Density-dependent fitness interactions in the bean rust fungus

Several paired comparisons of urediniospore production per uredinium at ranges of densities of five isolates of Uromyces appendiculatus on two susceptible bean cultivars were made. They revealed that isolates with very large (to 1.4 mm diameter) uredinia were more strongly influenced by crowding than were isolates with large (to 1 mm diameter) uredinia. Negative regression slopes of log of spore production per uredium over log of density (uredinia per unit area) of two isolates with very large uredinia were significantly steeper than were slopes of three isolates with large uredinia in all five comparisons that were made of the two classes of isolates. Slopes of log of reproduction on log of density were also significantly steeper in a more favorable growth chamber environment than in a less favorable greenhouse environment for a single isolate of each of the uredinial size classes. Hence, uredinial size and reproductive potential at low densities (about 1 uredinium/cm2) cannot be used alone to describe selective advantage of a very large uredinium; at high densities such a reproductive advantage is obviated. The results suggest an upper limit on spore reproductive rate per unit area of host tissue for the bean rust fungus