Competition in mixtures of susceptible and resistant genotypes of Chondrilla juncea differentially infected with rust

Plants of two genotypes of Chondrilla juncea (skeleton weed), one susceptible to and the other resistant to one isolate of the rust Puccinia chondrillina, were grown as pure and mixed populations both in the presence and absence of rust. Weights of individual plants were obtained at two harvests, one when the plants were rosettes and the other when flowering had begun. Distributions of plant weights of each genotype became progressively more positively skewed with time, with rust infection of plants of the susceptible genotype and with increasing competition between plants of both genotypes. The results show that genetic differences may be an important factor in determining which individuals become dominant or are suppressed in competing mixtures, and that differential disease pressure may alter dominance of individuals in plant populations.