Growth promotion of tomato plants by rhizobacteria and imposition of energy stress on Rhizoctonia solani

Abstract

Rhizobacteria isolated from the rhizosphere of tomato plants were used to inoculate seeds and roots in greenhouse assays. Three bacteria, Azospirillum sp., Azotobacter chroococcum and Pseudomonas fluorescens, provided a significant ( P = 0.05) increase in seedling emergence rate. A. chroococcum was found most effective in increasing the total dry weight, root and shoot length of tomato plants, followed by P. fluorescens and Azospirillum sp. Variations in incubation temperature and soil moisture significantly ( P = 0.01) influenced the total plant growth. Temperatures >30°C were not favourable for bacterial activity or plant growth. Inoculated seedlings at −5 kPa (25°C) showed maximum plant growth, whereas least response was observed at −15 kPa. Inoculated seedlings significantly ( P = 0.05) reduced the disease incidence and severity of Rhizoctonia damping-off of tomato seedlings. Colonization of Rhizoctonia solani sclerotia by rhizobacteria was negatively correlated with germination of sclerotia in patato dextrose broth ( r = −0.84) and in Pfeffer's salts solution ( r = 0.85). Bacteria were able to release 14C-labelled endogenous compounds from labelled sclerotia. Sclerotia exuded 37% (percentage of total label) 14C-labelled carbonaceous compounds in soil inoculated with P. fluorescens, 29% with A. chroococcum and 19% with Azospirillum sp. Loss of endogenous C compounds from sclerotia was correlated ( r = −0.97) with repression of germination.