Root traits
Plant roots, like leaves, are also thought to follow a root economic spectrum that influences soil pathogen susceptibility, but root systems are less well studied than aboveground traits and some evidence suggests that the root economic spectrum is only partially correlated with the leaf economic spectrum (Kramer-Walter et al. 2016; Laliberté 2017). Larger root systems can improve disease tolerance of plants when roots serve as storage organs for regrowth after damage/defoliation (Erbet al. 2009). As seen with leaves, faster growing roots with high turnover rates are less susceptible to pathogen attack (Yanai & Eissenstat 2002; Atucha et al. 2014), and root surface area and secondary root diameter have been shown to be positively correlated with disease resistance (Sollaet al. 2011; Singh et al. 2019). There is limited information on the mechanisms driving these correlations and studies across a broad range of species are needed to determine the full range of variability in disease susceptibility.
It is well documented that arbuscular mycorrhizal (AM) colonization can protect plants from soil-borne pathogens (Borowicz 2001; Whipps 2004; Pozo & Azcón-Aguilar 2007; Wehner et al.2011; Lewandowski et al. 2013). Potential mechanisms for mycorrhizal mediated pathogen protection include; 1) direct competition with or inhibition of pathogens (Newsham et al.1995); 2) promotion or changes in plant growth, nutrition, and morphology (Wehner et al. 2011); 3) modulation of biochemical plant defense (Van Wees et al.2008); and 4) facilitation of pathogen suppressive microbiomes (Whipps 2004). These mechanisms can be influenced by root architecture such as root branching (Sikes et al.2009; Maherali 2014). Ectomycorrhizal (ECM) fungi can also protect their hosts from pathogens and are thought to provide better protection against pathogens than AM fungi because of the sheath, hyphal network, and antimicrobial substances associated with ECM fungi (Garrett 1956; Marx 1972; Stack 1975; Duchesne et al. 1989). Evidence has been accumulating to suggest that ECM fungal protection against soil-borne pathogens can be a major mechanism behind more positive plant-soil feedbacks among ECM driving mono-specific stands of ECM compared to AM species (Connell & Lowman 1989; Laliberté et al. 2015; Liang et al. 2020). These findings suggest that ECM invasive plants may be less susceptible to pathogens than AM or non-mycorrhizal invaders, provided they encounter compatible ECM fungi.