Limits to selection for disease resistance from natural tree populations

It is demonstrated that for diseases characterized by a small number of discrete infections per tree, random placement of spores on trees results in substantial variation in the number of infections per tree. The consequences of this phenomenon for selection of resistant individuals and for natural selection are examined, using a previously reported distribution of susceptibility model for the western gall rust (Endocronartium harknessii (J.P. Moore) Y. Hiratsuka)-lodgepole pine (Pinus contorta Dougl.) pathosystem as an example