Effect of plant biostimulants on root and plant health and the rhizosphere microbiome of citrus trees in huanglongbing-endemic conditions

KEY MESSAGE: Plant biostimulants did not improve citrus health and productivity in an HLB-endemic environment after two years, but fulvic acids increased concentrations of some nutrients and influenced the rhizosphere microbial community. Fibrous root loss is one of the main factors associated with citrus tree decline due to Huanglongbing (HLB) disease in Florida, and there is increased interest to use soil amendments such as plant biostimulants to improve root health and crop production. The effect of individual and combined applications of seaweed extracts, humic substances, and microbial inoculants on root and tree health, tree nutritional status, and productivity was investigated in an HLB-endemic environment. Experiments were conducted in two differently managed commercial citrus orchards. The effect of the treatments on the composition and structure of the citrus rhizosphere bacterial and fungal community was examined and linked to variations in root macro- and micronutrient concentrations. Overall, treatments had no effect on disease index ratings, total and specific root lengths, root physiological responses, and fruit quality and yield after two years of application. Only the individual applications of fulvic acids resulted in statistically significant increases in root and leaf macro- and micronutrient concentrations compared to the control treatment, an effect that differed between trials. These increases in root macro- and micronutrient concentrations were linked to variations in the relative abundance of specific bacterial and fungal taxa in the citrus rhizosphere. Our results suggest that longer-term and higher application rates of fulvic acids may be necessary to translate rhizosphere effects into measurable effects on tree health and productivity in HLB-endemic conditions. We show that factors such as different management practices and soil and environmental conditions may determine efficacy of biostimulants on root and tree health.