Effect of timing of application and population dynamics on the degree of biological control of Sclerotinia sclerotiorum by bacterial antagonists.

Antagonistic Pseudomonas spp. (DF-41 and PA-23) were evaluated for inhibition of germination of ascospores, and for the effect of timing of application and its effect on biological control of Sclerotinia sclerotiorum (Lib.) de Bary, causal agent of stem rot of canola. Population dynamics were also assessed. In all studies, a petal inoculation technique was used. Significant inhibition (P < 0.05) of germination of ascospores was observed at both log 4 and log 8 cfu (colony forming units) ml(-1) of bacterial populations. In the population study, the pathogen had no significant effect (P < 0.05) on bacterial populations; however, a significant (P < 0.05) increase in bacterial populations was observed after 24 h and a decrease occurred between 96 and 120 h. Significant differences in disease severity (P < 0.05) were found with respect to timing of ascospore applications in the control treatments (ascospores only). One isolate completely suppressed disease when co-applied with ascospores, while only minor suppression occurred when applied 24 or 48 h after. Results from all studies indicate PA-23 and DF-41 to be effective biocontrol agents against S. sclerotiorum of canola and to have practical implications for biological control of this disease by bacteria in the field.