Biology and biocontrol of Sclerotinia sclerotiorum (Lib.) de Bary in oilseed Brassicas

Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic plant pathogen infecting over 500 host species including oilseed Brassicas. The fungus forms sclerotia which are the asexual resting structures that can survive in the soil for several years and infect host plants by producing ascospores or mycelium. Therefore, disease management is difficult due to the long term survivability of sclerotia. Biological control with antagonistic fungi, including Coniothyrium minitans and Trichoderma spp, has been reported, however, efficacy of these mycoparasites is not consistent in the field. In contrast, a number of bacterial species, such as Pseudomonas and Bacillus display potential antagonism against S. sclerotiorum. More recently, the sclerotia-inhabiting strain Bacillus cereus SC-1, demonstrated potential in reducing stem rot disease incidence of canola both in controlled and natural field conditions via antibiosis. Therefore, biocontrol agents based on bacteria could pave the way for sustainable management of S. sclerotiorum in oilseed cropping systems

Rapid marker-assisted selection of antifungal Bacillus species from the canola rhizosphere

A marker-assisted approach was adopted to search for Bacillus spp. with potential as biocontrol agents against stem rot disease of canola caused by Sclerotinia sclerotiorum. Bacterial strains were isolated from the rhizosphere of canola and screened using multiplex PCR for the presence of surfactin, iturin A and bacillomycin D peptide synthetase biosynthetic genes. Among the 96 isolates screened, only CS-42 harbored all three genes and was subsequently identified as Bacillus cereus using 16S rRNA gene sequencing. This strain was found to be effective in significantly inhibiting the growth of S. sclerotiorum in vitro and in planta. Scanning electron microscopy studies at the dual culture interaction region revealed that mycelial growth was curtailed in the vicinity of bacterial metabolites. Complete destruction of the outmost melanised rind layer of sclerotia was observed when treated with the bacterium. Transmission electron microscopy of ultrathin sections challenged with CS-42 showed partially vacuolated hyphae as well as degradation of organelles in the sclerotial cells. These findings suggested that genetic marker-assisted selection may provide opportunities for rapid and efficient selection of pathogen-suppressing. Bacillus strains for the development of microbial biopesticides

Molecular mapping of qualitative and quantitative loci for resistance to Leptosphaeria maculans causing blackleg disease in canola (Brassica napus L.)

Blackleg, caused by Leptosphaeria maculans, is one of the most important diseases of oilseed and vegetable crucifiers worldwide. The present study describes (1) the construction of a genetic linkage map, comprising 255 markers, based upon simple sequence repeats (SSR), sequence-related amplified polymorphism, sequence tagged sites, and EST-SSRs and (2) the localization of qualitative (race-specific) and quantitative (race non-specific) trait loci controlling blackleg resistance in a doubled-haploid population derived from the Australian canola (Brassica napus L.) cultivars Skipton and Ag-Spectrum using the whole-genome average interval mapping approach. Marker regression analyses revealed that at least 14 genomic regions with LOD ≥ 2.0 were associated with qualitative and quantitative blackleg resistance, explaining 4.6–88.9 % of genotypic variation. A major qualitative locus, designated RlmSkipton (Rlm4), was mapped on chromosome A7, within 0.8 cM of the SSR marker Xbrms075. Alignment of the molecular markers underlying this QTL region with the genome sequence data of B. rapa L. suggests that RlmSkipton is located approximately 80 kb from the Xbrms075 locus. Molecular marker-RlmSkipton linkage was further validated in an F2 population from Skipton/Ag-Spectrum. Our results show that SSR markers linked to consistent genomic regions are suitable for enrichment of favourable alleles for blackleg resistance in canola breeding programs.Rosy Raman, Belinda Taylor, Steve Marcroft, Jiri Stiller, Paul Eckermann, Neil Coombes, Ata Rehman, Kurt Lindbeck, David Luckett, Neil Wratten, Jacqueline Batley, David Edwards, Xiaowu Wang, Harsh Rama