Soybean rust, caused by the fungus Phakopsora pachyrhizi, was detected in the continental United States in 2004. Several new sources of resistance to P. pachyrhizi have been identified in soybean (Glycine max); however, there is limited information about their resistance when challenged with additional U.S. and international isolates. Resistance of 20 soybean (G. max) entries was compared after inoculation with 10 P. pachyrhizi isolates, representing different geographic and temporal origins. Soybean entries included 2 universal susceptible cultivars, 4 sources of soybean rust resistance genes (Rpp1–4), and 4 and 10 resistant entries selected from field trials in Paraguay and Vietnam, respectively. Of the known Rpp1–4 sources of resistance, plant introduction (PI) 459025B (Rpp4) produced reddish-brown (RB) lesions in response to all of the P. pachyrhizi isolates, while PI 230970 (Rpp2) produced RB lesions to all isolates except one from Taiwan, in response to which it produced a susceptible tan (TAN) lesion. PI 200492 (Rpp1) and PI 462312 (Rpp3) produced TAN lesions in response to most P. pachyrhizi isolates. The resistant entries selected from Paraguay and Vietnam varied considerably in their responses to the 10 P. pachyrhizi isolates, with M 103 the most susceptible and GC 84058-18-4 the most resistant. The reaction patterns on these resistant entries to the P. pachyrhizi isolates were different compared with the four soybean accessions with the Rpp genes, indicating that they contain novel sources of rust resistance. Among the P. pachyrhizi isolates, TW 72-1 from Taiwan and IN 73-1 from India produced the most susceptible and resistant reactions, respectively, on the soybean entries.

Phakopsora pachyrhizi Sydow, the causal fungus of soybean (Glycine max (L.) Merrill) rust, occurs in most soybean-growing areas of the world except continental North America. Initial studies on soybean rust isolates from the Western Hemisphere indicated that they were different than isolates from the Eastern Hemisphere. In 1992, the Eastern Hemisphere species, P. pachyrhizi, and the Western Hemisphere species, P. meibomiae, were established for the soybean rust fungi based on morphological differences. The first molecular differentiation of the two species was reported in 2002. A number of studies have reported the occurrence of race in P. pachyrhizi either on soybeans or on other hosts. In 1984, a set of four native Australian Glycine species were used to identify six different virulence combinations of P. pachyrhizi. Much of the research on differentiating isolates on soybean was completed in a containment facility at in the U.S. Genetic characterization on four plant introductions (PIs) indicated the occurrence of four independently inherited dominant genes. These genes are known to be effective to a limited number of isolates. There are many studies that need to be completed to determine if all isolates respond equally in terms of survival, urediniospore production, telia formation, and host range under different environments. Over the next few years, our understanding of pathogen diversity will increase as more concerted research efforts take place in different parts of the world.

Soybean rust, caused by Phakopsora pachyrhizi, may be the most important foliar disease of soybean. Within the last 10 years, the fungus has moved to many new geographical locations via spread of airborne urediniospores. The objective of this study was to determine the relationship between urediniospore viability and exposure to solar radiation. Urediniospores of P. pachyrhizi were exposed in Capitán Miranda, Paraguay, to determine the deleterious effects of sunlight. Concomitant total solar (0.285 to 2.8 µm) and ultraviolet (0.295 to 0.385 µm) irradiance measurements were used to predict urediniospore germination. Urediniospores exposed to doses of solar and ultraviolet (UV) radiation ≥27.3 MJ/m2 and ≥1.2 MJ/m2, respectively, did not germinate. The proportions of urediniospores that germinated, normalized with respect to the germination proportion for unexposed urediniospores from the same collections, were a linear function of solar irradiance (R2 = 0.83). UV measurements predicted normalized germination proportions equally well. Results of inoculation experiments with exposed P. pachyrhizi urediniospores supported the results of the germination trials, although the effects of moderate levels of irradiance varied. The relationship between urediniospore viability and exposure to solar radiation has been incorporated into the U.S. Department of Agriculture’s soybean rust aerobiological model that provides North American soybean growers with decision support for managing soybean rust.

Soybean rust, caused by Phakopsora pachyrhizi, is an important disease in Nigeria and many other soybean-producing countries world-wide. To determine the geographical distribution of soybean rust in Nigeria, soybean fields were surveyed in the Derived Savanna (DS), Northern Guinea Savanna (NGS), and Southern Guinea Savanna (SGS) agroecological zones in Nigeria between 2004 and 2006. Disease severity in each zone was determined and analyzed using geostatistics. Prevalence of infected fields and disease severity in surveyed fields were significantly (P < 0.05) different between geographical zones with both variables being higher in the DS zone than in either NGS or SGS zones. Geostatistical analysis indicated that the spatial influence of disease severity at one location on severity at other locations was between 75 and 120 km. An exponential model best described the relationship between semivariance and lag distance when rust severity was high. Spatial interpolation of rust severity showed that locations in the DS zone were more conducive for the rust epidemic compared to areas in the NGS zone. In the 2005 survey, 116 purified isolates were established in culture on detached soybean leaves. To establish the nature of pathogenic variation in P. pachyrhizi, a set of four soybean accessions with Rpp(1), Rpp(2), Rpp(3), and Rpp(4) resistance genes, two highly resistant and two highly susceptible genotypes were inoculated with single uredinial isolates. Principal component analysis on the number of uredinia per square centimeter of leaf tissue for 116 isolates indicated that an adequate summary of pathogenic variation was obtained using only four genotypes. Of these four, PI 459025B (with Rpp(4) gene) and TG× 1485-1D had the lowest and highest number of uredinia per square centimeter, respectively. Based on cluster analysis of the number of uredinia per square centimeter, seven pathotype clusters were determined. Isolates in cluster III were the most virulent, while those in cluster IV were the least virulent. Shannon’s index (H) revealed a more diverse pathogen population in the DS zone (H = 1.21) compared to the rust population in SGS and NGS with H values of 1.08 and 0.91, respectively. This work will be useful in breeding and management of soybean rust by facilitating identification of resistant genotypes and targeting cultivars with specific resistance to match prevailing P. pachyrhizi pathotypes in a given geographical zone.

Phakopsora pachyrhizi Syd. & P. Syd., the cause of soybean rust, was first observed in the continental United States in November 2004 (2). During the growing season of 2005, P. pachyrhizi was confirmed on soybean (Glycine max) and/or kudzu (Pueraria montana) in nine states in the southern United States. It is known that P. pachyrhizi has a much larger host range within the Fabaceae family. On 29 September 2005 in Quincy, FL, 45 entries of mostly large-seeded legumes were planted next to soybeans that were infected with P. pachyrhizi. Several seeds of each entry were planted on one hill. Soybean plants growing adjacent to these potential hosts had 15 to 25% of the leaf area affected, 95% incidence, and 73% defoliation on 16 November. On 7 December 2005, all the plants of Phaseolus coccineus L. (scarlet runner bean, PI311827), Phaseolus lunatus L. (lima bean, PI583558), and two Phaseolus vulgaris L. (kidney bean) cvs. Red Hawk and California Early Light Red Kidney (CELRK) were found to have leaves with suspected rust lesions. These plants were at physiological maturity but had not senesced. None of the hosts had been inoculated other than from spores produced by the adjacent rust-infected soybean plants or from unknown locations. On the basis of microscopic examination, suspected infected leaves from plants of the Phaseolus spp. had rust pustules characteristic of P. pachyrhizi uredinia. Uredinia were counted within a randomly selected 2-cm(^2) area of one leaf of each sample. The mean and range of uredinia per lesion for Phaseolus coccineus was 29 uredinia with a range of 0 to 3 uredinia per lesion, Phaseolus lunatus had 2 uredinia with 0 to 1 uredinium per lesion, Phaseolus vulgaris cv. Red Hawk had 22 uredinia with 0 to 5 uredinia per lesion, and Phaseolus vulgaris cv. CELRK had 43 uredinia with 0 to 4 uredinia per lesion. Polymerase chain reactions using two sets of primers (Ppa1/Ppa2 and Pme1/Pme2) were performed on DNA extracted from leaves of the three species with sporulating rust pustules (1). The results of these tests and further tests conducted by the USDA/APHIS confirmed that P. pachyrhizi was the causal organism for the observed rust.

Asian soybean rust is one of the most devastating diseases of soybean with yield losses of 10 to 100% reported. The disease is found primarily in the lower canopy before flowering and in the middle and upper canopy after flowering. Heavily infected plants often prematurely defoliate causing significant yield losses. Until useful genetic resistance can be identified and moved into commercial cultivars, fungicides will be the primary means to control the disease. There is not much information on fungicide application in soybean. Fungicide use in soybean has been limited to seed treatments and a single late season foliar. With soybean rust the canopy needs to be protected from onset of flowering through pod fill. The research presented is a preliminary summary of the measurement of canopy penetration using high and low water volumes with two fungicides, Bravo and Quadris. Fungicides were applied aerially at 5 and 10 gal/ac in six locations in the southern US and by ground in three locations in the midwest US. In the ground application experiment, air induction, flat fan pointed down, flat fans on drops set to spray 105° and twin jet on forward facing right angle drops set to spray 80° nozzles were compared. Field design was set up as a strip plot with at least three replications per location. Three water sensitive paper strips were placed at mid canopy across the spray swath in three locations the length of the plot. Increased water volume in both ground and aerial application improved fungicide coverage when compared to the lower application volume. Among the nozzle tips evaluated in ground applications, overhead flat fans provided the least fungicide coverage in mid canopy.

This paper focuses on the Asian soybean rust (Phakopsora pachyrhizi), the fungicides so far evaluated to control this disease, including the corresponding country where each fungicide was tested, as well as the individual summaries of application trials and recommendations. The importance of the timing and number of applications to control the disease, and the factors determining the number of applications (length of the reproductive phase of the crop, persistence of the compound and severity of the epidemic) are also emphasized. Among the most effective fungicides from trial done in Southern Africa were flusilazole + carbendazim, difenoconazole, and triadimenol. Trials in South America have identified several triazoles (tebuconazole and tetraconazole) as well as several strobularins and strobularin mixes ( azoxystrobin, pyraclostrobin, pyraclostrobin + boscalid and trifloxystrobin + propoconazole). The recommendations for control of the disease based on the experience in Southern Africa were two applications with the first application times 50 days after planting, at or before flowering, and a second application 20 days later.

The efficacy of fungicides in managing soybean rust was evaluated in 12 environments in South America and southern Africa over three growing seasons from 2002 to 2005. There were differences in final soybean rust severity, defoliation, and yield among the treatments at most locations. In locations where soybean rust was not severe, all the fungicides evaluated reduced severity. In locations where soybean rust was severe, applications of triazole and triazole + strobilurin fungicides resulted in lower severity and higher yields compared with other fungicides. The strobilurin fungicides provided the highest yields in many locations; however, severity tended to be higher than that of the triazole fungicides. There also were differences in yield and severity between the trials with two and three applications of several fungicides, with three applications resulting in less severe soybean rust and higher yields. However, the third application of tebuconazole, tetraconazole, and the mixtures containing azoxystrobin and pyraclostrobin was not needed to maintain yield. These fungicides were among the most effective for managing soybean rust and maintaining yield over most locations.

Soybean rust, caused by Phakopsora pachyrhizi, is a major disease limiting soybean production primarily in the tropics and subtropics of Asia. Research at the Asian Vegetable Research and Development Center (AVRDC) has focused on monitoring disease development; evaluating yield losses; obtaining basic information on the biology of the fungus; and on finding sources of resistance and developing these sources into breeding lines. Rust was monitored on one moderately resistant and three susceptible lines at five locations in Taiwan during three separate seasons. Apparent infection rates were similar within lines over locations and seasons. Several experiments showed that soybean maturation was significantly positively correlated to the rate of rust development, whereas effects due to the environment and the host genotype were not as highly correlated. To compare soybean lines, methods were developed to compensate for differences in host maturities. The best method used the relative soybean life time (RLT) as a time element from 0 to 100. The time between planting and maturity was converted to a percentage of the soybean life cycle completed. Factors related to pathogenic diversity of the fungus, and the effect of environmental parameters were studied. Nine races were identified from forty-two isolates using a differential set consisting of 11 lines. The predominant races were complex with multiple virulence factors for compatibility on the differentials. Studies on leaf wetness and temperature indicated that the optimum temperature for uredospore germination was 15~ 25°C ; the minimal dew period for infection was 6 hours at 20~ 25°C and 8-10 hours at 15~ 17.5°C ; and a mean night temperature below 15°C greatly reduced lesion numbers or completely prevented lesion development. Field studies showed that precipitation was a critical factor in the development of epidemics. It was used to predict rust severity, and was more important than frequency and intensity of the infection period which consisted of leaf wetness, temperature, and their interaction. Difficulties associated with identifying and quantifying rate-reducing resistance and the ineffectiveness of race-specific resistance have brought about techniques to develop higher soybean yields with tolerance to rust. Also techniques were developed to better quantify and understand the components involved in partial resistance. In other studies, new sources of resistance were identified in accessions of the wild perennial Glycine species. (Key words: Phakopsora pachyrhizi, infection rates, perennial Glycine spp., yield losses, races)

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi and recently discovered for the first time in continental United States, has been of concern to the U.S. agricultural industry for more than 30 years. Since little soybean rust resistance is known, and resistance is often difficult to detect or quantitate, we initiated a project to develop a better, more quantitative, method. The methodology determined the average numbers and diameters of uredinia in lesions that developed on leaves of inoculated plants 14 days after inoculation. It was used to compare virulence of P. pachyrhizi isolates from Asia and Australia and P. meibomiae from Puerto Rico and Brazil, collected as many as 30 years earlier, with isolates of P. pachyrhizi recently collected from Africa or South America. Susceptible reactions to P. pachyrhizi resulted in tan-colored lesions containing 1 to 14 uredinia varying greatly in size within individual lesions. In contrast, on these same genotypes at the same time of year, resistance to other P. pachyrhizi isolates was typified by 0 to 6 small uredinia in reddish-brown to dark-brown lesions. Using appropriate rust resistant and rust susceptible genotypes as standards, examination of uredinia 14 days after inoculation allowed quantitative comparisons of sporulation capacities, one measure of susceptibility or resistance to soybean rust. The study verified the presence and ability to detect all known major genes for resistance to soybean rust in the original sources of resistance. It demonstrated that soybean lines derived from the original PI sources, and presumed to possess the resistance genes, in actuality may lack the gene or express an intermediate reaction to the rust pathogen. We suggest that a determination of numbers and sizes of uredinia will detect both major gene and partial resistance to soybean rust.

Accessions of 12 perennial Glycine species were evaluated for resistance to Phakopsorapachyrhizi, the causal agent of soybean rust. A total of 23% of the accessions were resistant, 18% were moderately resistant, and 58% were susceptible. In two experiments, 59 and 40% of the accessions of G. tabacina (2/n=80) were resistant. Resistance to P. pachyrhizi was identified in accessions of G. argyrea, G. canescens, G. clandestina, G. latifolia, G. microphylla, and G. tomentella, but not in accessions of G. arenaria, G. cyrtoloba, G. curvata, and G. falcata.

Phakopsora pachyrhizi was inoculated on two soybean (Glycine max) genotypes at three different reproductive growth stages (GS) in four trials. Leaf rust was more severe on Taita Kaohsiung No. 5 (TK 5), a commercial cultivar, than on SRE-B15-A (B15 A), a genotype selected for tolerance to leaf rust. At GS R6, the percentage of leaf area infected ranged from 14 to 95% for TK 5 and from 0 to 34% for B15 A. Values for area under disease progress curve (AUDPC) were significantly greater for TK 5 than B15 A. Yields in fungicide-protected plots ranged from 2,312 to 3,546 kg/ ha and were not significantly different between the genotypes. Average yields of plants inoculated at GS Rl were reduced by 62 and 22% and seed weights by 35 and 14% for TK 5 and B15 A, respectively, compared with fungicide-protected plots. Regressions of yield percentage of fungicide-protected plants on disease severity assessments at GS R6, AUDPC, and area under the green leaf area curve were significant for both genotypes.

Soybean rust (SBR), caused by Phakopsora pachyrhizi, was first discovered in North America in 2004 and has the potential to become a major soybean [Glycine max (L.) Merr.] disease in the USA. Currently, four SBR resistance genes have been identified but not mapped on the soybean genetic linkage map. One of these resistance genes is the Rpp1 gene, which is present in the soybean accession PI 200492. The availability of molecular markers associated with Rpp1 will permit marker-assisted selection and expedite the incorporation of this gene into U.S. cultivars. We compared simple sequence repeat (SSR) markers between ‘Williams 82’ and the BC5 Williams 82 isoline L85-2378, which contains the Rpp1 resistance allele from the soybean accession PI 200492, for candidate regions that might contain Rpp1. One candidate region was found with the SSR marker BARC_Sct_187 on linkage group G. A population of BC6F2:3 lines segregating for the Rpp1 resistance locus was genotyped in this region on linkage group G followed by inoculation with the P. pachyrhizi isolate India 73-1 in the USDA-ARS Biosafety Level 3 Plant Pathogen Containment Facility at Ft. Detrick, MD. The Rpp1 gene was mapped between SSR markers BARC_Sct_187 and BARC_Sat_064 on linkage group G.

Five hundred thirty soybean accessions from maturity groups (MG) III through IX were evaluated for resistance to Phakopsora pachyrhizi in a replicated field trial at Centro Regional de Investigación Agrícola in Capitán Miranda, Itapúa, Paraguay during the 2005–06 season. Soybean rust severities of individual accessions ranged from 0% (resistant) to 30.0% (susceptible). In MG III and IV, the most resistant accessions were PI 506863, PI 567341, and PI 567351B, with severities less than 1.2%. In MG V, the most resistant accessions were PI 181456, PI 398288, PI 404134B, and PI 507305, with severities less than 0.3%. In MG VI, the most resistant accessions were PI 587886, PI 587880A, and PI 587880B, with severities less than 0.3%. In MG VII and VIII, the most resistant were PI 587905 and PI 605779E, with severities less than 1.0%. In MG IX, the most resistant accessions were PI 594754, PI 605833, PI 576102B, and PI 567104B, with severities less than 1.0%. The resistance in 10 selected accessions from MG VI, VII, VIII, and XI was confirmed in subsequent greenhouse and field experiments where severities of 0.4% or less and reddish-brown lesions with sporulation levels less than 3.0 were observed. These accessions, with low severities in the adult plant field evaluation, may be new sources of resistance to P. pachyrhizi.

Asian soybean rust, Phakopsora pachyrhizi, has been an important pathogen of soybean in Asia with yield losses of 40 to 80% commonly reported. The pathogen has moved into Africa, where it was reported in Uganda in 1996, then Zimbabwe (1998) and South Africa (2001). The pathogen was first found South America in Paraguay and then Brazil during the 2001 growing season. A set of 174 soybean accessions was evaluated against local soybean rust populations in field or greenhouse studies in Brazil, China, Paraguay, and Thailand. The materials were also evaluated in the USDA BSL-3 containment greenhouse in Ft. Detrick, MD against a mixed collection of P. pachyrhizi from Brazil, Paraguay, Thailand and Zimbabwe. Among the set were soybeans that had previously been reported to have resistance to either P. pachyrhizi or P. meibomia, including the sources of the four identified resistance genes. The pathogen is known to have a complex and diverse virulence pattern with many phenotypes seen within a field collection. This was observed as mixed resistant (RB) and susceptible reactions on several lines within each location. Disease severity and reaction phenotypes on individual lines differed by location. These differences were due to local environmental conditions, which reduced rust severity as well as differences in the virulence of the rust population at each location. No lines were found to be resistant at all locations.

Soybean rust (Phakopsora pachyrhizi) has been reported on common bean (Phaseolus vulgaris) in Asia, South Africa, and the United States. However, there is little information on the interaction of individual isolates of Phakopsora pachyrhizi with common bean germplasm. A set of 16 common bean cultivars with known genes for resistance to Uromyces appendiculatus, the causal agent of common bean rust, three soybean accessions that were sources of the single gene resistance to P. pachyrhizi, and the moderately susceptible soybean ‘Ina’ were evaluated using seedlings inoculated with six isolates of P. pachyrhizi. Among the common bean cultivars, Aurora, Compuesto Negro Chimaltenango, and Pinto 114, were the most resistant to all six P. pachyrhizi isolates, with lower severity, less sporulation, and consistent reddish-brown (RB) lesions associated with resistance in soybean. A differential response was observed among the common bean cultivars, with a cultivar-isolate interaction for both severity and sporulation levels, as well as the presence or absence of the RB lesion type. This differential response was independent of the known genes that condition resistance to U. appendiculatus, suggesting that resistance to P. pachyrhizi was independent of resistance to U. appendiculatus.

The accessions in the USDA Germplasm Collection at the Univ. of Illinois were evaluated to identify soybean germplasm with resistance to soybean rust. The inoculation program used a mixture of four P. pachyrhizi isolates. Of 16,595 accessions rated for rust severity, 3,215 were selected for a second round of evaluation. Of these, 805 were selected for further evaluation. Some of these selected accessions have the potential to provide soybean rust resistance genes that may be useful for incorporation into commercial soybean cultivars.

Soybean rust, caused by Phakopsora pachyrhizi, is one of the most important constraints to soybean production worldwide. The absence of high levels of host resistance to the pathogen has necessitated the continued search and identification of sources of resistance. In one set of experiments, 178 soybean breeding lines from the International Institute of Tropical Agriculture were rated for rust severity in the field in 2002 and 2003 at Ile-Ife, Yandev, and Ibadan, Nigeria. Thirty-six lines with disease severity ≤3 (based on a 0-to-5 scale) were selected for a second round of evaluation in 2004 at Ibadan. In the third round of evaluation under inoculated field conditions, 11 breeding lines with disease severity ≤2 were further evaluated for rust resistance at Ibadan in 2005 and 2006. The breeding lines TGx 1835-10E, TGx 1895-50F, and TGx 1903-3F consistently had the lowest level of disease severity across years and locations. In another set of experiments, 101 accessions from the United States Department of Agriculture–Agricultural Research Service and National Agriculture Research Organization (Uganda) were evaluated in the first round in 2005 under inoculated conditions in the screenhouse; 12 accessions with disease severity ≤20% leaf area infected were selected for evaluation in the second round in 2005 and 2006 under inoculated field conditions at Ibadan. Highly significant differences (P < 0.0001 in disease severity were observed among the 101 accessions during this first round of rust evaluation. Significant (P < 0.0001) differences in rust severity and sporulation also were observed among the 12 selected accessions. Accessions PI 594538A, PI 417089A, and UG-5 had significantly (P < 0.05) lower disease severity than all other selected accessions in both years of evaluation, with rust severities ranging from 0.1 to 2.4%. These results indicate that some of the breeding lines (TGx 1835-10E, TGx 1895-50F, and TGx 1903-3F) and accessions (PI 594538A, PI 417089A, and UG-5) would be useful sources of soybean rust resistance genes for incorporation into high-yielding and adapted cultivars.

Fourteen soybean accessions and breeding lines were evaluated for resistance to soybean rust caused by the fungus Phakopsora pachyrhizi. Evaluations were conducted in replicated experiments in growth chambers using detached leaves and under greenhouse and field conditions. In growth-chamber experiments, inoculation of detached leaves with 1 × 106 spores/ml resulted in a significantly (P < 0.0001) higher total number of pustules and spores per unit leaf area than inoculations with lower spore concentrations. Amending agar medium with plant hormones significantly (P < 0.0001) aided retention of green leaf color in detached leaves. Leaf pieces on a medium containing kinetin at 10 mg/liter had 5% chlorosis at 18 days after plating compared with leaf pieces on media amended with all other plant hormones, which had higher levels of chlorosis. Leaf age significantly affected number of pustules (P = 0.0146) and number of spores per pustule (P = 0.0088), and 3- to 4-week-old leaves had a higher number of pustules and number of spores per pustule compared with leaves that were either 1 to 2 or 5 to 6 weeks old. In detached-leaf and greenhouse screening, plants were evaluated for days to lesion appearance, days to pustule formation, days to pustule eruption, lesion number, lesion diameter, lesion type, number of pustules, and spores per pustule in 1-cm2 leaf area. Plants also were evaluated for diseased leaf area (in greenhouse and field screening) and sporulation (in field screening) at growth stage R6. There were significant (P < 0.0001) differences among genotypes in their response to P. pachyrhizi infection in the detached-leaf, greenhouse, and field evaluations. Accessions PI 594538A, PI 417089A, and UG-5 had very low levels of disease compared with the susceptible checks and all other genotypes. Detached-leaf, greenhouse, and field results were comparable, and there were significant correlations between detached-leaf and greenhouse (absolute r = 0.79; P < 0.0001) and between detached-leaf and field resistance (absolute r = 0.83; P< 0.0001) across genotypes. The overall results show the utility of detached-leaf assay for screening soybean for rust resistance.