ABSTRACT
Epidemiology is concerned simultaneously with populations of pathogens and host plants as they occur in an evolving environment, i.e., the classic disease triangle. Some plant disease epidemics, e.g., wheat rusts, southern corn leaf blight, and grape downy mildew, have caused tremendous losses of produce over rather large areas. Knowledge of epidemiology is useful in forecasting of a disease and also for the management of a disease. The disease triangle can be expanded to include two more components, time and humans. The amount of each of the three components of disease and their interaction in the development of the disease are influenced by the fourth component, time. Three components are absolutely necessary in order for a disease to occur in any plant system. The three components are susceptible host plant, virulent pathogen and favorable environment. Pathogens are typically restricted to an area based on the conditions of the macroclimate. A microclimate is the prevailing climatic conditions in a certain geographical area.
