Viewing Fragmentation through the Disease Triangle: Implications for Pathogen Pressure on Seeds

Disease and landscape fragmentation each shape plant diversity, but how and why pathogen pressure varies across fragmented landscapes is not well understood. Landscape fragmentation likely influences all three components of the classic plant pathology "disease triangle"-hosts, pathogens, and abiotic environments. Thus, we would expect pathogen pressure to vary with fragment size. We focus on data from an experimentally fragmented landscape in Kansas, USA as a case study for ways in which fragmentation may impact soilborne plant disease. We surveyed plant communities in permanent plots stratified across three habitat types: small patches, large patch edges, and large patch interiors. We also surveyed fungal communities by isolating and identifying fungi from seeds buried for a year near survey plots. To assess the impact of pathogens on seed hosts, we buried seeds of six plant species; half of the seeds were treated with fungicide. After one year, we retrieved the seeds to conduct germination trials and isolate fungi. We found that habitat type influenced all vertices of the disease triangle. Plant communities were compositionally distinct among habitat types, with large patch interiors containing the most species. Fungal communities isolated from seeds varied according to both host species and habitat type. Mean daily temperature range was highest in small patches. More extreme temperature ranges decreased germination across the landscape for two plant species, and habitat type influenced germination success for two additional species. Fungicide increased seed germination success across habitat types in two species, but for four species, benefits of fungicide depended on where in the landscape seeds were buried. Altogether, our study demonstrates that patch size alters the community context for disease by impacting all three vertices of the "disease triangle," with consequences for seed survival. Our study also shows that impacts of disease on seed germination are not uniform across habitat types. This study provides a foundation for future work to mechanistically explore how fungal-mediated seed survival contributes to biodiversity in fragmented landscapes.