  A simple mean field model is proposed that captures the two scales of dynamic
processes involved in the spread of the coffee rust disease. At a local level
the state variable is the fraction of coffee plants on an average farm that are
infested with the disease and at the regional level the state variable is the
fraction of farms that are infested with the disease. These two levels are
interactive with one another in obvious ways, producing qualitatively
interesting results. Specifically there are three distinct outcomes: disease
disappears, disease is potentially persistent, and disease is inevitably
persistent. The basic structure of the model generates the potential for
tipping point behavior due to the presence of a blue sky bifurcation,
suggesting that the emergence of epizootics of the disease may be
unpredictable.
