Multifunctional Bacillus Strains for Integrated Fruit Rot Management and Improved Soil Phosphorus Availability in Longan Orchards

Fruit rot caused by Lasiodiplodia pseudotheobromae poses a serious threat to longan (Dimocarpus longan Lour.) production. The present study evaluated antagonistic and phosphate-solubilizing bacteria for their potential use in environmentally responsible disease and soil management. Two complementary Bacillus isolates were selected. Bacillus amyloliquefaciens UPBA63 exhibited strong antagonistic activity against L. pseudotheobromae, whereas Bacillus subtilis SBP04 demonstrated superior phosphate-solubilizing and plant-growth-promoting traits. Their combined application provided complementary benefits for disease suppression and soil phosphorus availability. Biochemical assays and molecular identification using concatenated 16S rDNA and gyrB sequences assigned UPBA63 to Bacillus amyloliquefaciens and SBP04 to B. subtilis. Both isolates produced extracellular protease, cellulase, and amylase. UPBA63 significantly reduced fruit rot incidence in planta and during field trials, whereas SBP04 exhibited the greatest phosphate-solubilizing activity. Field application of UPBA63 and SBP04 combined with organic matter and a chemical fertilizer and fungicide at half the recommended doses reduced disease incidence to 15.40 ± 7.42% and improved yield relative to the untreated control. The integrated treatment combining Bacillus spp. with organic matter, half-rate chemical fertilizer, and half-rate fungicide resulted in a soil pH of 4.56, an organic matter content of 2.92%, and an available phosphorus of 149.25 mg kg−1. Multifunctional Bacillus strains demonstrated dual capacity for pathogen suppression and nutrient mobilization, supporting their potential application in sustainable disease control and soil fertility management in longan orchards.