where the first term represents the number of spores that successfully germinate in location 1 and the second term represents the number of spores that successfully germinate in location 2. Here, γ i is the probability that a spore deposited on location i will germinate, α i is the number of spores produced per time unit on location i, τ i is the infectious period of a spore-producing lesion on location i, ρ is the probability that a spore is deposited on a susceptible site, and H is the density of susceptible sites in a host population.