Microbial soil amendment to improve apple tree establishment following planting

Symptoms due to Apple Replant Disease (ARD) complex typically appear shortly after replanting when apple trees are replanted in soils where the same, or a closely related species, has grown previously. The individual and combined use of a single-strain of arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR), and biocontrol agents (BCAs) at planting to support tree establishment were investigated. A Brassicaceae seed meal product (Biofence), and a formulated product of a five AMF species/strains mix were also included as two treatments. Trees were planted in spring 2020 with the soil amendment applied at planting. Tree growth was monitored annually until Spring 2024. Amendment with the single-strain AMF and BCAs resulted in an overall significant increase of 11% and 7% in the annual girth expansion rate, respectively. There were no significant interactions between the single-strain AMF and BCAs regarding the benefit conferred to tree development. Overall, only the single-strain AMF led to significant increases in both plant height and projected canopy area. PGPR failed to improve tree development and, in several cases, interacted negatively with AMF and BCA amendment. Furthermore, annual re-application of BCA and PGPR did not result in any significant additional benefit. The single-strain AMF resulted in similar benefits to plant growth as the five-species AMF mix. Biofence resulted in a significant increase in plant height and canopy cover, but not girth expansion. Present results suggest that amending soils with AMF alone, or jointly with BCAs at planting, can improve tree development and should be exploited in practice.