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First Report of Phakopsora pachyrhizi Adapting to Soybean Genotypes with Rpp1 or Rpp6 Rust Resistance Genes in Field Plots in the United States

During the years following 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 exhibited high levels of resistance there (1,2,3). When challenged with 72 different American isolates collected between 2006 and 2009, PI 200492 (source of Rpp1) produced no sporulating lesions (2). In 2011 and 2012, however, field populations of P. pachyrhizi from Gadsden County, FL, caused higher rust severity on plants with Rpp1 or Rpp6 than in previous years. To assess aggressiveness, sporulation ratings were made using a 1 to 5 scale (no sporulation to profuse sporulation) on leaflets collected from field plants at or near the R6 (full seed) stage of development. A dissecting microscope was used to examine 3 replications of 5 leaflets each in 2009 or 2 replications of 10 leaflets each in 2012. The sporulation ratings increased on PI 200492 (from 1.1 ± 0.1 in 2009 to 4.1 ± 0.4 in 2012), PI 567102B (Rpp6; from 1.1 ± 0.1 in 2009 to 2.4 ± 0.2 in 2012), and L85-2378, a ‘Williams 82’ isoline carrying the Rpp1 gene (from 1.0 ± 0 in 2009 to 4.0 ± 0.3 in 2012). The mean 2009 and 2012 sporulation ratings for susceptible control Williams 82 were 5.0 ± 0 and 4.2 ± 0.1, respectively. Single-uredinium-derived isolates were purified from bulk isolates 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. Detached leaflets from 3-week-old seedlings of Williams 82, PI 200492, PI 567102B, and L85-2378 were inoculated by pipetting 15-μl drops of a 30 to 40 urediniospore μl^–1 suspension onto the abaxial side of 3 to 4 leaflets per genotype, which were then sealed in Petri plates and incubated in a growth chamber at 20 to 22°C. Plates were kept in the dark for 12 h following inoculation. For the greenhouse assay, the first trifoliolate leaves of at least 3 seedlings were each sprayed with 1.5 ml of a 40 urediniospore μl^–1 suspension and incubated 24 h at 22 to 24°C in a dark mist chamber. The plants were then maintained at 22 to 24°C and 76 to 86% relative humidity in a greenhouse with 10 h of daylight on average. 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 reddish-brown (RB) lesions 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 United States.

References: (1) S. Li. Crop Sci. 49:887, 2009. (2) Twizeyimana and Hartman. Plant Dis. 96:75, 2012. (3) Walker et al. Crop Sci. 51:678, 2011.