diseases in perennials are slower than in annuals. Environmental factors - Water and Temperature. Cultural practices, including site preparation, propagative material, sanitation, plant nutrition, monoculture 5. Introduction of new pathogens or new races or into new areas.