Epidemiology is the science of disease in populations, in this article plant populations. It is a quantitative discipline with strong conceptual foundations and practical applications. Epidemics of plant diseases have had major impacts on agricultural and horticultural crops and on forest-tree populations with major socio-economic and political consequences. An epidemiological tool-kit includes methodologies for analysing disease progress in time and space, the derivation of key parameters determining the rate of epidemic progress, and the extent of control required for disease management. The complexity of biotic interactions affecting plant epidemics is shown by introducing genetic variation in host and pathogen, the role of disease vectors in transmission, and higher level interactions involved in biological control. The importance of disease in wild plant communities is assessed. With the increased recognition of the landscape connectedness of crop plants, natural communities and invasive species, some additions to the epidemiological tool-kit are proposed.

Key concepts

• Infection cycle: The components of the pathogen life cycle in a host population. For a fungal pathogen this would include the sequence infection, colonization, sporulation and dispersal (to a new host, completing the cycle).
  
• Integrated disease control: The use of different methods of disease control – chemical, cultural, biological and host resistance – within a decision-based approach to disease management.
  
• Disease triangle: A recognition that a disease (epidemic) is a consequence of the interaction of a virulent pathogen population with a susceptible host population in a conducive (biotic and abiotic) environment. With plant virus diseases the ‘triangle’ is often extended to include explicity the vector.
  
• Basic reproductive number: The number of secondary infected units (plants, plant parts) caused by the introduction of one infectious unit into a susceptible host population, curing that unit's period of infectiousness.
  
• Intraspecific diversity: Genetic variation occurring within a plant host and/or pathogen population; such variation can determine the level of compatibility in a host–pathogen interaction as expressed in gene-for-gene recognition systems. It is used in disease control strategies based on multilines and varietal mixtures.
  
• Evolutionary epidemiology: An attempt to integrate population genetic and population dynamic approaches in epidemiology so that the evolutionary dynamics of plant pathogen populations can be studied and managed.
  
• Landscape epidemiology: A recognition that plant disease epidemics occur at the landscape scale, and are influenced by aspects of the wider environment including features such as topography, watersheds, human activity and the conjunction of cropland and semi-natural vegetation.