Epidemiology of Soybean Rust in Soybean Sentinel Plots in Florida

Since its discovery in the southeastern United States in 2004, soybean rust (SBR) has been variable from year to year. Caused by Phakopsora pachyrhizi, SBR epidemics in Florida are important to understand, as they may serve as an inoculum source for other areas of the country. This study examined the first disease detection date, incidence, and severity of SBR in relation to environmental data, growth stage, and maturity group (MG3, MG5, MG7) in soybean sentinel plots (225 m2) across north Florida from 2005 through 2008. The majority (91%) of the initial infections were observed in MG5 and MG7 soybeans, with plots not becoming infected until growth stage R4 or later. Precipitation was the principle factor affecting disease progress, where disease increased rapidly after rain events and was suppressed during dry periods. On average, plots became infected 30 days earlier in 2008 than 2005. In 2008, there was a significant increase in disease incidence and severity associated with the occurrence of Tropical Storm Fay, which deposited up to 380 mm of rainfall in north Florida. The results of this study indicate that climatic and environmental factors are important in determining the development of SBR in north Florida.

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Comparative Susceptibility of Kudzu Accessions from the Southeastern United States to Infection by Phakopsora pachyrhizi

Plant Disease ◽

10.1094/pdis-93-6-0593 ◽

2009 ◽

Vol 93 (6) ◽

pp. 593-598 ◽

Cited By ~ 12

Author(s):

M. R. Bonde ◽

S. E. Nester ◽

W. F. Moore ◽

T. W. Allen

Keyword(s):

United States ◽

Southeastern United States ◽

Average Diameter ◽

Economic Loss ◽

The United States ◽

Soybean Rust ◽

Phakopsora Pachyrhizi ◽

Wide Range ◽

The Individual ◽

A Site

Soybean rust, caused by Phakopsora pachyrhizi, was first discovered in the continental United States in the fall of 2004. The potential for economic loss in the United States hinges largely on whether or not the pathogen can survive winters in the absence of soybean. Kudzu (Pueraria lobata) is known to be a host for P. pachyrhizi in Asia and South America and is widely distributed in the southern United States. This study examined reactions of kudzu collected from several areas of the southeastern United States to three isolates of P. pachyrhizi, one each from Alabama, Louisiana, and Brazil. Susceptible tan (TAN) lesions, resistant reddish-brown (RB) lesions, and immune (IM) response, previously described on soybean, were produced on kudzu based on the evaluation of 125 plants. However, in contrast to soybean, the RB response on kudzu was common, with approximately 50% frequency. IM responses to at least one isolate were observed on five individual plants, and two plants were immune to all three pathogen isolates used in the test. TAN lesions averaged 3.2 uredinia per lesion with an average diameter per uredinium of 121 μm. In contrast, RB lesions had an average of 0.3 uredinia per lesion with an average uredinial diameter of 77 μm. In 25 of 39 (64%) instances in which multiple plants were tested from a site, each reacted the same to the individual pathogen isolates. This suggested a tendency for plants at specific sites to be genetically identical with respect to rust reaction. Only 19 of 125 (15%) individual plants produced a different reaction to one isolate than to the other two isolates. When four kudzu plants previously shown to produce only TAN lesions to P. pachyrhizi isolates Alabama 04-1, Brazil 01-1, and Louisiana 04-1 were inoculated with eight additional isolates from several areas of the world, all 11 isolates produced only TAN lesions. Likewise, when five other plants previously shown to produce only RB lesions when inoculated with the three isolates were inoculated with the 11 isolates, all produced only RB lesions. These results suggest that susceptibility or resistance to P. pachyrhizi in individual kudzu plants often is broad, extending over a wide range of P. pachyrhizi isolates.

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Effects of Daily Temperature Highs on Development of Phakopsora pachyrhizi on Soybean

Phytopathology ◽

10.1094/phyto-01-12-0011-r ◽

2012 ◽

Vol 102 (8) ◽

pp. 761-768 ◽

Cited By ~ 11

Author(s):

M. R. Bonde ◽

S. E. Nester ◽

D. K. Berner

Keyword(s):

United States ◽

Southeastern United States ◽

The United States ◽

Daily Temperature ◽

Soybean Rust ◽

Limiting Factor ◽

Climatic Data ◽

Phakopsora Pachyrhizi ◽

Diurnal Temperature ◽

National Climatic Data Center

Although considerable information exists regarding the importance of moisture in the development of soybean rust, little is known about the influence of temperature. The purpose of our study was to determine whether temperature might be a significant limiting factor in the development of soybean rust in the southeastern United States. Soybean plants infected with Phakopsora pachyrhizi were incubated in temperature-controlled growth chambers simulating day and night diurnal temperature patterns representative of the southeastern United States during the growing season. At 3-day intervals beginning 12 days after inoculation, urediniospores were collected from each plant and counted. The highest numbers of urediniospores were produced when day temperatures peaked at 21 or 25°C and night temperatures dipped to 8 or 12°C. When day temperatures peaked at 29, 33, or 37°C for a minimum of 1 h/day, urediniospore production was reduced to 36, 19, and 0%, respectively, compared with urediniospore production at the optimum diurnal temperature conditions. Essentially, no lesions developed when the daily temperature high was 37°C or above. Temperature data obtained from the National Climatic Data Center showed that temperature highs during July and August in several southeastern states were too high for significant urediniospore production on 55 to 77% of days. The inhibition of temperature highs on soybean rust development in southeastern states not only limits disease locally but also has implications pertaining to spread of soybean rust into and development of disease in the major soybean-producing regions of the Midwestern and northern states. We concluded from our results that temperature highs common to southeastern states are a factor in the delay or absence of soybean rust in much of the United States.

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Pathogenic Variation of Phakopsora pachyrhizi Isolates on Soybean in the United States from 2006 to 2009

Plant Disease ◽

10.1094/pdis-05-11-0379 ◽

2012 ◽

Vol 96 (1) ◽

pp. 75-81 ◽

Cited By ~ 30

Author(s):

M. Twizeyimana ◽

G. L. Hartman

Keyword(s):

United States ◽

Resistance Genes ◽

The United States ◽

Soybean Rust ◽

Phakopsora Pachyrhizi ◽

Soybean Genotype ◽

Pathogenic Variation ◽

Differential Set ◽

Soybean Genotypes ◽

Soybean Leaves

The introduction of Phakopsora pachyrhizi, the cause of soybean rust, into the United States is a classic case of a pathogen introduction that became established in a new geographical region overwintering on a perennial host (kudzu, Pueraria lobata). The objective of our study was to classify the pathogenic variation of P. pachyrhizi isolates collected in the United States, and to determine the spatial and temporal associations. In total, 72 isolates of P. pachyrhizi collected from infected kudzu and soybean leaves in the United States were purified, then established and increased on detached soybean leaves. These isolates were tested for virulence and aggressiveness on a differential set of soybean genotypes that included six genotypes with known resistance genes (Rpp), one resistant genotype without any known characterized resistance gene, and a susceptible genotype. Three pathotypes were identified among the 72 U.S. P. pachyrhizi isolates based on the virulence of these isolates on the genotypes in the differential set. Six aggressiveness groups were established based on sporulating-uredinia production recorded for each isolate on each soybean genotype. All three pathotypes and all six aggressiveness groups were found in isolates collected from the southern region and from both hosts (kudzu or soybean) in 2008. Shannon's index based on the number of pathotypes indicated that isolates from the South region were more diverse (H = 0.83) compared with the isolates collected in other regions. This study establishes a baseline of pathogenic variation of P. pachyrhizi in the United States that can be further compared with variation reported in other regions of the world and in future studies that monitor P. pachyrhizi virulence in association to deployment of rust resistance genes.

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First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Pachyrhizus erosus in the United States

Plant Disease ◽

10.1094/pdis-07-10-0483 ◽

2011 ◽

Vol 95 (8) ◽

pp. 1034-1034

Author(s):

M. A. Delaney ◽

E. J. Sikora ◽

D. P. Delaney ◽

M. E. Palm ◽

J. Roscoe ◽

...

Keyword(s):

United States ◽

Ribosomal Rna ◽

Agricultural Research ◽

The United States ◽

University Teaching ◽

Soybean Rust ◽

Infected Leaf ◽

Phakopsora Pachyrhizi ◽

First Report ◽

Pachyrhizus Erosus

Soybean rust, caused by the fungus Phakopsora pachyrhizi, was detected on jicama (Pachyrhizus erosus L. Urban) for the first time in the United States in November 2009. The pathogen was observed on leaves of a single, potted jicama plant grown outdoors in a residential area and on leaves of all plants in a 12-m2 demonstration plot located at the Auburn University Teaching Garden in Auburn, AL. Symptoms on the upper leaf surfaces were isolated chlorotic areas near the leaf edges in the lower part of the canopy. The abaxial surface was first observed to exhibit brown lesions and subsequently produced volcano-shaped uredinia. These symptoms are consistent with a rust previously described on jicama in Mexico (1). Representative symptomatic plant tissue was sent to the USDA National Identification Services (Mycology) Laboratory in Beltsville, MD for diagnostic confirmation at both the Urbana, IL lab and the USDA National Plant Germplasm and Biotechnology Laboratory for DNA testing. From an infected leaf, samples of approximately 5 mm2 were excised from a microscopically observed rust lesion and an apparently noninfected area. Total DNA was purified with the FastDNA Spin Kit (MP Biomedicals, Solon, OH) followed by the E.Z.N.A. MicroElute DNA Clean-Up Kit (Omega Bio-tek, Inc, Doraville, GA) per manufacturer's instructions. Detection of P. pachyrhizi and P. meibomiae DNA was achieved by quantitative PCR using the method of Frederick et al. (2) and a DNA standard of previously prepared P. pachyrhizi spores. The observed rust pustule was found to contain P. pachyrhizi DNA in excess of 28,000 genomes, while no P. pachyrhizi DNA was observed from the asymptomatic sample. Both samples were negative for P. meibomiae. The fungal structures present were confirmed to be Phakopsora spp. DNA was extracted from sori aseptically removed from leaves with a Qiagen (Valencia, CA) DNeasy Plant Mini Kit and amplified with primers Ppa1 and NL4. The resulting partial ITS2 and 28S ribosomal RNA sequences were 100% identical to GenBank entry DQ354537 P. pachyrhizi internal transcribed spacer 2 and 28S ribosomal RNA gene, partial sequence. Sequences from jicama from Alabama were deposited in GenBank. Voucher specimens were deposited in the USDA Agricultural Research Service, National Fungus Collection (BPI). To our knowledge, this is the first report of the disease on jicama in the United States. References: (1) A. Cárcamo Rodriguez et al. Plant Dis. 90:1260, 2006. (2) R. D. Frederick et al. Phytopathology 92:217, 2002.

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Quantitative Aspects of the Spread of Asian Soybean Rust in the Southeastern United States, 2005 to 2006

Phytopathology ◽

10.1094/phyto-97-11-1428 ◽

2007 ◽

Vol 97 (11) ◽

pp. 1428-1433 ◽

Cited By ~ 37

Author(s):

R. S. C. Christiano ◽

H. Scherm

Keyword(s):

United States ◽

South Carolina ◽

Average Rate ◽

Southeastern United States ◽

Monitoring Program ◽

Early Summer ◽

Soybean Rust ◽

Initial Inoculum ◽

Disease Progress ◽

Short Period

The regional dynamics of soybean rust, caused by Phakopsora pachyrhizi, in six southeastern states (Florida, Georgia, Alabama, South Carolina, North Carolina, and Virginia) in 2005 and 2006 were analyzed based on disease records collected as part of U.S. Department of Agriculture's soybean rust surveillance and monitoring program. The season-long rate of temporal disease progress averaged ≈0.5 new cases day–1 and was higher in nonsentinel soybean (Glycine max) plots than in sentinel soybean plots and kudzu (Pueraria lobata) plots. Despite the early detection of rust on kudzu in January and/or February each year (representing the final phase of the previous year's epidemic), the disease developed slowly during the spring and early summer on this host species and did not enter its exponential phase until late August, more than 1 month after it did so on soybean. On soybean, cases occurred very sporadically before the beginning of July, after which their number increased rapidly. Thus, while kudzu likely provides the initial inoculum for epidemics on soybean, the rapid increase in disease prevalence on kudzu toward the end of the season appears to be driven by inoculum produced on soybean. Of 112 soybean cases with growth stage data, only one occurred during vegetative crop development while ≈75% occurred at stage R6 (full seed) or higher. The median nearest-neighbor distance of spread among cases was ≈70 km in both years, with 10% of the distances each being below ≈30 km and above ≈200 km. Considering only the epidemic on soybean, the disease expanded at an average rate of 8.8 and 10.4 km day–1 in 2005 and 2006, respectively. These rates are at the lower range of those reported for the annual spread of tobacco blue mold from the Caribbean Basin through the southeastern United States. Regional spread of soybean rust may be limited by the slow disease progress on kudzu during the first half of the year combined with the short period available for disease establishment on soybean during the vulnerable phase of host reproductive development, although low inoculum availability in 2005 and dry conditions in 2006 also may have reduced epidemic potential.

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Characterization of Fusarium Yellows Resistance in Collard

Plant Disease ◽

10.1094/pdis.2001.85.8.890 ◽

2001 ◽

Vol 85 (8) ◽

pp. 890-894 ◽

Cited By ~ 16

Author(s):

Mark W. Farnham ◽

Anthony P. Keinath ◽

J. Powell Smith

Keyword(s):

United States ◽

Disease Incidence ◽

Southeastern United States ◽

Critical Factor ◽

Controlled Environment ◽

Infested Soil ◽

Cole Crops ◽

Commercial Cultivars ◽

Significant Temperature

The yellows disease of cole crops, caused by Fusarium oxysporum f. sp. conglutinans, can be very damaging to collard. Growers in the southeastern United States frequently produce collard in hot, summer months when conditions for yellows development are favorable, and thus, incidence of this disease is increasing. A collection of essentially all U.S. commercial cultivars of collard, various landraces of collard, and other representative cole crops was evaluated for response to artificial inoculation with F. oxysporum f. sp. conglutinans under controlled-temperature conditions. In addition, the same collection was evaluated following transplanting for response to naturally infested soil in the field during summer 1997 and 1998. In all trials, genotype had the most significant effect on percentage of diseased plants, and genotype responses ranged from resistant (0 to 20% diseased) to susceptible (61 to 100% diseased). There was a significant temperature effect on percentage of diseased plants in one growth chamber experiment with five genotypes that resulted primarily from an increase in disease incidence for the cultivar Blue Max at 30°C compared with 25°C. Temperature was not significant in a second experiment with 20 genotypes. In the field, although significant differences were observed among genotypes and between years, a significant genotype × year interaction was not detected for percentage of diseased plants, indicating a similar ranking of genotypes for resistance between years. There was a significant correlation between results from controlled-environment studies and the field. A resistant response to F. oxysporum f. sp. conglutinans was expressed in certain cultivars of collard, including Flash, Heavicrop, and Morris Heading, and also in specific landraces. This resistance was stable in relatively high temperature environments used in evaluations. Results of this research indicate that choice of cultivar is a critical factor in producing collard where conditions favor infection by F. oxysporum f. sp. conglutinans. This information will aid in development of new yellows-resistant cultivars.

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Assessment of Epidemic Potential of Soybean Rust in the United States

Plant Disease ◽

10.1094/pd-89-0678 ◽

2005 ◽

Vol 89 (6) ◽

pp. 678-682 ◽

Cited By ~ 29

Author(s):

S. Pivonia ◽

X. B. Yang ◽

Z. Pan

Keyword(s):

United States ◽

The United States ◽

Soybean Rust ◽

Phakopsora Pachyrhizi ◽

Long Distance Dispersal ◽

Long Distance ◽

Climate Conditions ◽

Soybean Production ◽

Critical Components ◽

Production Areas

This article assesses the epidemic potential of soybean rust (Phakopsora pachyrhizi) in the United States. In the assessment, there are three critical components of uncertainty: (i) suitability of climate conditions in production areas for soybean rust epidemics; (ii) likelihood of establishment of the fungus in North America; and (iii) the seasonal dispersal potential of the pathogen from overwintering regions to major soybean production regions. Assessments on the first and second components suggest soybean rust epidemics are likely in the United States, and the certainty of the third component is yet to be determined. Comparison of epidemiological factors for soybean rust in soybean production regions between China and the United States shows a complicated picture with the United States having factors that both increase and decrease risk. Future investigation of risk components—incipience in the field and long-distance dispersal—is needed.

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Evaluation of Soybean Germplasm Accessions for Resistance to Phakopsora pachyrhizi Populations in the Southeastern United States, 2009-2012

Crop Science ◽

10.2135/cropsci2013.08.0513 ◽

2014 ◽

Vol 54 (4) ◽

pp. 1673-1689 ◽

Cited By ~ 16

Author(s):

David R. Walker ◽

Donna K. Harris ◽

Zachary R. King ◽

Zenglu Li ◽

H. Roger Boerma ◽

...

Keyword(s):

United States ◽

Southeastern United States ◽

Phakopsora Pachyrhizi ◽

Soybean Germplasm ◽

Germplasm Accessions

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Reaction of Exotic Soybean Germplasm to Phakopsora pachyrhizi in Uganda

Plant Disease ◽

10.1094/pdis-92-11-1493 ◽

2008 ◽

Vol 92 (11) ◽

pp. 1493-1496 ◽

Cited By ~ 14

Author(s):

H. K. Oloka ◽

P. Tukamuhabwa ◽

T. Sengooba ◽

S. Shanmugasundram

Keyword(s):

Research And Development ◽

Resistance Genes ◽

Rust Resistance ◽

Growth Stage ◽

Resistance Breeding ◽

Soybean Rust ◽

Phakopsora Pachyrhizi ◽

Moderate Resistance ◽

Rust Resistance Genes

Host plant resistance is the best long-term strategy for managing soybean rust (Phakopsora pachyrhizi) in endemic areas. Resistance breeding efforts are hampered by the presence of several races of the pathogen that often overcome single resistance genes deployed against them. In Uganda, only two soybean cultivars show moderate resistance to Phakopsora pachyrhizi, but this is likely to break down given the aggressive nature of the pathogen. A total of 25 rust tolerant or resistant accessions were imported from the Asian Vegetable Research and Development Centre and screened at Namulonge, in central Uganda. Only 10 accessions, G 33, G 8527, G8586, G 8587, GC 60020-8-7-7-18, GC 87016-11-B-2, GC 87021-26-B-1, SRE-D-14A, SRE-D-14B, and SS 86045-23-2, showed no rust symptoms at growth stage R6 during the three seasons of testing. Soybean rust resistance genes Rpp1, Rpp3, and Rpp4 did not confer resistance at Namulonge; gene Rpp2 was effective.

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First Report of Phakopsora pachyrhizi, the Causal Agent of Asian Soybean Rust, on Florida Beggarweed in the United States

Plant Disease ◽

10.1094/pd-90-0972a ◽

2006 ◽

Vol 90 (7) ◽

pp. 972-972 ◽

Cited By ~ 7

Author(s):

L. E. Sconyers ◽

R. C. Kemerait ◽

J. H. Brock ◽

R. D. Gitaitis ◽

F. H. Sanders ◽

...

Keyword(s):

United States ◽

Pcr Primers ◽

The United States ◽

Soybean Rust ◽

Phakopsora Pachyrhizi ◽

Asian Soybean Rust ◽

Abaxial Leaf Surface ◽

Humidity Chamber ◽

Average Size ◽

Soybean Diseases

Phakopsora pachyrhizi Syd. & P. Syd., which causes Asian soybean rust (SBR), was observed on Florida beggarweed, Desmodium tortuosum (Sw) DC., in Attapulgus, GA during late October and early November 2005. Tan to brown lesions (<1.0 mm in diameter) consistent with symptoms of SBR (2) were observed on older leaves of several plants collected near an SBR-infected soybean trial. Dissection (40 to 60×) and compound microscopy (×200 to 400) revealed conical pustules and ellipsoid, echinulate urediniospores (average size 15 × 20 μm) on the abaxial leaf surface. Polymerase chain reaction (PCR) (primers Ppm1 and Ppa2) (1) was conducted on four samples to confirm identification of P. pachyrhizi or P. meibomiae. Three were positive for P. pachyrhizi, and one was negative for both species. Using morphology and real-time PCR, SBR was confirmed as P. pachyrhizi by the USDA/APHIS in Beltsville, MD. Six noninfected Florida beggarweed plants were transplanted to pots during December 2005 and grown at 22 to 24°C in a greenhouse. On 11 January 2006, a water suspension of urediniospores collected from SBR-infected soybeans (1 × 105 spores per ml) was spray inoculated on all leaves to almost runoff and incubated for 48 h in a plastic humidity chamber. Lesions, pustules, and urediniospores consistent with SBR (2) were observed on 3 February 2006. A PCR assay was conducted on six samples from the infected greenhouse plants and all were positive for P. pachyrhizi. Florida beggarweed is widespread in the southern United States and may serve as an additional overwintering source for P. pachyrhizi and a potential inoculum source for the soybean crop. References: (1) R. D. Fredrick et al. Phytopathology 92:217, 2002. (2) J. B. Sinclair and G. L. Hartman. Soybean rust. Pages 25–26 in: Compendium of Soybean Diseases. 4th ed. G. L. Hartman et al., eds. The American Phytopathological Society, St. Paul, MN, 1999.

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