Archaeophytopathology of Phakopsora pachyrhizi, the soybean rust pathogen

Authors: Haudenshield, James; Hartman, Glen

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2014
Publication Date: 5/1/2015
Citation: Haudenshield, J.S., Hartman, G.L. 2015. Archaeophytopathology of Phakopsora pachyrhizi, the soybean rust pathogen. Plant Disease. 99:575-579.

Interpretive Summary: Soybean rust (SBR) can be a devastating disease of soybean. There are two species of the SBR fungus that are known to infect soybean, but neither had been present in the continental United States until 2004, when one, P. pachyrhizi, was detected in the southern states. The other species, P. meibomiae, has not yet been detected. However, molecular detection tests have been devised which permit identification of the two species based upon their genetic sequence. To better understand the patterns of spread and genetic variation of this fungus, we attempted to purify its DNA from dried specimens of infected plant tissue collected around the world and preserved in a botanical museum referred to as a herbarium. Some of these plant specimens were nearly, or over, 100 years old. We were able to successfully extract intact DNA from the SBR present, and use that DNA to determine the exact genotype of the fungus inside the dried specimen. The results suggested that the fungus may have spread only relatively recently to Africa, and is perhaps not as suited to transcontinental movement in air currents as initially believed. This information is of interest to epidemiologists, plant pathologists, and others interested in geograpical and historical evidence of plant pathogens that have been preserved in herbariums.

Technical Abstract: Phakopsora pachyrhizi and P. meibomiae are two rust species that infect soybean (Glycine max). Traditionally, these two species are said to differ in geographic distribution, with P. pachyrhizi confined to Asia, Africa & Australia, and P. meibomiae confined to South & Central America. Several herbaria have accessions reported to contain one of the two species and include specimens from those locations, some nearly or over 100 years old. We sampled 38 of these archival specimens, and extracted & speciated the DNA of the fungus, if present, using quantitative PCR specific to either P. pachyrhizi, P. meibomiae, or to a third group inclusive of many rust species. Of the archival specimens, 11 were positive for P. pachyrhizi, including a 1912 specimen from Japan, but no P. pachyrhizi was found in specimens from before 1994 outside of Asia or Australia. Fifteen specimens were positive for P. meibomiae, including a 1928 specimen from Brazil and two 1923 specimens from the Philippines. Twelve specimens (including all African accessions) were found to be negative for both species, but six were positive in the more inclusive rust assay, and included specimens from Tanzania, Sao Tome, Nigeria, Sierra Leone, and China; all had been labeled as P. pachyrhizi and none were on G. max. These results demonstrate the feasibility of DNA genotyping in archaeophytopathological investigations and suggest that P. pachyrhizi may have been more recently introduced to Africa than previously believed.