The spread of a viral infection in a plantation

We consider a plantation subject to viral infection, in which infected plants are replaced by plants from a nursery, also subject to a minor degree of infection. A discrete time Markov chain model for the spread of infection in the plantation is proposed, and its consequences derived. The expected number of healthy plants in the plantation is obtained in the stationary case, and some conclusions drawn about the effect of the level of infection in the nursery on that in the plantation.