Host genotype diversity and its effects on pathogen populations

1. Host genotype diversity and its effects on pathogen populations Mixtures of red-grained and white-grained sorghum (courtesy of Dr. Henry Ngugi, Pennsylvania State University)

2. Host genotype diversity = “host heterogeneity” • What is it, why the interest in it • Mechanisms: • why does it work • factors that make it more / less effective • Empirical experience • Effects on pathogen populations • does it select for complexity?

3. Technically… • Mixture: different species • fescue + bluegrass • Blend: different varieties, same species • wheat varieties NC-Neuse + USG3592 • Multiline: mixtures of near-isogenic lines (NILs) • 1950s: Browning developed oat multilines for managing oat crown rust in Iowa

4. Basic idea: if monoculture maximizes selection for new virulences, take a step back

5. Host genotype diversity • Mechanisms • Dilution of inoculum • Barrier effect • Induced resistance

6. “Mid-component”: the mean of the mixture’s component cultivars when they are grown as pure stands • Mixture performance is typically evaluated by comparison to the mid-component • Disease severity • Yield • Yield stability • Quality

7. Mechanisms of disease reduction in small-grain blends • Induced resistance -- accounted for 27% of total stripe rust reduction in wheat cultivar mixtures (Calonnec et al, 1996, Eur. J. Plant Pathol. 5:733-741) • Differences in genetic background among cultivars (partial resistance) -- led to additional 33% reduction of powdery mildew beyond that accounted for by R-gene differences (Wolfe et al., 1981, in Jenkyn and Plumb, eds, Strategies for the Control of Cereal Disease, Oxford: Blackwell) • Compensationof resistant cultivars through increased tillering -- accounted for 6% of total disease reduction by club wheat mixtures inoculated with stripe rust (Finckh & Mundt, 1992, Phytopathology 82:905-913) • Disease reductions below mid-component were statistically significant in 20 of 58 two-, three-, and four-component mixtures

8. Club wheat mixture: components can be distinguished by head color

9. From data in Finckh & Mundt, 1992, Stripe rust, yield, and plant competition in wheat cultivar mixtures, Phytopathology 82:905-913

10. Characteristics that predict whether blends will reduce disease (Garrett & Mundt, 1999) • Things that maximize allo- vs. autodeposition • Genotype unit area • Shallow dispersal gradient of pathogen spores • Small lesion size • Short pathogen generation time • Strong host specialization Small GUA Large GUA

11. Different plant sizes relative to pathogen dispersal gradient (Garrett & Mundt, 1999. Epidemiology in mixed populations. Phytopathology 89:984-990.)

12. Garrett & Mundt, 1999. Epidemiology in mixed populations. Phytopathology 89:984-990.

13. Spatial scale is predicted by theory to be important to blend effectiveness • Research plots may underestimate blend effects in controlling spread from disease foci that would be seen on a commercial scale • Host-diversity effects of reducing disease should be larger for larger fields of mixtures than for smaller fields, because the difference in epidemic velocity between pure and mixed stands will increase with distance from the focus • Large-plot wheat / stripe rust experiments in eastern Oregon: Cowger et al, 2005. Velocity of spread of wheat stripe rust epidemics. Phytopathology 95:972-982

14. Cowger et al, 2005. Velocity of spread of wheat stripe rust epidemics. Phytopathology 95:972-982.