Location: Foreign Disease-weed Science ResearchTitle: First report of Phakopsora pachyrhizi on soybean in Costa Rica Author
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2014
Publication Date: 11/25/2014
Publication URL: http://handle.nal.usda.gov/10113/61540
Citation: Murillo-Williams, A., Esker, P.D., Allen, T.W., Stone, C.L., Frederick, R.D. 2014. First report of Phakopsora pachyrhizi on soybean in Costa Rica. Plant Disease. 99(3)418. Interpretive Summary: Soybean rust (SBR) caused by the fungus Phakopsora pachyrhizi has not been reported to occur in Costa Rica in Central America. Soybean leaves with SBR symptoms were observed in fields in La Garita, Alajuela, Costa Rica in late December 2012 and early January 2013. Soybean plants displayed typical symptoms of SBR infection with yellow and brown spots, and micropscopic analysis revealed spores on the undersurface of infected leaves. Infected leaf samples were sent to the USDA-ARS Foreign Disease-Weed Science Research Unit for molecular characterization and identification. Real-time polymerase chain reaction assays and nucleotide sequence analysis confirmed the pathogen identity as P. pachyrhizi. This information about the spread of SBR to other countries is important to soybean pathologists and epidemiologist who are studying the movement of soybean rust throughout the world.
Technical Abstract: American soybean rust, caused by Phakopsora meibomiae, has been reported to occur in several legume species in the tropical regions of Central and South America. In Costa Rica, this pathogen was initially reported as P. pachyrhizi; however, to our knowledge P. pachyrhizi has not been detected in the country. In routine evaluations of a soybean field in La Garita, Alajuela, Costa Rica, symptoms similar to Asian soybean rust were observed in late December 2012 and early January 2013. Soybean plants were at growth stages R4-R5 when these symptoms were observed, which were of the form of medium size yellow spots on leaves with brown spots on the abaxial surface. Further evaluations were made at growth stage R5-R6 where it was noted that the spots had coalesced, turned grayish-brown, with substantial defoliation. Routine and microscopic inspections indicated the presence of uredinia and urediniospores on the lower surface of the leaf. Infected leaf samples were packaged and submitted to the USDA-ARS Foreign Disease-Weed Science Research Unit under the appropriate USDA-Animal Plant Health Inspection Service permit for molecular characterization and identification. Urediniospores were collected by washing infected leaves with sterile water and pelleted by centrifugation. DNA was extracted from urediniospore pellets and excised leaf pieces, and all 8 samples amplified in real-time PCR with the P. pachyrhizi-specific primers Ppm1 and Ppa2 but not with the P. meibomiae-specific primers Ppm1 and Pme2. Nucleotide sequence alignment of the internal transcribed spacer regions 1 and 2 amplified by PCR using the primers Ppa1 and Ppa2 further confirmed the identification as P. pachyrhizi. To the best of our knowledge, this is the first known confirmation of Asian soybean rust, caused by P. pachyrhizi, in Costa Rica.