Abstract
Clubroot disease caused by Plasmodiophora brassicae can lead to serious yield losses in crucifers such as Brassica napus. In this study, 323 bacterial strains were isolated from the rhizosphere of severely diseased B. napus in Dangyang county, Hubei province, China. Antagonistic strains were first identified based on dual culture inhibition zones with Fusarium oxysporum and Magnaporthe oryzae. These were then further screened in germination inhibition and viability assays of resting spores of P. brassicae. Finally, eight of the antagonistic strains were found to significantly reduce the disease severity of clubroot by more than 40% under greenhouse conditions, and two strains, F85 and T113, were found to have efficacy of more than 80%. Root hair infection experiments showed that F85 and T113 can inhibit early infection of root hairs, reduce the differentiation of primary plasmodia of P. brassicae, and inhibit formation of secondary zoosporangia. Based on sequence analysis of 16S rDNA gene, gyrA gene and 22 housekeeping genes as well as carbon source utilization analysis, the F85 was identified as Bacillus velezensis and T113 as Bacillus amyloliquefaciens. Genome analysis, PCR and RT-PCR detection revealed that both F85 and T113 harbor various antibiotic biosynthesis gene clusters required to form peptides with antimicrobial activity. To our knowledge, this is the first report of B. velezensis as a biocontrol agent against clubroot disease.
Highlights
Clubroot, caused by soil-borne obligate parasite Plasmodiophora brassicae, is a serious disease on Brassica spp., especially oil-seed crops, and causes severe yield losses worldwide (Dixon and Page, 1998)
P. brassicae is an obligate parasite that cannot be cultured in vitro, which presents a bottleneck for the large-scale screening of biocontrol bacteria against P. brassicae (Bi et al, 2016)
For screening of biocontrol bacteria against clubroot, a variety of pathogenic fungi could be used as indicator fungi for antagonistic test, the pathogenic fungus F. oxysporum
Summary
Clubroot, caused by soil-borne obligate parasite Plasmodiophora brassicae, is a serious disease on Brassica spp., especially oil-seed crops, and causes severe yield losses worldwide (Dixon and Page, 1998). After rapeseed roots become infected, root cells proliferate abnormally, forming tumor-like bulges. The enlarged roots become cracked and rotten due to infection by other pathogens and saprophytes. Life cycle of the pathogen consists of two key phases: in the initial phase, resting spores penetrate root hairs and epidermal cells and form primary plasmodia; in the second phase, primary plasmodia release secondary zoospores, which. New Biocontrol Agents Against Clubroot penetrate the root cortex and form galls. The mature secondary plasmodia of root cortex can form many resting spores that are released into soil (Schwelm et al, 2015)
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