The contribution of foliar endophytes to quantitative resistance to Melampsora rust

Foliar endophytes of Populus do not induce the hypersensitive response associated with major genes for resistance to Melampsora leaf rust. But they could contribute to the quantitative resistance that represents a second line of defense. Quantitative resistance is thought to be determined by suites of minor genes in both host and pathogen that are influenced by the abiotic environment. Here, we determined the relative importance to quantitative resistance of foliar endophytes, one element of the biotic environment. Leaves of six host genotypes differing in genetic resistance to Melampsora × columbiana were inoculated first with one of four foliar endophytes (Stachybotrys sp., Trichoderma atroviride, Ulocladium atrum or Truncatella angustata), and then with Melampsora. These endophytes greatly reduced rust severity within inoculated leaves (i.e. local effects), but they had no systemic effect on rust of leaves not inoculated with endophytes. Differences among endophytes and their controls explained 54% of the total variation in quantitative resistance (i.e. rust severity); the six host/pathogen genotypes explained just 5%. In terms of magnitude of effect on rust severity, Stachybotrys, Trichoderma, Ulocladium and Truncatella were ranked in this order on all host/pathogen genotypes. Endophytes may contribute significantly to quantitative resistance to Melampsora in leaves of Populus.