Competition and density-dependent fitness in a plant parasitic fungus

Inter- and intrastrain competitive interactions and their effects on fitness were quantified for coexisting strains of Puccinia graminis f.sp. tritici (Pst) on wheat leaves. Urediniospores of two strains were inoculated onto leaves singly and in a 1:1 mixture over a range of inoculum densities. Per-leaf relationships among inoculum density, uredinial formation, and urediniospore production were quanitified and mathematically modeled. From the single-strain data, values of carrying capacity for uredinia on leaves, infection efficiency of urediniospores, maximum sporulation capacity on leaves, and sporulation efficiency of uredinia were estimated for both strains. From the mixed-strain data, interstrain competitive effects of each strain on the other's uredinial formation and urediniospore production were evaluated. Although one strain had competitive advantages in both uredinial formation and urediniospore production, the other strain was able to dominate in mixture due to its substantially higher carrying capacity, maximum sporulation capacity, and infection efficiency. This illustrates that in coexisting strains or species, competitive advantages do not necessarily translate into an advantage in fitness. The methods of competition analysis have potential applications for the study of pathogen populations, as well as other systems of coexisting organisms.