A.9. Plant Pathogens. van den Bosch et al. [6] described a systematic method to calculate the basic reproduction ratio from knowledge of a pathogen's life cycle and its interactions with the host plant. They developed a system of linear difference equations and rearranged the dominant eigenvalue to find R 0 . A fraction q of infected spores are deposited in location 1, while a fraction 1 − q are deposited in location 2. This results in R 0 = q γ 1 α 1 τ 1 ρH + 1 − q γ 2 α 2 τ 2 ρH , (A.9)