The effectiveness of chlorine dioxide as a soil disinfectant for the prevention and control of apple replant disease (ARD) remains largely unexplored. This study aimed to determine the optimal concentration of chlorine dioxide for soil disinfection and to evaluate its impact on the prevention and control of ARD. The experimental results indicated that a high concentration of 600 mg·L⁻¹ chlorine dioxide exhibited a potent lethal effect on both the hyphae and spores of pathogenic Fusarium. Results from pot experiments demonstrated that, compared with the replant control, the height, ground diameter, fresh weight, and dry weight of Malus hupehensis Rehd. seedlings treated with 600 mg·L⁻¹ chlorine dioxide increased by 27.20%, 15.95%, 100.70%, and 76.28%, respectively. Additionally, the root length, surface area, volume, and number of root tips of the seedlings increased by 31.74%, 96.54%, 257.29%, and 85.29%, respectively. The activities of root-protective enzymes, including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), also increased to varying degrees, whereas the malondialdehyde (MDA) content was significantly reduced compared with the control treatment. Furthermore, the number of soil fungi treated with chlorine dioxide and the concentration of phenolic acid compounds in replant soil were significantly reduced. Treatment with 600 mg·L⁻¹ chlorine dioxide significantly decreased the detected copy number of genes associated with soil-borne pathogenic Fusarium, optimized the soil microbial community structure, and reduced the relative abundance of pathogenic fungi. In summary, disinfection of replant soil using 600 mg·L⁻¹ chlorine dioxide can enhance the growth of M. hupehensis Rehd. seedlings, inhibit the growth and reproduction of pathogenic Fusarium fungi, improve the soil environment, and effectively prevent and control ARD.

The crude metabolic extract from plant biocontrol bacteria plays a very important role in sustainable agricultural production. These extracts help maintain healthy plants and have very important application prospects in biotechnology related to alleviating apple replant disease (ARD). In this study, Bacillus velezensis XC1 (T1), Bacillus amyloliquefaciens QSB-6 (T2), and Lactobacillus reuteri LBR (T3) were examined to characterize the ability of their crude metabolic extracts to alleviate ARD. The high-throughput sequencing data of the soil microbial community structure were analyzed in relation to LBR crude metabolic extracts, and an extensive untargeted metabolomic analysis of UHPLC-Qex active components was performed. Active LC-MS/MS revealed that the main secondary metabolites involved in the biological control exerted by L. reuteri included 3-hydroxypropionaldehyde, extracellular polysaccharides (EPS), p-hydroxybenzoic acid, and azelaic acid. These crude metabolic extracts significantly inhibited the growth of soil pathogenic fungi, reduced the abundance of Fusarium, promoted the abundance of beneficial bacteria such as Pseudomonas, and optimized the soil microbial community structure. Improved modern extraction and purification technologies will be able to offer additional insights into the mechanism of action of these secondary metabolites and enable them to be used in biological preparations to prevent and control ARD in the future, as well as to allow harmful chemical fumigants to be discontinued.

Xanthoceras sorbifolium Bunge, a unique oil crop native to northern China, has a long history of cultivation. In this study, X. sorbifolium Bunge was continuously planted in Zibo and Weifang, Shandong Province, to explore the factors that cause a decline in X. sorbifolium Bunge yield and fruit quality after long-term continuous planting. The results showed that the continuous cropping of X. sorbifolium Bunge led to a significant decrease in the biomass of the plant's seedlings, markedly reduced the root activity, and reduced the soil nutrient content. A significant change in the soil microbial community structure was observed after years of X. sorbifolia Bunge monoculture. At the genus level, the relative abundance of soil pathogenic fungi, such as Neocosmospora, Aspergillus, and Penicillium, significantly increased after continuous cropping, with the relative abundance of Neocosmospora increasing significantly in the three study sites. The abundance of common soil bacterial genera, such as Mortierella, Bacillus, and Streptomyces, is significantly lower under continuous cropping than in regular soil. The number of soil-specific bacteria was also reduced. The results showed that the accumulation of fungal pathogens, particularly Neocosmospora, may be the main challenge in the continuous cropping of X. sorbifolium Bunge, as it reduces the abundance of beneficial bacteria, primarily Bacillus, Streptomyces, and Mortierella.