Pseudomonas syringae pv. Porri: A new pathogen of Australian onions

Abstract

The first record of Pseudomonas syringae pv. porri on onion in Australia was confirmed in September 2012. This pathogen was previously described in Australia on leek, and pathogenicity tests determined it could infect other allium species Isolates were recovered from water-soaked lesions on onion leaf material collected from the Lockyer Valley, Queensland, Australia. The disease was observed on onion and shallot in the same area the previous year. The isolates were characterized as P. syringae by biochemical tests and confirmed as the causal pathogen by Koch's postulates. Sequences of the CFL and 16S genes from these isolates had high identity to P. syringae pv. porri from Australian leek plants and a non-fluorescent pathovar of P. syringae on onion reported from the United States. Surveys of the Lockyer Valley and preliminary epidemiology studies were conducted to determine the range and severity of field infections. The disease was widespread across the region and favoured cool, humid conditions. A period of heat may effectively halt disease development, though precise effects of temperature require further investigation. A distinct 'yellow leaf' symptom was observed in many field infections and may be a response to toxins, as toxin production has been reported in several other pathovars of P. syringae. This study developed a reliable method for inoculation in brown onion, red onion, and leek. Opportunities for further study will be detailed, including seed testing methods, efficacy of control methods, pathogen survivability, and toxin production. The outcomes of this project will ultimately improve the productivity of the onion industry, and develop diagnostic and response protocols for endemic and exotic bacterial pathogens of allium crops.