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What is Pathogen Aggressiveness?
- 1. What is pathogen aggressiveness? Does it matter? If it does what should we do about it? Chair: Chris Gilligan University of Cambridge
- 2. What is aggressiveness? • Panellists – Gene Milus, University of Arkansas – Mogens Hovmøller, Aarhus University – Robert Park, University of Sydney
- 3. Organisation of discussion • Definitions • Case studies • Big questions • Scientific challenges • Epidemiological • Surveillance • Evolutionary • Measurement • Genetical & Breeding • Management • Crop loss • (Relations with human epidemiology) • Actions
- 4. Aggressiveness: definition • Virulence – Pathotype either infects or does not – Remarkably important, relatively easy to work with “This compatibility relationship has so far largely monopolized the attention of plant pathologists and shaped the discipline.” Periaud et al. (2009) • Aggressiveness – Quantitative measure of pathogenicity & spread – Manifest by spread of new strains – Population level: rate of spread – Host plant level: yield loss & infectiousness
- 5. Aggressiveness: case studies • Puccinia striiformis • Puccinia graminis • Puccinia triticina • Many others: – Phytophthora infestans – Gaeumannomyces graminis – Ophiostoma novo-ulmi – Melampsora lini Invasion? –… Persistence? Co-existence?
- 6. Components of aggressiveness • Latent period • Speeds up infection cycle • Infection efficiency • Spore production rate • Increases rate of transmission • Lesion size • Infectious period • Increases host damage • Production of toxins • Reduction of photosynthetic activity • Necrosis
- 7. What are the big questions? • Epidemic perspective: – How do changes in aggressiveness affect epidemic dynamics? » Invasion, rate of spread, persistence • Evolutionary perspective: – What causes evolutionary change in aggressiveness? » How aggressive can a pathogen go? » Are evolutionary changes affected by crop management? • Genetical and breeding perspective – Is aggressiveness heritable and predictable? – How should it affect breeding strategies? • Yield perspective: – How does increased aggressiveness affect crop loss? » Trade-off: crop loss vs spread
- 8. Epidemiological perspective Adjusting the components of aggressiveness can lead to all or nothing responses • Infection efficiency R0 > 1 for epidemic invasion R0 < 1 no epidemic invasion • Spore production rate • Lesion size Phase transition from no • Infectious period invasion to sudden invasion by adjusting components of aggressiveness Ieq Ieq R0 “Reproductive 0 0 1 2 3 4 number” R0 R0
- 9. Epidemiological perspective Adjusting the components of aggressiveness can lead to all or nothing responses • Infection efficiency R0 just greater than 1: variability matters • Spore production rate • Lesion size • Infectious period Infecteds R0 “Reproductive number” Gilligan, C.A. & van den Bosch, F. (2008) Ann rev. Phytopathol 46: 385-418
- 10. Evolutionary perspectives Trade-offs matter: accounting for absence of excessively aggressive pathotypes selection of generalists and specialists Consider trade-off in transmission rates Infectivity to host 2 Infectivity to host 2 Infectivity to host 1 Gudelj, I., van den Bosch, F. and Gilligan, C. A. (2004). Proc.Roy. Soc. Ser. B 271: 2187-2194. van den Berg, F. Gilligan C.A. Bailey D.J. and van den Bosch, F. (2010). Phytopathology 100: 1169-1175
- 11. Organisation of discussion • Definitions • Case studies • Big questions • Scientific challenges • Epidemiological • Surveillance • Evolutionary • Measurement • Genetical & Breeding • Management • Crop loss • (Relations with human epidemiology) • Actions
- 12. What are the big questions? • Epidemic perspective: – How do changes in aggressiveness affect epidemic dynamics? » Invasion, rate of spread, persistence • Evolutionary perspective: – What causes evolutionary change in aggressiveness? » How aggressive can a pathogen go? » Are evolutionary changes affected by crop management? • Genetical and breeding perspective – Is aggressiveness heritable and predictable? – How should it affect breeding strategies? • Yield perspective: – How does increased aggressiveness affect crop loss? » Trade-off: crop loss vs spread
- 14. Relations with human epidemiology Human pathogens Within hosts • ‘Virulence’ • Quantity of damage induced by pathogen on host • Driven by within-host pathogen multiplication Between hosts • Pathogen fitness • Host => Host transmission • Usually assessed by R0 • Includes multiplication and survival Galvani, A.P. (2003) Trends in Ecology & Evolution 18: 132-139
- 15. Adaptive potential of aggressiveness components • Variability within pathogen populations • Heritability of aggressiveness components • Differential Cv x Isolate interactions • Trade-off: virulence vs aggressiveness • Genetic correlations between quantitative traits • Adaptation to environmental conditions Periaud, B., Ravigné, V., Halkett,F., Goyeau, H. Carlier, J. Lannou, C. (2009) Plant Pathology 58: 409-424
- 17. Evolutionary perspectives 2) Quantitatively-resistant crops could result in selection for increased pathogen aggressiveness Trade-off: for P. triticina Total spore production Total number of spores produced 250 a 200 Susceptible c b 150 Resistant 100 50 0 0.0 0.2 0.4 0.6 0.8 1.0 Lesion size (mm2) Lesion size van den Berg, F., Lannou, C. Pariaud, B., Gilligan, C. A. and van den Bosch, F. Quantitative resistance can select for increased aggressiveness through pathogen traits not related to resistance breaking in Puccinia triticina (Under review)