Call number: LD2668 .T4 1949 I71 / Master of Science

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Disease has been implied as an important selective force acting in plant populations. This study was conducted to determine the effects of stripe rust (Puccinia striiformis) on the population dynamics of wheat (Triticum aestivum) cultivar mixtures. Five wheat cultivars were grown in pure stands and all possible mixtures at three and two locations in 1987 and 1988, respectively. In 1989, four replacement series and their component pure stands were grown in two locations. All treatments were exposed to or protected from two stripe rust races. Disease severity and yield were determined on a per-cultivar basis for mixtures and also for pure stands. In all but one mixture, disease severity relative to the pure stands was reduced between 6 and 97%. Disease severity changes could be separated into two effects: First, selection for the more resistant or susceptible genotype reduced or increased disease in mixtures as compared to their pure stands by up to 47 and 11%, respectively. Second, epidemiological effects of host diversity reduced disease severity on individual cultivars below that of their pure stands. Disease severity on a genotype was often frequency-dependent. However, interactions among plant genotypes sometimes appeared to alter susceptibility and obscured the relationship. Non-diseased and diseased mixtures yielded 0 to 8% and 8 to 15% more than pure stands, respectively. overall, mixture yields were more influenced by plant-plant interactions than by disease. Population dynamics over time were studied by applying variable disease pressure to populations of four wheat cultivars for one-to-three generations in two locations. Fitnesses of genotypes were calculated by regressing the legit of a genotype's frequency on generation. Fitnesses were affected by disease and location and appeared constant over time. However, genotype frequency-changes were negatively correlated with planting frequencies, suggesting that fitnesses were frequency-dependent. Analysis of data from longer-term studies in the literature indicated that three generations may not have been sufficient to detect frequency-dependence. Stable equilibria may more likely exist for mixtures of genotypes that are closely related and adapted to the environment in which they are grown than for randomly selected genotypes. / Graduation date: 1992

Thesis (M. Sc.)--University of Hong Kong, 1969. / Typewritten.

Thesis--Ph. D., University of Hong Kong. / Typewritten.