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ARS Home » Southeast Area » Tifton, Georgia » Crop Protection and Management Research » Research » Publications at this Location » Publication #295838
Title: Distribution and survival of Pseudomonas sp. on Italian ryegrass and Curly dock in Georgia

Author
item DUTTA, B - University Of Georgia
item GITAITIS, R - University Of Georgia
item Webster, Theodore
item SANDERS, H - University Of Georgia
item SMITH, S - University Of Georgia
item LANGSTON, D - University Of Georgia

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2013
Publication Date: 5/1/2014
Citation: Dutta, B., Gitaitis, R.D., Webster, T.M., Sanders, H., Smith, S., Langston, D.B. 2014. Distribution and survival of Pseudomonas sp. on Italian ryegrass and Curly dock in Georgia. Journal of Phytopathology. 98(5):660-666.

Interpretive Summary: Yellow bud, caused by Pseudomonas sp. is an emerging bacterial disease of onion. We identified two weed species as potential sources of inoculum and assessed if the bacterium could survive on these weeds within and around the Vidalia onion-growing zone in Georgia between spring onion harvest autumn seeding of the next crop. Currently, we lack basic information on the possible sources of inoculum, survival and spread of the pathogen and biology of bacterial infection. The identification of inoculum sources for the disease could result in the development of management practices that will reduce disease severity and minimize economic losses. The identification of different sources of inocula is an important step in understanding the epidemiology of a disease and helps in devising strategies for its management. This study showed that the Pseudomomas sp. that causes yellow bud can survive as an epiphyte on at least 10 different weed species in the Vidalia onion-growing zone of Georgia. The identified weeds included cutleaf geranium, wild mustard, cutleaf evening primrose, Virginia pepperweed, black cherry, soybean, Italian ryegrass, curly dock, Pennsylvania smartweed, and Carolina geranium. Among the positive weed species, the yellow bud bacterium was recovered from more than 73% of the samples of Italian ryegrass and curly dock. Also, samples of these weeds were positive from all three sample sites including a site adjacent to a fallow field where yellow bud was first observed on onions in 2007. Although the host range of this Pseudomonas sp. is unknown, evidence provided by both PCR and recovery on NA+ medium suggest that common weeds may serve as asymptomatic hosts of epiphytic populations of bacteria. This implicates weeds as a possible source of inoculum for yellow bud outbreaks in onion. In addition, our data indicated the bacterium could be seed-borne. Multiple sources of inocula will make it more difficult to develop control strategies, since both seeds and weeds will have to be targeted for exclusion and eradication.

Technical Abstract: Yellow bud, caused by Pseudomonas sp. is an emerging bacterial disease of onion. Polymerase chain reaction (PCR) assay based on the coronafacate ligase (cfl) and HrpZ genes were 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 rRNA 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 resident hosts, Italian ryegrass (Lolium multiflorum) and curly dock (Rumex crispus) were prominent as =73% of the samples from five sample sites was positive for Pseudomonas sp. These weeds are commonly found throughout Georgia, 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 of 2012, which represents the time interval between onion growing seasons. Samples (n = 10/ weed species/site) were collected and processed for PCR assay and recovery on agar medium. In June, 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, the bacterium was recovered from Italian ryegrass in 60 and 10% of the sample sites. In contrast, during the same time period, the bacteria were recovered from curly dock in 40 and 80% of the sample sites. These data demonstrate that the Pseudomonas sp. responsible for yellow bud can survive as epiphytes on Italian ryegrass and curly dock between onion crops. Furthermore, using artificially infested onion seeds, we demonstrated that Pseudomonas sp. can be transmitted through contaminated seed.