1989 Volume 55 Issue 5 Pages 643-652
Strains RB425 and RB3292 of Pseudomonas cepacia, isolated originally from roots of lettuce and Campanula sp. and known producers of the antibiotics pyrrolnitrin and two pseudane compounds, both were suppressive to damping-off of radish caused by Rhizoctonia solani AG4 when applied on radish seeds. Two other naturally occurring strains A2 and A4 of P. cepacia, causal agents of bacterial brown spot of Cymbidium spp., and known producers of pyrrolnitrin (but not the two pseudanes), also were suppressive to damping-off of radish. A fifth strain, P. cepacia ATCC No.25416, produced no detectable quantity of any of the three antibiotics and was not suppressive to damping-off of radish. Eight mutants of strain RB425, induced by nitrosoguanidine (NTG), were distinguished from the wild-type parent by colony morphology. Three of these mutants produced no detectable quantities of either pyrrolnitrin or the two pseudanes in liquid culture and provided no suppression of damping-off. The other five mutants produced the three antibiotics in amounts similar to the parent and were suppressive to damping-off. Tests with derivative strains resistant to rifampicin and nalidixic acid showed that the mutants were not significantly different from the parent in the ability to colonize the roots of radish seedlings. Both colonization of radish seedlings by R. solani and severity of damping-off of radish caused by this fungus were suppressed by about 50% when the radish seeds were coated with either living cells of RB425 (107cfu/seed) or purified pyrrolnitrin (1.0μg/seed). Damping-off was suppressed only slightly or not at all following seed treatment with either of the two pseudane compounds, even at 40μg/seed. Observations by SEM confirmed that the effective strains multiplied in the spermosphere and the rhizosphere of radish seedlings. These results indicate that pyrrolnitrin plays an important role in suppression of radish damping-off by seed bacterization with P. cepacia.