Colonization of the Rhizosphere by Biological Control Agents Applied to Seeds

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Abstract
The ability of various fungi and bacteria to move from inoculated seeds [Pisum sativum] to developing roots was studied. Plant root growing into soil-free moist chambers were not colonized by Trichoderma spp., but were colonized by Enterobacter cloaceae. When roots were grown in sterile soil, Trichoderma harzianum was detected in the rhizosphere of the upper half of the roots, while E. cloacae colonized the entire rhizosphere. In untreated soil, none of the organisms studied (T. harzianum, T. koningii, Gliocladium virens, Penicillium funiculosum, E. cloacae, Pseudomonas fluorescens, P. putida, or another fluorescent Pseudomonas sp.) could be detected in the rhizosphere more than 3 cm below the planted seed. In autoclaved soil to which fungi were added, E. cloacae colonized roots well, while T. harzianum was inhibited. Conversely, in autoclaved soil to which soil bacteria were added. E. cloacae was inhibited, and T. harzianum grew well. Percolating water enhanced the downward movement of both bacterial and fungal propagules.