We isolated and identified a bacterium that could produce IAA and degrade phloridzin in the rhizosphere soil of healthy replanted apple (the rootstock is M9T337 and the scion is Yanfu 3), providing a theoretical basis for reducing the obstacles associated with apple replant disease (ARD). Isolates were screened using Salkowski colorimetry and screening medium for phloridzin. The isolate of interest (W6) was identified as Ochrobactrum haematophilum based on morphological analysis, physiological and biochemical tests, and 16S rDNA sequencing. In a laboratory experiment, W6 produced auxin and promoted the growth of Arabidopsis thaliana roots, and its degradation rate of 100 mg·L⁻¹ phloridzin was 62.0%. In a pot experiment, W6 significantly reduced the phenolic acid contents of replanted soil, lowered the abundance of the harmful fungus Fusarium solani, and increased soil enzyme activities, thereby improving the micro-ecological environment of replant soil. W6 increased the root antioxidant enzyme activity and leaf photosynthetic pigment content of replanted Malus hupehensis Rehd. seedlings, effectively alleviating the decrease in net photosynthetic rate, transpiration rate and stomatal conductance caused by ARD. In a field experiment, W6 also promoted the growth of replanted apple (the rootstock is M9T337 and the scion is Yanfu 3) saplings. Therefore, W6 can promote apple growth and degrade phenolic acids, and it can be used as an effective treatment for the reduction of ARD.

This study investigated the effects of six crop rotation combinations on the soil quality of old apple orchard and seedling growth of Malus hupehensis Rehd. (apple rootstock) under pot conditions. The inhibitory effects of crops such as Allium fistulosum, Brassica juncea, and Triticum aestivum on four species of Fusarium were observed and compared in six treatments. These were continuous cropping (CK), fumigation with the methyl bromide (FM), rotating A. fistulosum only (R1), rotating A. fistulosum and T. aestivum (R2), rotating A. fistulosum, B. juncea, and T. aestivum (R3), and fallow (FC) in a year. The results showed that the biomass of Malus hupehensis Rehd. seedlings increased significantly. The root length increased and the root architecture was optimized. The respiration rate of the root system was increased by about 1 time after rotation. The treatments of R1, R2, R3, and FC increased bacterial count by 232.17%, 96.04%, 316.21%, and 60.02%, respectively. However, the fungi were reduced in varying degrees and bacteria/fungi ratio was increased by 5–10 times. The enzyme activities, pH, and organic matter were increased, but soil bulk density was decreased. Phenolic acids such as phloridzin was decreased significantly. The copy number of four Fusarium species declined by 85.59%, 74.94%, 69.68%, and 54.41% after rotating three different crops (R3 treatment). The root volatiles of three plants inhibited mycelial growth and spore germination of four Fusarium species.

A two-year field experiment was carried out in order to study the occurrence degree and mechanism of apple replant disease (ARD) in the apple orchards with different soil textures. So we can adopt appropriate control measures according to the severity of ARD. Healthy two-year-old seedlings with consistent growth were selected, of which the root stock was T337 and the scion was Yanfu 3. There were significant differences in biomass between methyl bromide fumigation and replanted treatments, and the difference was the largest in clay loam, followed by sandy loam, and loam, which verified ARD in clay loam was most serious, followed by sandy loam and loam. Based on high-throughput sequencing of fungi in soil samples, fungal richness and diversity were the highest in clay loam, followed by sandy loam, and loam. The relative abundance of Fusarium in SX, SL, FX, FL, WX and WL was 7.33%, 19.32%, 2.70%, 4.24%, 10.71% and 23.87%, respectively. Based on Real-time quantitative analysis, there were significant differences in the number of Fusarium oxysporum and Fusarium solani between methyl bromide fumigation and replanted treatments, i.e., clay loam > sandy loam > loam. Fusarium was the main pathogen causing ARD. This shows that ARD is the most serious under replanted clay loam condition. High-throughput sequencing technology was used to prove the difference in Fusarium was one of the important reasons for ARD under different soil textures. This technology provides a new idea for the prevention and control of ARD.