Promoting Strawberry (Fragaria × ananassa) Stress Resistance, Growth, and Yield Using Native Bacterial Biostimulants

Strawberry production is challenged by several abiotic and biotic stresses, such as drought, soil salinity, and the angular leaf spot (ALS) disease caused by Xanthomonas fragariae. In recent decades, the development of commercial products containing combinations of different Plant-Growth-Promoting (PGP) microorganisms has been one of the main focuses of agricultural research. However, their results are often erratic depending on crop species, environmental conditions, and competition among the different strains or indigenous plant microbiota. The use of beneficial microorganisms selected from the crop-specific microbiota may help overcome this limitation, promoting their utilization for sustainable agriculture. The culturable bacteriota of strawberry plants was screened to identify PGP activities in vitro. Bacterial isolates were tested in vivo on strawberry plants in both optimal and stress (X. fragariae infection or salinity) conditions, allowing the selection of strains of Pseudomonas fluorescens, Stenotrophomonas rhizophila, and Agrobacterium rubi whose application showed a significant increase in plant growth and fruit production (up to seven-fold), even under stress conditions, and the ability to control ALS by over 50%. Potential synergistic effects among PGP isolates were tested by coordinated inoculation. However, plant growth and fruit quality were not promoted, except for fruit weight and size, by coordinate inoculation in comparison to m23 and m27 single-strain treatment.