Distribution and Survival of Pseudomonas sp. on Italian Ryegrass and Curly Dock in Georgia

Yellow bud, caused by Pseudomonas sp., is an emerging bacterial disease of onion. A polymerase chain reaction assay based on the coronafacate ligase (cfl) and HrpZ genes was used to detect initial suspected bacteria on weeds. Growth on an agar medium, ability to cause a hypersensitive response in tobacco, pathogenicity on onion, and sequence analysis of 16S ribosomal RNA and cfl genes were used to confirm the identity of Pseudomonas sp. recovered from 10 asymptomatic weed species in the Vidalia onion-growing zone (VOZ) of Georgia. Among the weeds identified as epiphytic hosts for Pseudomonas sp., Italian ryegrass (Lolium multiflorum) and curly dock (Rumex crispus) were prominent because ≥73% of the samples from five sample sites were positive for the bacterium. These weeds are commonly found throughout Georgia and, thus, were selected to assess their role in yellow bud epidemiology. Samples of the two weed species were collected from sites along the perimeter of and within the VOZ (n = 5 sites) during late June, August, and September 2012 and 2013, which represented the time interval between onion growing seasons. Samples (n = 10/weed species/site) were collected and processed for bacterial detection as described above. In June (2012 and 2013), Pseudomonas sp. was detected from Italian ryegrass and curly dock in 100 and 40% of the sample sites, respectively. During the months of August and September (2012), the bacterium was recovered from Italian ryegrass in 60 and 10% of the sample sites, respectively; whereas, in August (2013), Pseudomonas sp. was recovered from 40% of the sample sites. However, the bacterium was not recovered from any of the sites in September (2013). In contrast, during August and September (2012), Pseudomonas sp. was recovered from curly dock in 20 and 80% of the sample sites, respectively. Similarly, in August and September (2013), the bacterium was detected from 40 and 100% of the sample sites, respectively. These data demonstrated that the Pseudomonas sp. responsible for yellow bud can survive as an epiphyte on Italian ryegrass and curly dock between onion crops. Furthermore, using artificially infested onion seed, we demonstrated that Pseudomonas sp. can be transmitted through contaminated seed.