Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/28/1997
Publication Date: N/A
Citation: Interpretive Summary: Many of the most damaging diseases of crop plant are caused by pathogenic fungi that are genetically diverse. For example, hundreds of different races of the wheat stem rust fungus have been identified. Each race can attack a different combination of wheat varieties with different types of rust resistance. The multitude of rust races that potentially could attack commercial wheat varieties in the field makes it difficult to breed wheat varieties that will remain healthy and avoid severe yield loss to rust attack. Toward the goal of improving predictions of which rust races are most likely to be encountered by wheat varieties in the future, we studied the basic nature of competition between stem rust races in mixed infections of wheat. We found that races differ in ability to infect wheat, in the numbers of spores that they produce within each infection, and in the maximum numbers of infection and spores they can form on single wheat plants. Knowing these characteristics, however, is not enough to predict which race will win the competition. We also found that some races are better than others in inhibiting the infection efficiency and spore production of their competitors. This information will be used by scientists studying rust populations to anticipate changes in rust races in the field. Their predictions, in turn, will be used by wheat, barley, and oat breeders to select the best new types of rust resistance to add to their new varieties to attain long-term protection against rust diseases, which are capable of causing hundreds of millions of dollars in losses to susceptible cereal crops.
Technical Abstract: In mathematical models, we investigated how infection and sporulation characteristics of competing plant pathogens determine density- and frequency-dependence of relative fitness. Two models, for the infection and sporulation stages of the pathogen life cycle, describe reproductive output of pathogen strains in mixture as a function of the strains; population densities. Model parameters include infection and sporulation efficiencies, carrying capacities on leaves for sporulating lesions and spore production, and coefficients of inter-strain competitive effects in both life cycle stages. Paired hypothetical strains were assigned equal, baseline parameter values. Parameters were then altered one at a time for one or both strains, and relative fitness was calculated over a range of densities and strain frequencies. Except for infection efficiency, the fitness benefit conferred by advantages in single parameters was density- dependent. Relative fitness was frequency-dependent whenever inter- and intra-strain competitive effects were not equal.