Effects of Crystal Lime Sulfur Fumigation and Application of Root-Growth-Promoting Agents on the Control of Apple Replant Disease

Apple replant disease (ARD) is seriously hindering the development of the apple industry. This experiment assessed the effects of two different root-growth-promoting agents (Indoleacetic acid and nutrient elements) on the microbial environment of apple-replanted soil and the growth of apple rootstock <i>Malus hupehensis</i> Rehd. seedlings after fumigation with crystal lime sulfur. The results showed that the simultaneous application of crystal lime sulfur, indoleacetic acid, and nutrient elements (T4) improved the biomass of <i>Malus hupehensis</i> Rehd. seedlings. It also enhanced the activities of soil enzymes and root antioxidant enzymes (SOD, POD, CAT). Their activities were significantly higher than in the individual treatments and resulted in a decrease in malondialdehyde (MDA) content. The T4 treatment significantly increased the net photosynthetic rate and chlorophyll content of the plant, thus effectively increasing the plant growth status. After fumigation, the amount of soil microorganisms was reduced, and the amount of bacteria and actinomycetes was increased after mixed application with the root-growth-promoting agent. The abundance of different species such as <i>Pseudallescheria</i>, <i>Guehomyces</i>, <i>Trichoderma</i>, <i>Bacillus</i>, <i>Gaiella</i>, and <i>Sphingomonas</i> was effectively increased, and the amount of <i>Fusarium oxysporum</i> was reduced. Through correlation analysis between different species and plant and soil enzymes, we found that the different species were positively correlated with root respiration rate and SOD activity and negatively correlated with MDA content. The differentially accumulated microbial species may be the key microorganism that promotes plant growth. Therefore, the simultaneous application of crystal lime sulfur, indoleacetic acid, and nutrient elements can optimize the apple replant soil environment and promote the growth of <i>Malus hupehensis</i> Rehd. seedlings, and can be used to control apple replant disease.