Background/Question/Methods  Across a broad range of growth forms, variability in plant physiological traits can be mapped onto a continuum that represents life history trade-offs between fast growth and tissue longevity, or between rapid resource acquisition and resource conservation. These traits have long been used to understand and predict plant adaptation/acclimation to resource environments, but whether these traits mediate plant interaction with natural enemies is less established. In general, conservative species have low tissue nitrogen concentrations and tougher leaves, and consequently experience lower rates of herbivory.  We predict that when herbivores act as pathogen vectors, lower rates of herbivore contact or feeding should decrease the likelihood of infection (susceptibility). Similarly, if conservative species do become infected, low herbivore contact rates will decrease the likelihood of infecting new vectors (host competence).  Host susceptibility and competence are chief determinants of pathogen transmission rate.  If physiological traits mediate both plant response to resource availability and plant pathogen transmission, they may help predict pathogen emergence and prevalence under global change scenarios of increasing resource availability.
We tested four trait-based hypotheses of plant-resource and plant-pathogen interactions:  1) physiological traits covary as predicted by a resource acquisition-conservation tradeoff; 2) nitrogen addition shifts plant species traits toward a less conservative strategy, as evidenced by changes in physiological trait values; 3) plants with conservative traits are less susceptible and competent hosts;  4) by shifting physiology to a less conservative strategy, nitrogen addition increases host susceptibility and competence. We used 20 California grassland hosts of generalist, vector-borne barley yellow dwarf viruses (BYDVs) as a model host-pathogen system. In a factorial greenhouse experiment, we manipulated host species, nitrogen supply, and virus infection (inoculation with infected vs. uninfected aphids). Host species included both native and non-native species with both annual and perennial life-histories.      
Results/Conclusions Results support hypotheses 1 and 2. We found a significant, positive relationship between LMA and leaf toughness, as predicted by a resource acquisition-conservation tradeoff. Nitrogen addition significantly decreased leaf toughness and increased photosynthetic capacity, consistent with a phenotypic shift towards less conservative strategies in nitrogen addition treatments. Annuals had lower LMA and leaf toughness, and were more likely to become infected when inoculated (i.e. they were more susceptible to infection). However, susceptibility to infection did not depend on either ecophysiological traits or nitrogen supply. Life history was the only significant predictor of inoculation success.  Susceptibility to infection did not differ between native and non-native host species.