[Figure: see text] ▪ Abstract  Farmers motivated and inspired me to pursue a career in plant disease epidemiology. A taste for field observation and a need for reflexion led to a theory of disease foci, which grow in space and time. Quantitative effects are modulated by qualitative factors such as race differentiation of the pathogen and partial resistance of the host plant. Moving to larger scales of operation, within- and between-crop diversity become important. Diversity may be amenable to management. Whereas reductionistic experimentation will remain necessary to explain details, this paper makes a plea to use modern techniques for tackling complexity in crop protection.