Rhizosphere colonization by Serratia marcescens for the control of Sclerotium rolfsii

Two-hundred and three different strains of bacteria were isolated from the rhizosphere of bean, peanut and chickpea plants grown in Sclerotium rolfsii infested soil. A bacterium, identified as Serratia marcescens, was found to be the best biocontrol agent of the pathogen, under greenhouse conditions (up to 75% disease reduction). Populations of 10⁵ or 10⁶ CFU g^-1 soil were the most effective in disease control. The drench and drip application of S. marcescens suspension were more effective in controlling S. rolfsii than spraying, mixing in soil or seed coating. This bacterium significantly reduced damping-off incidence of bean, caused by Rhizoctonia solani, by 50%, but was not effective against Pythium aphanidematum in cucumber. A natural mutant of S. marcescens, resistant to the antibiotic rifampicin, was isolated. The mutant, effective as the wild type, was used to study rhizosphere colonization. The highest population density of the bacteria was found on the proximal portion of the root, decreasing significantly until the tips, where they increased again.