Characterisation of rice-associated antagonistic pseudomonads and their application in combination with plant resistance inducer molecules for the control of sheath blight disease of rice

Rice sheath blight caused by Rhizoctonia solani AG1-IA is one of the most important diseases of high-yielding rice (Oryza sativa L.) cultivars in rice-growing areas of the world including Iran. This study aimed to determine the efficacy of native fluorescent pseudomonads in combination with plant defence activators in controlling rice sheath blight disease. Sampling was done from local and high-yielding rice cultivars grown in Mazandaran province, Iran. A total of 291 fluorescent strains were isolated and screened for antagonism against R. solani. The isolates Ps-275, Ps-128, Ps-287, Ps-240 and Ps-243 reduced the disease symptoms up to 53.1­%, 49.3%, 44.76%, 40.83% and 35.7­%, respectively, only when applied in combination with potassium silicate at the booting stage of rice growth. The isolate Ps-3 reduced the disease symptoms when applied alone and in combination with oxalic acid by 60.5% and 58.7%, respectively. Multilocus sequence analysis of house-keeping genes, rpoD, oprI and oprL identified the most effective antagonistic isolates, Ps-275, Ps-3 and Ps-287, as endemic Pseudomonas strains affiliated with the P. protegens, P. mosselii and P. aeruginosa (sub) groups, respectively. These strains significantly inhibited the growth of R. solani AG1-IA in vitro and reduced the disease symptoms at the booting stage of the rice plant as well.