Evaluation of biological control of Sclerotinia sclerotiorum on canola (Brassica napus) in the laboratory, in the greenhouse, and in the field

Abstract

Sclerotinia sclerotiorum (Lib.) de Bary, causal agent of stem rot on canola, is the most economically important disease of canola in Western Canada. Control of this pathogen using conventional methods such as crop rotation, and breeding for resistant cultivars is extremely challenging and as a result biological control (biocontrol) has been extensively researched as an alternative to chemical control, the latter of which can have negative impacts on both environmental and human health. A commercial formulation of a biocontrol agent is currently in use, however given that its target is the survival structures of the pathogen, disease outbreaks are still possible given favourable environmental conditions. Our focus for biocontrol in this study (where the bacteria were applied) was therefore the initial infection site, the petals. In the present study, 197 bacterial isolates from canola and related plant species were screened for biocontrol efficacy using plating techniques to further elucidate the mechanisms being employed by the isolates. Forty two percent of the 197 isolates screened exhibited significant inhibition in the plate screenings. Some of the more promising isolates were identified using the Biology software. A select group of four isolates was then tested for efficacy in the greenhouse, and one of the four (isolate 41, a Pseudomonas spp.) was evaluated at different concentrations and temperatures with bacterial treatments preceeding the pathogen inoculations. One isolate was also evaluated in the greenhouse using different inoculation regimes to test the relative competitive ability of the bacterial strain. One hundred percent disease suppression was observed for all greenhouse trials, except when different inoculation regimes were employed. In this case there was 100% suppression with a co-inoculation treatment, but significantly less suppression was observed when the bacterial antagonist was applied after the pathogen. A field study was conducted in 2001 at the Carman Research Station in Carman, Manitoba in which two of the bacterial strains were tested for efficacy in the field using a foliar spray application at 30% bloom, with one treatment being a second spray of one of the isolates at 50% bloom. The field study revealed significant inhibition (P < 0.05) between the control and plots treated with fungicide, isolate 41 or isolate PA-23 at two applications at the time of stem infection. Population dynamics were then assessed for this isolate, both in the presence and absence of the pathogen and a microscopic study of inhibition at two different concentrations was conducted. Population dynamics for the bacteria did not differ significantly in the presence of the pathogen, and relatively high numbers of bacteria (log 2 to log 3 cfu/ml) were maintained throughout the course of the study. Microscopic examinations revealed total inhibition of ascospore germination at log 8 cfu/ml and partial inhibition at log 4 cfu/ml. Results from these studies suggest that biological control targeted to canola petals could significantly limit infection by Sclerotinia sclerotiorum in the field.