1 INTRODUCTION

Despite evidence that many plant pathogens are generalists and interact with multiple hosts in the community (Parker & Gilbert, 2007; Semchenko et al., 2022), interactions between plants and pathogens are typically studied in isolation of the wider community (but see Hawksworth, 2001). In invaded communities, exotic plant species can benefit from supporting and tolerating generalist pathogens that are shared with native species (i.e. accumulation of Local Pathogens Hypothesis, Eppinga et al., 2006; Mitchell et al., 2010; Semchenko et al., 2022; Visscher et al., 2021), resulting in pathogen-mediated apparent competition between plants (Holt, 1977; Holt & Bonsall, 2017, also known as disease-mediated invasion, Strauss et al., 2012). Exotic plants may benefit from influencing pathogen communities in native plant communities through various pathways: (1) pathogens that are compatible with resident plants are introduced with exotic hosts and integrate into native plant-pathogen networks (Bufford et al., 2020), (2) novel pathogens are introduced with exotic plants and make a host jump following a lag phase (Gilbert & Parker, 2010) or (3) exotic plants may amplify native or exotic pathogens that are already present in the environment (Eppinga et al., 2006; Malmstrom et al., 2005; Strauss et al., 2012). The importance of pathogen spread from exotic plants to native plants in invaded communities is difficult to quantify experimentally and marks an important gap in our knowledge of plant community dynamics (Flory & Clay, 2013; Goss et al., 2020).

Research in simplified crop and cultivated systems has shown that exotic invaders can serve as ‘reservoir hosts’ (asymptomatic or mildly symptomatic plants, also known as amplification hosts), spreading pathogens to agricultural plants (Linde et al., 2016; Wisler & Norris, 2005). However, demonstrating that exotic plants may act as reservoir hosts in more complex, multi-host, multi-pathogen natural systems has proven to be more challenging (Paull et al., 2012). Studies that do adopt a microbial community approach often examine plant–soil feedbacks (Bever et al., 1997), comparing the influence of different plants on and their response to pathogen communities in soils. However, plant–soil feedback experiments rarely involve more than 1–2 plant hosts or characterise specific microbial functional groups, thereby failing to determine whether effects of soil biota are due to pathogens or parasitic mutualists (e.g. mycorrhizal fungi, Klironomos, 2003). Furthermore, these tests cannot distinguish between the Enemy Release (Keane & Crawley, 2002) and Accumulation of Local Pathogens Hypotheses, as the absence of a growth depression from antagonistic soil biotic communities could indicate that plants have either escaped or are simply tolerating pathogens. Finally, it remains unclear whether plant–soil feedback effects observed in monoculture translate to a community, where the sharing of interaction partners may result in influential indirect interactions (Allen, 2020).

Several characteristics of exotic plants could make them more competent hosts for pathogens compared to native species. For instance, many exotic invaders exhibit “quick-return” strategies, such as faster growth and high nitrogen in tissues (Leishman et al., 2014; van Kleunen et al., 2010), and “quick-return” plants are generally more competent hosts for antagonists, including pathogens, than “slow-return” species (Allen et al., 2021; Cappelli et al., 2020; Cronin et al., 2010; Fahey et al., 2022; Strauss & Agrawal, 1999). Second, many exotic plants can tolerate damage from enemies with minimal impact on plant fitness (Ashton & Lerdau, 2008; Goss et al., 2020; Roy & Kirchner, 2000), and/or can replace lost tissues faster than slow-growing species (Allen et al., 2021; Gianoli & Salgado-Luarte, 2017). Third, exotic plants can attain high local abundance, increasing opportunities for pathogen establishment and spread (Burdon & Chilvers, 1982; Gilbert, 2002). Thus, communities with a majority of tolerant, exotic host biomass likely experience maximum transmission rates by pathogens (Parker & Gilbert, 2004). Considering that many exotic plants possess quick-return strategies, exhibit disease-tolerance, and occur at high abundance, they are likely to meet these criteria for acting as reservoir hosts.

To examine whether exotic plants host more generalist fungi that are shared with native plants, we conducted a single-species and a community-level plant–soil feedback experiment. We characterised fungal communities in the roots of native and exotic plants growing together in the 8-species communities (n = 80), spanning a range of exotic dominance from 0% to 100%. Our focus was on soil-borne fungi, as they represent the most common and damaging pathogen group affecting plants (Delgado-Baquerizo et al., 2020). We restricted our analyses to include only fungal taxa previously recognised as “probable” or “highly probable” pathogens/pathotrophs in the FUNGuild database (Nguyen et al., 2016). We quantified the relative abundance of pathogens in plant roots, calculated the proportion of operational taxonomic units (OTUs) shared between native and exotic plant hosts, assessed the generality of associations for both plants and fungi (relative to other species in the community) and analysed the frequency at which generalist fungi are shared with other plants. We addressed the following research questions: (1) Do exotic plants and exotic-dominated communities host more generalist pathogens than native plants and native-dominated communities? (2) Given their higher generalism, do exotic plants share a greater proportion of putative fungal pathogens with natives than do other native plants? (3) Does pathogen sharing between native and exotic plants correlate with exotic plant dominance in communities? (4) Do plant–soil feedbacks in monoculture predict feedbacks in communities?