Location: Plant Science ResearchTitle: First report of bacterial stem blight of alfalfa caused by pseudomonas viridiflava in California and Utah
|Samac, Deborah - Debby|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2019
Publication Date: 6/22/2019
Citation: Lipps, S.M., Lenz, P., Samac, D.A. 2019. First report of bacterial stem blight of alfalfa caused by Pseudomonas viridiflava in California and Utah. Plant Disease. 103(12):3274. https://doi.org/10.1094/PDIS-05-19-1044-PDN.
Interpretive Summary: Bacterial stem blight of alfalfa was found to be widespread in northern California and northern Utah in the spring of 2016 and 2017. Although the disease has been known for many years, it was reported only sporadically to cause significant crop losses. A second species of bacteria was found to be associated with the disease, and methods were developed for rapidly identifying each species. Previously, the second bacterium was found causing root and crown rot of alfalfa but this is the first report of it causing stem and leaf blight. The presence of both bacteria may help to explain the emergence of bacterial stem blight in these fields. Identification of the pathogens will aid in developing methods for reducing damage from the disease.
Technical Abstract: In April and May of 2016 and 2017, widespread bacterial stem blight was observed on alfalfa (Medicago sativa) in northern California and northern Utah in the United States. Affected plants exhibited water soaking, chlorosis, and necrosis of leaves, necrotic lesions on petioles and stems, wilting and stunting. The symptoms were consistent with those caused by Pseudomonas syringae pv. syringae. Eight isolates were used for PCR amplification of 16S rRNA and all sequences had 100% match to multiple P. viridiflava strains. To facilitate identification of P. viridflava, PCR primers PV_Lipoprt_F (5´-TCGCCACAATACCCAGTTCC-3´) and PV_Lipoprt_R (5´-GATATTACGGCCTGACCCCG-3´) were developed to amplify a 360 bp fragment from a putative lipoprotein and monooxygenase specific to P. viridflava. These primers can be used in multiplex PCR reactions with primers for the lipodepsipeptide toxin gene (syrB) of P. syringae pv. syringae to rapidly distinguish between isolates of the two pathogens. Pathogenicity of the eight isolates was tested by on alfalfa cv. CUF101 by wounding and stems and inoculating with a bacterial suspension. The same symptoms of bacterial stem blight seen in the field were present among the P. viridiflava inoculated plants including stem necrosis, leaf necrosis and chlorosis, and wilting. Bacteria were reisolated from symptomatic plants on King’s B agar medium and colonies were confirmed as P. viridiflava by PCR, fulfilling Koch’s postulates. Previously, P. viridiflava was reported to cause root and crown rot on alfalfa with isolates causing chlorosis and wilting upon stem inoculation. This is the first report of P. viridiflava causing stem and leaf blight on alfalfa in the United States.