van der Kamp and Tait (3) developed a mathematical model to describe the lodgepole pine (Pinus contorta Douglas ex Loud.)-western gall rust (Endocronartium harknessii (J.P. Moore) Y. Hiratsuka) pathosystem. Although discussed in terms of this particular disease, the model concerns itself with the segregation of members of a natural population that are susceptible and resistant to diseases characterized by discrete infections. van der Kamp (2) applied the model to investigate the theoretical limits of selecting trees for resistance to a particular disease and concluded that "...the ability to identify resistant trees is significantly compromised by random placement of spores and by the large number of trees that remain uninfected". In contrast, our examination of the model predicts increased levels of resistance when disease-free trees are selected.