Biological control of an Australian noxious weed “Angled Onion” (Allium triquetrum L.) using molecular and traditional approaches

Angled onion (<i>Allium triquetrum</i> L.) is a noxious weed in Australia and is difficult to control, especially in natural habitats. Research on biological control of <i>A. triquetrum</i> began in mid-2008 at the School of Applied Sciences, RMIT University, in collaboration with the Department of Primary Industries, Frankston, Victoria. There was no report of biological control of this Australian noxious weed and no research was conducted on the genetic diversity of <i>A. triquetrum</i> over the Australian states. Genetic analysis of A. triquetrum provenances across Australia was performed using RAPDs, PCR-RFLP and sequencing, suggested that the degree of variation was relatively small, making it a suitable biological control target. The aim of the project was initially to evaluate <i>Stromatinia cepivora</i>, a fungal pathogen causing white rot disease of cultivated <i>Allium</i> species, as a biocontrol agent to control the weed in infestations. It was not known whether the fungi would be pathogenic on this weed or if genetic differences between fungal strains or plant provenances would affect the pathogenicity and virulence. This study was the first report of pathogenicity of <i>S. cepivora</i> on<i> A. triquetrum</i> in Australia. The results achieved in this study demonstrated that there was a difference in pathogenicity and virulence of S. cepivora isolates on test-tube-grown plants in that only the DPI fresh isolate was pathogenic to Wonthaggi provenance (VIC). These results reflected the genetic diversity of both biocontrol agent and the host plants. During this study two soft rotting bacteria <i>Pectobacterium carotovorum</i> subsp. carotovorum and bacterium close to Ochrobactrum sp. isolated from infected <i>A. triquetrum</i> bulbs were also evaluated as biocontrol agents for wetter areas where sclerotia of the fungus are reported as not germinating. <i>Pectobacterium carotovorum</i> subsp. <i>carotovorum</i> has not previously been considered as a potential biological control agent for <i>A. triquetrum</i> in Australia, yet. The pathogenicity testing results for both test-tube-grown and potted <i>A. triquetrum</i> indicated that this strain of the bacterium was highly virulent on <i>A. triquetrum</i> in vitro in 24 h and in vivo in 2 weeks and it was not pathogenic on cultivated <i>Allium</i> species. The isolated bacterium close to <i>Ochrobactrum</i> sp. was pathogenic and virulent in vitro but not in vivo in all <i>A. triquetrum</i> provenances and cultivated Allium species tested. In test-tube trials both the bacterium and the fungus were separately pathogenic and highly virulent; infected plants died. However; the bacterium inhibited the fungus from growing when tested together, though host plants still died. Therefore the novel <i>Ochrobactrum </i>sp. had potential for control of the fungus in cultivated <i>Allium</i> species. In this study <i>S. cepivora</i> and <i>P. carotovorum</i> subsp. <i>carotovorum</i> were evaluated as potential biocontrol agents for <i>A. triquetrum</i> in pot trials and this research is now proceeding to field trials.<br>