The effectiveness and costs of pathogen resistance strategies in a perennial plant

Abstract

Summary Plants have evolved different strategies to resist pathogens, but little is known about how effective, stable and costly these mechanisms are in perennial plants across multiple growing seasons. We conducted a laboratory experiment to assess resistance variation in Plantago lanceolata against the powdery mildew‐causing fungus Podosphaera plantaginis and to measure possible trade‐offs between the different resistance strategies. To test stability and costs of resistance, we established common garden populations of plants possessing three different resistance strategies: qualitative resistance that is the ability to block pathogen infection, quantitative resistance that is the ability to mitigate pathogen development and susceptibility. We measured their performance with and without disease for 3 years. There were no trade‐offs between qualitative and quantitative resistance, and the components of quantitative resistance were positively correlated. Throughout the 3‐year common garden study, pathogen loads were significantly affected by host resistance in the study populations. Qualitative resistance efficiently blocked infections but contrary to laboratory obtained results; quantitative resistance did not hinder epidemic growth. We detected costs in plant performance for qualitative and quantitative resistance compared to susceptible plants in absence of the pathogen. The costs of infection varied according to resistance strategy, pathogen load and plant age. Synthesis. In perennial plants, the costs and benefits of resistance need to be evaluated over multiple years as they may change with plant age. Our results give new insights into how polymorphism in resistance can be maintained through costs of resistance in plant performance and through shifts in resource allocation between vegetative growth and reproduction under infection.