Print PageIn natural ecosystems plant pathogens play key roles in regulating plant community dynamics. This is particularly relevant in invasion biology as some invasive plants gain an advantage over competitors through their interactions with components of the soil biota, especially soil pathogens. Phragmites australis is one of the most important invasive plants in estuarine and freshwater wetlands. Whereas both introduced and native genotypes are widespread in North America, the introduced genotype is extremely aggressive and a major problem for wetland management. Seed germination and seedling recruitment are central to the initial colonization and establishment of invaded habitats by P. australis. Oomycete pathogens are of special interest because of their negative impacts on seed and seedling mortality along with the abundant and diverse oomycete communities known to exist in wetland soils. We hypothesize that pathogen spillback, the amplification of native pathogens by a non-native competent reservoir host, contributes to the success of introduced P. australis. To test this hypothesis we compared the structure of soil-inhabiting oomycete communities associated with four populations each of native and introduced P. australis using both culture-based and culture-independent approaches. We also assessed the virulence of individual isolates obtained from each P. australis population.
Oomycete communities associated with introduced P. australis genotypes were distinct from those associated with native genotypes. Communities were dominated by species of Pythium, many of which are known pathogens of native and domestic plant species. The distribution of oomycete taxa across sites was extremely variable, yet some taxa were significantly less abundant in native than in introduced P. australis soils. In pathogenicity bioassays, recovered isolates displayed a range of virulence to seeds and seedlings of both native and introduced P. australis as well as other native wetland plant species. When tested on native P. australis seeds and seedlings, the most virulent species from native P. australis populations differed from the most virulent species from introduced P. australis populations. We found no evidence that individual isolates exhibit differential virulence on native and introduced P. australis. Our results indicate that introduced genotypes of P. australis differentially influence the species composition of oomycete soil communities and their virulence to native P. australis and a range of other important native wetland plant species. These differential responses may facilitate P. australis invasion dynamics by making pathogen-compromised native plant species less competitive during P. australis invasions.
