Sequential intercropping boosts apple sapling growth and suppresses soil pathogens

Apple replant disease is commonly associated with soil degradation and microbial imbalance in aged orchards. This study investigated the effects of sequential intercropping on soil properties and microbial communities during orchard renovation. One year before replanting, Tagetes patula, Brassica juncea and Triticum aestivum were successively intercropped between tree rows, followed by the removal of aged trees and the replanting of young apple saplings. Intercropping increased urease, sucrase, phosphatase and catalase activities, reduced phlorizin and phloretin accumulation, increased cultivable bacterial abundance and decreased fungal populations, resulting in a higher bacteria-to-fungi ratio. Sequencing analysis showed a shift in fungal community composition, with reduced relative abundance of pathogenic genera and enrichment of beneficial taxa. Brassica root exudates inhibited four Fusarium spp. by 10.8–32.4% and pot experiments showed improved seedling biomass and root development in intercropped soil. In the field, replanted trees under sequential intercropping had greater plant height and stem diameter than the non-intercropped control. These results indicate that sequential intercropping improved soil biological properties and supported early growth of replanted apple trees.