Bulb rot in live Allium triquetrum by Pectobacterium carotovorum subsp. carotovorum.

Allium triquetrum L. (angled onion) is an invasive weed that threatens native ground flora such as orchids, lilies and grasses in natural habitats, especially in damp situations. A soft-rotting bacterium isolated from rotting A. triquetrum bulbs after 2 months of storage at 4°C (collected from Horsnell Gully, South Australia) was assessed for its potential as a biocontrol agent. The bacterium was identified as Pectobacterium carotovorum subsp. carotovorum Waldee by 16S rDNA sequencing and physiological tests. In test-tube trials, the bacterium produced severe soft rot symptoms of bulbs 12 h post-inoculation and rotten young seedlings collapsed after 24 h incubation at 25°C. Identical symptoms were observed at 15°C and 4°C, but with a longer development period at 4°C. Histology of infected plants revealed that the bacterium invaded both the cortical and vascular tissue. In glasshouse tests, all A. triquetrum provenances inoculated with 108 CFU of the bacterium per plant showed soft rot symptoms 20 days post-inoculation, but cultivated Allium species and Australian native monocots were not affected 3 months post-inoculation. The soft-rotting bacterium was reisolated from infected A. triquetrum bulbs and leaves in the glasshouse, fulfilling Koch postulates. Although this bacterium is normally associated only with storage rots, it is potentially an effective biological control agent for A. triquetrum in the field, as it can attack live plants too. Field trials to demonstrate efficacy are currently in progress.