Abstract
An understanding of the evolution of pathogen quantitative traits in response to host selective pressures is essential for the development of durable management strategies for resistant crops. However, we still lack experimental data on the effects of partial host resistance on multiple phenotypic traits (aggressiveness) and evolutionary strategies in pathogens. We performed a cross‐inoculation experiment with four grapevine hosts and 103 isolates of grapevine downy mildew (Plasmopara viticola) sampled from susceptible and partially resistant grapevine varieties. We analysed the neutral and adaptive genetic differentiation of five quantitative traits relating to pathogen transmission. Isolates from resistant hosts were more aggressive than isolates from susceptible hosts, as they had a shorter latency period and higher levels of spore production. This pattern of adaptation contrasted with the lack of neutral genetic differentiation, providing evidence for directional selection. No specificity for a particular host variety was detected. Adapted isolates had traits that were advantageous on all resistant varieties. There was no fitness cost associated with this genetic adaptation, but several trade‐offs between pathogen traits were observed. These results should improve the accuracy of prediction of fitness trajectories for this biotrophic pathogen, an essential element for the modelling of durable deployment strategies for resistant varieties.
Highlights
The use of quantitative resistance to control pathogens is increasingly being seen as a valuable and durable approach to crop protection (Brun et al 2010; L^e Van et al 2013; Mundt 2014; Niks et al 2015)
Gandon and Michalakis (2000) showed, in a theoretical study, that host quantitative resistance selected for greater virulence in parasites, assuming that quantitative resistance had no direct effect on pathogen transmission
We found no apparent differences in quantitative traits between isolates of ‘resistant varieties (RES)’ origin and of ‘susceptible hosts (SUS)’ origin on the susceptible host, suggesting that there was no fitness cost associated with aggressiveness
Summary
The use of quantitative resistance (i.e. partial resistance) to control pathogens is increasingly being seen as a valuable and durable approach to crop protection (Brun et al 2010; L^e Van et al 2013; Mundt 2014; Niks et al 2015). Aggressiveness can be assessed by evaluating multiple phenotypic quantitative traits of the pathogen directly linked to its fitness These traits are likely to be under selection, resulting in differential adaptive patterns according to the environment (Pariaud et al 2009; Lannou 2012). Gandon and Michalakis (2000) showed, in a theoretical study, that host quantitative resistance selected for greater virulence (i.e. the amount of damage caused to the host) in parasites, assuming that quantitative resistance had no direct effect on pathogen transmission. This model predicted that once quantitative plant resistance is eroded, pathogens exhibited
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