The grape-powdery mildew pathosystem is characterized by a polycyclic pathogen capable of explosive multiplication and a host population with a high degree of spatial structure at the field level and with a complex architecture of leaf and berry structure at the individual plant level exhibiting rapid changes over time. As well as environmental differences, the high degree of human interference during vine development and the wide diversity of cropping systems further enhances variability from one crop to another and hence, variability between epidemics.