Selecting Plant Growth-Promoting Rhizobacteria (PGPR) for Both Biological Control of Multiple Plant Diseases and Plant Growth Promotion in the Presence of Pathogens

Abstract

A study was conducted to select PGPR strains for broad-spectrum disease suppression and growth promotion in the presence of plant pathogens. First, 198 strains were tested for antibiosis capacity against nine different pathogens in vitro, including Pythium ultimum, P. aphanidermatum, three different isolates of Rhizoctonia solani, Fusarium oxysporum f. sp. lycopersici and f. sp. vasinfectum, Xanthomonas campestris pv. vesicatoria (Xcv), and Pseudomonas syringae pv. tomato (Pst). Thirty elite strains which inhibited 8 or 9 pathogens were then tested for traits often related to plant growth promotion, including N-fixation, IAA production, siderophore production, phosphate solubilization, phytase production, biofilm formation, and biosurfactant activity. All strains exhibited at least one trait of these traits. Second, these 30 PGPR strains were tested for biological control of four different plant diseases in the growth chamber. Five strains reduced the incidence or severity of 3 out of 4 tested diseases. AP69, AP199, AP200 significantly reduced two foliar bacterial diseases (Pst and Xcv) on tomato and P. ultimum on cucumber. AP197 and AP298 significantly reduced two foliar bacterial diseases on tomato (Pst and Xcv) and R. solani on pepper. Lastly, two separate experiments were conducted in the greenhouse, and each experiment included two individual PGPR strains and their mixtures, which were tested for biological control of three different diseases and for plant growth promotion in presence of pathogens. Mixtures exhibited better disease reduction and increases in growth (shoot dry weight and root dry weight) and root morphology parameters (root volume, total root length, root surface area, and fine roots) compared with individual PGPR strains. In summary, selected individual PGPR strains and some mixtures exhibited both biological control of multiple plant diseases and plant growth promotion, and results were better with mixtures than with individual PGPR strains.