Endophytic Microorganisms Associated with Reversion of Silverleaf Disease Symptoms in Apple

Silverleaf is caused by the fungus Chondrostereum purpureum, which produces wood necrosis and foliar silvering in woody plants. Field observations and studies in apple have shown the reversion of foliar symptoms. Because plants were clones and received identical agronomical management, it was hypothesized that reversion is driven by endophytic microbiota. Thus, the objectives of this study were to compare healthy, diseased, and reverted plants with respect to their physiology, endophytic microbial communities, antagonistic ability of their endophytes against C. purpureum, and defense genes expression. Water potential, stomatal conductance, chlorophyll content, and fluorescence were measured. Endophytic bacterial and fungal DNA were analyzed by denaturing gradient gel electrophoresis, and community richness and similarity were calculated. Wood cores were collected and bacterial and fungal endophytes were isolated and confronted with C. purpureum-virulent strains in dual-culture assays. Defense genes expression was measured by quantitative PCR. Results indicated that there were no differences in physiological parameters between healthy and reverted plants, except for fluorescence, and both type of plants differed from diseased ones. Bacterial and fungal community richness was similar in healthy and reverted plants and higher than in diseased ones. Endophytes from reverted and healthy plants showed high antagonism to C. purpureum. Furthermore, nonexpressor of pathogenesis-related gene 1 expression was upregulated in reverted plants, whereas phenylalanine ammonia lyase and polygalacturonase-inhibiting protein genes showed higher values in diseased plants. Overall, physiological, molecular, and microbial characteristics were similar between healthy and reverted plants, and both differed from diseased ones. Therefore, reversion of symptoms is associated with changes in the endophytic microbiota, which seems to be a promising source of biological control agents against C. purpureum.