Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/5/2013
Publication Date: 10/1/2013
Citation: Paul, C., Hartman, G.L., Marois, J., Wright, D., Walker, D.R. 2013. First report of Phakopsora pachyrhizi adapting to soybean genotypes with Rpp1 or Rpp6 rust resistance genes in field plots in the United States. Plant Disease. 97:1379. Interpretive Summary: Soybean rust has been reported in most countries in the tropical and subtropical regions of the world that grow soybeans. After reports of its first occurrence in Brazil in 2001 and the continental United States of America in 2004, research on the disease and its pathogen has greatly increased. Genes for rust resistance have been found in soybean, and initially soybean genotypes with genes known as Rpp1 or Rpp6 exhibited high levels of resistance in the United States. However, in 2011 and 2012, isolates of the rust pathogen were found to be attacking and causing rust symptoms on soybean genotypes with the genes Rpp1 or Rpp6. In addition to the field records, isolates were purified and tested on greenhouse grown plants and detached leaves. Isolates collected prior to 2011 did not cause a susceptible reaction on soybean genotypes with Rpp1 and Rpp6, but some isolates collected in 2011 and 2012 did cause susceptible rust symptoms on soybean genotypes with these two resistance genes. This indicates that a shift in the rust fungus population occurred allowing for infection and colonization of soybean genotypes that were initially resistant to the fungus. This information is important to the seed industry, growers, and researchers that are interested in soybean production, host plant resistance, and disease epidemiology.
Technical Abstract: Since the first detection of soybean rust, caused by Phakopsora pachyrhizi Syd., in the continental United States in November, 2004, soybean [Glycine max (L.) Merr.] genotypes with the Rpp1 or Rpp6 resistance genes have exhibited high levels of resistance in the United States. In 2011 and 2012, however, field populations of P. pachyrhizi from Gadsden County, FL were more virulent on plants with Rpp1 or Rpp6 than in previous years. Sporulation ratings were made using a 1 to 5 scale on leaflets collected from field plants at or near the R6 (full seed) stage of development. The sporulation ratings indicated an increase in the virulence of the field populations on PI 200492 (Rpp1), PI 567102B (Rpp6), and L85-2378, a ‘Williams 82’ isoline carrying the Rpp1 gene. Single-uredinium-increased isolates were purified from infected soybean leaves collected from field plots in 2009 (FL-Q09-1), 2011 (FL-Q11-1) and 2012 (FL-Q12-1). Greenhouse and detached leaflet assays were then used to test the virulence of these isolates under controlled conditions. Two weeks after inoculation with FL-Q11-1 or FL-Q12-1, all of the genotypes had developed TAN lesions with abundant sporulation, indicating susceptibility. On leaves inoculated with FL-Q09-1, however, no visible reaction was observed on PI 200492, and PI 567102B developed the reddish-brown (RB) lesion type associated with incomplete resistance. Although the lesions on Rpp1 and Rpp6 greenhouse seedlings inoculated with the FL-Q11-1 and FL-Q12-1 isolates were slightly darker than those that developed on Williams 82 plants or on detached leaflets, the profuse sporulation that is characteristic of the TAN infection type was observed. The higher virulence of the 2011 and 2012 Florida isolates on two soybean genotypes with Rpp1 and one with Rpp6 confirmed the presence of a P. pachyrhizi pathotype in north-central Florida that is more virulent against these genes than earlier populations from the southeastern U.S.