Survey of bacterial foliage diseases in onions, their virulence, epidemiology and management

Michigan has been one of major onion producing states, with an estimate production of more than 36 million kilograms annually. Bacterial diseases are major problems for onion growers both in fields and storages, with losses reached 100% for some fields in Michigan. During 2013 and 2014, symptomatic plants showing leaf blight with water-soak lesions were collected from six different counties in Michigan for isolation and identification of bacterial pathogens. Bacterial isolates were identified by using BIOLOG (Hayward, CA) and sequencing of universal 16s rDNA gene and diagnostic primers. A total of 414 isolates were obtained; Pantoea agglomerans, P. anantis and Enterobacter cowanii were most prevalent. Subsets of bacterial isolates were chosen for pathogenicity test on onion sets and bulbs, and copper hydroxide sensitivity testing. On onion sets, almost all of P. ananatis were pathogenic, while only approximately 50% of P. agglomerans and E. cowanii were pathogenic. When tested in onion bulbs, almost all of isolates were pathogenically positive. For copper hydroxide sensitivity test, 40% of P. agglomerans, and approximately 20% of P. anantis and E. cowanii isolates were tolerant to copper hydroxide at (200ug/ml). This study suggests that copper hydroxide might not be very effective to control these bacterial diseases in onions.To understand the epidemiology of P. agglomerans, P. ananatis and E. cowanii, effects of temperature, relative humidity (RH) and plant ages on the disease development were investigated. The three bacteria significantly resulted in disease at temperatures ranging from 25 to 30oC. Disease progressed rapidly on onion foliage under RH of 80 to 100%, but decline approximately 30% or greater if the RH was reduced to ≤60%. Inoculating plants aged between 6 to 14 weeks old with bacterial pathogens indicated that onions become significantly (P <.0001) more susceptible as they aged.Nine-teen long-day onion cultivars were evaluated in greenhouse condition for susceptibility to foliar blight incited by P. agglomerans, P. ananatis and E. cowanii. The area under the disease progress curve (AUDPC) data differed significantly among cultivars tested in each of the two trials (P<0.0001). Overall, cv. ‘Sherman’, ‘Mandras’, ‘Moondance’ and ‘Milestone’ displayed more frequently partial resistance to the three bacterial pathogens, whereas ‘Highlander’, ‘Delgado’, ‘Patterson’, ‘Pulsar’ and ‘Red-defender’ were often highly susceptible to the three tested bacterial pathogens causing leaf blight and rot in onions.In Cambodia, sixteen short-day onion cultivars were also evaluated for adoptability and stress response under field condition in two different locations. Number of stand count, bulb diameter, bulb weight and stress rating were significantly different in both trials (P<0.01). Overall, cv. ‘Texas Early Grano’, ‘Yellow Granex’, and ‘AVON1073’ were phenomenal in almost all measuring parameters. Results indicated that select short-day onion cultivars may be suitable for the tropical agro-climate of Cambodia.