Biological control of Pythium damping-off of pea and sugar beet by Rhizobium leguminosarum bv. viceae

Rhizobium leguminosarum Jordan bv. viceae strains from pea and lentil root nodules were tested for control of damping-off of pea (Pisum sativum L., host) and sugar beet (Beta vulgaris L., nonhost) crops caused by Pythium sp. group G. Of the 18 Rhizobium isolates tested, only strain R5 inhibited mycelial growth of Pythium sp. group G. None of the strains showed any protease activity. Results of indoor experiments in soil artificially infested with Pythium sp. group G showed that 10 strains of R. leguminosarum bv. viceae were effective in increasing sugar beet emergence compared with the untreated control, when bacteria were coated onto seeds. Three of the most promising strains, R12, R20, and R21, were further tested for control of damping-off of field pea and sugar beet in a field naturally infested with Pythium spp. R12 and R20 significantly increased seedling emergence of field pea in the two field tests, compared with the untreated control. The efficacy of strains R12 and R20 was similar to that of Pseudomonas fluorescens Migula 708, a biological control agent of Pythium sp. group G. Rhizobium leguminosarum bv. viceae strains R12 and R21 were the most effective biological control agents for control of sugar beet damping-off in the field experiments. They were as effective as seed treatment with the fungicide Thiram(TM) in one field experiment. The present study reveals that some R. leguminosarum bv. viceae strains, in addition to their use as biofertilizer, also have the potential to be used for biological control of Pythium damping-off of field pea and sugar beet.