Epidemic Development and the Measurement of Disease Levels and Crop Losses

Abstract

Epidemiology has been the subject of much recent experimentation and thought by pathologists. Basically, this is an attempt to quantify in scientific terms the reasons why epidemics develop in particular ways and to differentiate between different forms of epidemic. From this work has developed a set of equations describing disease incidence and relating this to the likely success of control techniques, especially the use of various forms of host resistance. The topic has evolved a mathematical basis, and hence prediction of epidemics has changed from an art to a science. In order to control plant pathogens by host resistance, by husbandry methods or by chemicals, it is essential to understand that host-parasite interactions differ in the manner of their development and their interrelation with the environmental conditions. Techniques of disease measurement and prediction need to become objective so that they may be applied on the widest possible basis. Hence disease assessment keys need to be internationally standardised, allowing uniform and unbiased evaluation of disease amount. Epidemiological studies now permit advance calculation of the likely life expectancy of novel forms of host resistance in relation to the capacity of the pathogen population to develop compatible virulences at high frequency levels. Such work is now becoming integrated with studies of population genetics of host and pathogen. Notice is also being taken of the need to quantify losses due to pathogens.