Objectives Continuous cropping obstacles limit the growth and profit of Lanzhou lily (Lilium davidii var. unicolor). This study investigated the mitigating mechanism of foliar application of the bio-stimulant 5-aminolevulinic acid (ALA) by screening the main pathogenic and beneficial microorganisms in the rhizosphere soil.

Methods The experiment was conducted in a field continuously planting Lanzhou lily for three years, in the main production area of Lanzhou lily. The treatments included a spraying-water control (CK) and a treatment of spraying a 200 mg/L ALA solution (ALA). Plant growth indicators were measured using conventional methods. Endogenous hormones (IAA, JA, SA) were detected using LC-MS/MS. Rhizosphere soil microbial communities were analyzed using 16S rRNA and ITS gene amplicon sequencing. Conventional methods were used to detect soil enzyme activities, the contents of available nutrients, and the total content of auto-toxic phenolic acids.

Results Compared to the CK, the ALA treatment significantly increased chlorophyll content by 170% and thousand-grain weight by 41%; significantly increased the JA and IAA content and decreased SA content; the population of pathogenic fungi Fusarium declined from 11.36% to 7.54%, and that of Alternaria declined from 4.27% to 0.86%. Meanwhile, the saprophytic nutrient fungi were significantly enriched, along with nitrogen cycle-related bacteria, organic matter decomposition, fermentation, and self-toxic substance degradation-related functional bacteria. Gemmatimonas and Chaetomium were the key indicator microorganisms; they were positively correlated with each other, and Gemmatimonas was negatively correlated with pathogenic fungi. Rhizosphere soil physicochemical properties were significantly improved, with alkaline N content significantly increased by 39%, total phenolic acid content of self-toxic substances significantly decreased by 48%, and catalase and alkaline xylanase activities significantly increased.

Conclusions The foliar application of 200 mg/L 5-aminolevulinic acid (ALA) can regulate the balance of endogenous hormone metabolism, optimize the rhizosphere microbial community structure and function, improve soil nutrient status, reduce phytotoxic substance accumulation, alleviate yellowing symptoms in Lanzhou lily, mitigate continuous cropping disorders, and achieve disease resistance and yield increase.