Obligate parasites are more active on vigorously growing plants, while non-obligates prefer weakened tissue and wounds. N, P, K, Ca levels significantly effect disease. The study of epidemics or epiphytotics. 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.