Abstract
Soil pathogens affect plant community structure and function through negative plant–soil feedbacks that may contribute to the invasiveness of non-native plant species. Our understanding of these pathogen-induced soil feedbacks has relied largely on observations of the collective impact of the soil biota on plant populations, with few observations of accompanying changes in populations of specific soil pathogens and their impacts on invasive and noninvasive species. As a result, the roles of specific soil pathogens in plant invasions remain unknown. In this study, we examine the diversity and virulence of soil oomycete pathogens in freshwater wetland soils invaded by non-native Phragmites australis (European common reed) to better understand the potential for soil pathogen communities to impact a range of native and non-native species and influence invasiveness. We isolated oomycetes from four sites over a 2-year period, collecting nearly 500 isolates belonging to 36 different species. These sites were dominated by species of Pythium, many of which decreased seedling survival of a range of native and invasive plants. Despite any clear host specialization, many of the Pythium species were differentially virulent to the native and non-native plant species tested. Isolates from invaded and noninvaded soils were equally virulent to given individual plant species, and no apparent differences in susceptibility were observed between the collective groups of native and non-native plant species.
Highlights
Plant pathogens often have significant impacts on plant populations, where they may influence the diversity and structure of plant communities (Mangla and Callaway 2007; Beckstead et al 2010; Mordecai 2011)
While our results have confirmed the high prevalence of Pythium species in both invaded and noninvaded soils, they have revealed the wide differential virulence of many of these generalist pathogens to a range of wetland plant species
Perhaps among the more significant findings from our work was the observation that isolates of most Pythium species did not differ in their virulence to individual plant species regardless of whether they were isolated from invaded or noninvaded soils
Summary
Plant pathogens often have significant impacts on plant populations, where they may influence the diversity and structure of plant communities (Mangla and Callaway 2007; Beckstead et al 2010; Mordecai 2011). Many invasive species are thought to experience reduced negative impacts from pathogens in introduced ranges relative to their native ranges (Callaway et al 2011; Flory and Clay 2013; Maron et al 2013b), in part because the composition and relative abundance of pathogens in introduced ranges differ from those in native ranges as a result of geographic isolation and local evolution (Rout and Callaway 2012) This leads to pathogen interactions in the introduced range that could potentially contribute to invasiveness through a number of different mechanisms including (1) the inhibition of pathogens in the introduced range by the invading plant species (Zhang et al 2009, 2011), (2) reduced frequency or abundance of virulent taxa in the invaded range (Reinhart et al 2010b, 2011), and (3) decreased susceptibility of introduced invasive plants to pathogens endemic to the invaded range (Klironomos 2002; Beckstead et al 2010; Mordecai 2011).
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