Native perennial and non-native annual grasses shape pathogen community composition and disease severity in a California grassland

Abstract

O_LIThe densities of highly competent plant hosts may shape pathogen community composition and disease severity, altering disease risk and impacts. Life history and evolutionary history influence host competence: longer-lived species tend to be better defended than shorter-lived species and pathogens adapt to infect species with which they have longer evolutionary histories. It is unclear, however, how the densities of species that differ in competence due to life and evolutionary histories affect plant pathogen community composition and disease severity. C_LIO_LIWe examined foliar fungal pathogens on two host groups in a California grassland: native perennial and non-native annual grasses. We first characterized pathogen community composition and disease severity on the two host groups to approximate differences in competence. We then used observational and manipulated gradients of native perennial and non-native annual grass densities to assess the effects of each host group on pathogen community composition and disease severity in 1-m2 plots. C_LIO_LINative perennial and non-native annual grasses hosted distinct pathogen communities but shared generalist pathogens. Native perennial grasses hosted pathogens with larger host ranges and experienced greater disease severity (75% higher proportions of leaves with lesions) than non-native annuals. While the relative abundances of three common pathogens tended to shift, and disease severity of native perennial grasses tended to increase with increasing densities of both host groups, these changes were not statistically significant. C_LIO_LISynthesis. The life and evolutionary histories of grasses likely influence their competence for different pathogen species, leading to distinct pathogen communities and differences in disease severity. However, there was no evidence that the density of either host group significantly affected pathogen community composition or disease severity. Therefore, variation in competence for different pathogens likely shapes pathogen community composition and disease severity but may not interact with host density to alter disease risk and impacts at small scales. C_LI