Comparisons of Visual Rust Assessments and DNA Levels of Phakopsora pachyrhizi in Soybean Genotypes Varying in Rust Resistance

Reactions of Soybean Germplasm Accessions to Six Phakopsora pachyrhizi Isolates from the United States

Soybean rust, caused by Phakopsora pachyrhizi Syd. & P. Syd., is one of the most economically important foliar diseases of soybean. Resistant cultivars could reduce yield losses and management costs but considerable pathogenic diversity exists among populations of the fungus; thus, resistance to a range of pathotypes is essential. Seedling and detached-leaf assays were conducted to characterize the resistance of 55 soybean plant introductions (PIs) to six purified isolates of P. pachyrhizi originating from the southern United States. In the greenhouse resistance assays, the differentials Hyuuga (PI 506764) and PI 471904 and accessions PI 224268, PI 567025A, PI 567039, PI 567046A, and DT 2000 (PI 635999) were resistant to all six isolates, including Florida isolates from 2011 and 2012 that were able to defeat resistance conditioned by the Rpp1 through Rpp4 genes. Twenty-six other PIs were resistant to four or five of the six isolates. In the detached-leaf assays, eight accessions developed reddish-brown reactions to all six isolates, with an average of only 0.23 to 0.55 uredinia/lesion. These included Hyuuga, DT 2000, two differentials with a resistance allele at the Rpp5 locus, and accessions PI 224268, PI 423960B, PI 567025A, and PI 567046A. Many of the resistant accessions have subsequently been reported to have a resistance allele at the Rpp3 locus, and two others have resistance genes at the Rpp4 or Rpp6 locus. This study provided new information about resistance reaction phenotypes that can be useful for understanding mechanisms of resistance, which Rpp genes and alleles could be combined to obtain broader and more durable rust resistance in soybean cultivars, and pathotype diversity among the six isolates used.

Comparison of Pathogenic Variation among Phakopsora pachyrhizi Isolates Collected from the United States and International Locations, and Identification of Soybean Genotypes Resistant to the U.S. Isolates

A major constraint in breeding for resistance to soybean rust has been the virulence diversity in Phakopsora pachyrhizi populations. In greenhouse experiments, reactions of 18 soybean genotypes to 24 U.S. isolates from 2007 and 2008 and 4 foreign isolates were compared. Reactions of four differentials (Rpp1 to Rpp4) to these U.S. isolates were also compared with reactions to nine foreign isolates and three U.S. isolates from 2004. Principal component analysis (PCA) of the reaction types grouped the U.S. isolates into a single virulence group, whereas each of the foreign isolates had a unique virulence pattern. In another experiment, reactions of 11 differentials to the 24 U.S. isolates were compared and significant interactions (P < 0.001) were found between the isolates and host genotypes for rust severity and uredinia densities. PCA of these two measures of disease placed the 24 isolates into seven or six aggressiveness groups, respectively. In a third experiment, evaluation of 20 soybean genotypes for resistance to the previously established aggressive groups identified 10 genotypes resistant to isolates representing most of the groups. This study confirmed the pathogenic diversity in P. pachyrhizi populations and identified soybean germplasm with resistance to representative U.S. isolates that can be used in breeding.

Characterization and quantification of fungal colonization of Phakopsora pachyrhizi in soybean genotypes

Soybean rust, caused by the fungus Phakopsora pachyrhizi, is an economically important disease of soybean with potential to cause severe epidemics resulting in significant yield losses. Host resistance is one of the management tools to control this disease. This study compared soybean genotypes exhibiting immunity, complete and incomplete resistance, and susceptibility to an isolate of P. pachyrhizi based on visual assessment of reaction type, other visual traits such as sporulation, quantitative measurements of the amount of fungal DNA (FDNA) present in leaf tissues, and data on infection and colonization levels. Soybean genotype UG5 (immune), and plant introduction (PI) 567102B and PI 567104B (complete resistance) had lower quantities of uredinia and FDNA than four other genotypes with incomplete resistance. Based on microscopic observations, early events of spore germination, appressorium formation, and fungal penetration of the epidermis occurred within 24 h postinoculation and were similar among the tested soybean genotypes. Differences in infection among the genotypes were evident once the hyphae penetrated into the intercellular spaces between the mesophyll cells. At 2 days after inoculation (dai), soybean genotype Williams 82 had a significantly (P < 0.05) higher percentage of hyphae in the mesophyll tissue than other soybean genotypes, with UG5 having significantly (P < 0.05) lower percentages than all of the other soybean genotypes at 3, 4, and 5 dai. The percentage of interaction sites with mesophyll cell death was significantly (P < 0.05) higher in UG5 than other genotypes at 3, 4, and 5 dai. There was a significant positive correlation (r = 0.30, P < 0.001) between quantities of hyphae in the mesophyll cells and FDNA. These results demonstrated that incompatible soybean-P. pachyrhizi interaction results in restricted hyphal development in mesophyll cell tissue, likely due to hypersensitive apoptosis.

A multiplexed immunofluorescence method identifies Phakopsora pachyrhizi Urediniospores and determines their viability

Soybean rust, caused by Phakopsora pachyrhizi, occurs concomitantly wherever soybean is grown in the tropical and subtropical regions of the world. After reports of its first occurrence in Brazil in 2001 and the continental United States in 2004, research on the disease and its pathogen has greatly increased. One area of research has focused on capturing urediniospores, primarily by rain collection or wind traps, and detecting them either by microscopic observations or by immunological or molecular techniques. This system of detection has been touted for use as a potential warning system to recommend early applications of fungicides. One shortcoming of the method has been an inability to determine whether the spores are viable. Our study developed a method to detect viable P. pachyrhizi urediniospores using an immunofluorescence assay combined with propidium iodide (PI) staining. Antibodies reacted to P. pachyrhizi and other Phakopsora spp. but did not react with other common soybean pathogens or most other rust fungi tested, based on an indirect immunofluorescence assay using fluorescein isothiocyanate-labeled secondary antibodies. Two vital staining techniques were used to assess viability of P. pachyrhizi urediniospores: one combined carboxy fluorescein diacetate (CFDA) and PI, and the other utilized (2-chloro-4-[2,3-dihydro-3-methyl-(benzo-1,3-thiazol-2-yl)-methylidene]-1-phenylquinolinium iodide] (FUN 1). Using the CFDA-PI method, viable spores stained green with CFDA and nonviable spores counterstained red with PI. Using the FUN 1 method, cylindrical intravacuolar structures were induced to form within metabolically active urediniospores, causing them to fluoresce bright red to reddish-orange, whereas dead spores, with no metabolic activity, had an extremely diffused, faint fluorescence. An immunofluorescence technique in combination with PI counterstaining was developed to specifically detect viable P. pachyrhizi urediniospores. The method is rapid and reliable, with a potential for application in forecasting soybean rust based on the detection of viable urediniospores.