SOFT ROT OF ONION BULBS CAUSED BY BURKHOLDERIA GLADIOLI PV ALLIICOLA IN SLOVENIA
J. Lamovšek, B. Gerič Stare, M. Žerjav, G. Urek
In 2012, growers of an indigenous onion (Allium cepa L.) cultivar from Northeastern Slovenia began noticing an increase of rotting onion bulbs both in the field and in storage. In some instances up to 30% of the bulbs were affected. Externally, the bulbs appeared healthy, but internal layers were found to have water-soaked and brown coloured lesions. We obtained samples from two different onion storage locations in spring of 2013 and isolated 27 morphologically distinct bacterial colonies on King’s medium B. The isolates were identified based on 16 S rDNA sequences (Bandi et al., 1994). An identical 16 S rDNA sequence (GenBank accession No. LT547718) was obtained from four isolates found at both storage locations and identified as a potential onion pathogen, Burkholderia gladioli based on high sequence similarity to other B. gladioli sequences (100 %) in the database. The strains of B. gladioli exhibited two different colony morphologies. One had a slightly wrinkled surface while the other was smooth. Both exhibited typical white-yellowish colour and released a yellow diffusive pigment into the medium. Pathogenicity of B. gladioli strains was evaluated on the onion bulbs by injecting a bacterial suspension into the bulb’s neck. Only B. gladioli strains caused a light brown rot development on the bulb’s internal layers after 6 to 12 days at 25oC. These results suggested the isolated B. gladioli strains belonged to the pathovar alliicola which was recently associated with extensive onion rotting in Poland (Kowalska et al., 2015). This is the first report and identification of Burkholderia gladioli pv. alliicola as a causative agent of soft rot on onion in Slovenia.