Influence of a multi-species bacterial biostimulant on rhizosphere dynamics, yield earliness, and fruit quality of open-field winter melon (Cucumis melo L. var. inodorus)

The effect of a multi-species ( Bacillus tequilensis, Peribacillus frigoritolerans, Pseudomonas lini, Pseudomonas frederiksbergensis, Pseudomonas atacamensis, Pseudomonas reinekei , and Pseudomonas granadensis ) plant growth-promoting bacterial (PGPB) biostimulant on soil microbial dynamics and winter melon performance was evaluated over two growing seasons. In both trials PGPB were inoculated twice: at transplant and 30 days after transplant. Shortly after the first inoculation, total mesophilic bacteria increased by up to +1.8 log CFU g⁻¹ and total mesophilic anaerobic bacteria by +1.6 log CFU g⁻¹, while fungi rose from 4.35 to 5.15 log CFU g⁻¹ (p < 0.01). The second inoculation further enhanced bacterial abundance (+1.0 and +0.8 log units for TMB and TMaB, respectively), whereas fungal populations remained unchanged. By the end of the crop cycle, microbial counts converged between treatments. Culture-dependent tracking showed persistence of three out of the eight inoculated strains, with B. tequilensis and P. reinekei detected at harvest. Illumina profiling revealed strong early shifts in community structure, including an increase of Pseudomonas to 13.2 % and Bacillus to 13.8 %, followed by partial re-equilibration. Agronomically, inoculated plants exhibited enhanced vegetative growth (stem length +21.9 % at 20 days after transplant), earlier female flowering and accelerated fruit set. Total yield increased by 12.0 %, with early yield rising by 27.9 %, driven by a 37.6 % higher fruit number. Fruit quality also improved, showing a 15 % increase in firmness and higher soluble solid content (13.7 °Brix). These findings confirm that early microbial priming with this mixed inoculum drives lasting agronomic benefits, supporting efficient and sustainable winter melon production under field conditions.