Both environmental conditions and pathogen community structure are likely to mediate disease burden on hosts. However, how natural gradients of environmental variation influence pathogen community composition and pathogen pressure on host populations remains a major empirical gap. To better understand how environmental heterogeneity shapes the landscape of disease experienced by plant hosts, we took advantage of the occurrence of the California native bunchgrass Stipa pulchra in both benign greenstone and harsh serpentine soil types with differing chemistries and plant communities. We asked whether S. pulchra on different soil types found along the same ridgetop would have significantly different disease burdens caused by unique pathogen communities. To answer this question, we compared the amount of symptomatic foliar tissue and the fungal pathogen communities associated with foliar disease over three years of field surveys. Additionally, we analyzed the chemical makeup of plant tissue from serpentine- and nonserpentine-grown S. pulchra to understand potential effects of soil chemistry on plant-pathogen interactions.
Results/Conclusions
We found that distinct fungal pathogen communities associated with each soil type caused consistent, low disease burden on S. pulchra. Foliar tissue chemistry reflected the soil type plants grew in: serpentine plants had lower plant essential nutrient content and higher Ni and Mg content. Plant community structure and plant tissue chemistry represent two possible mechanisms by which soil type might shape unique pathogen communities at our study sites. Our findings that fungal pathogen communities causing minimal damage on S. pulchra experience high species turnover both between years and with soil type suggest that local variation in soil type and annual variation in climatic conditions, along with seasonal plant senescence, may protect S. pulchra from disease outbreaks and maintain diverse communities of relatively benign fungal pathogens. The processes that limit disease burden on S. pulchra suggest that differences in climate and growth form may explain contrasting conclusions from studies in semi-arid grasslands and in tropical forests that find differing roles for disease in plant community structure.