Phakopsora Pachyrhizi Syd Research Articles

Models and applications for risk assessment and prediction of Asian soybean rust epidemics

Asian rust of soybean [Glycine max (L.) Merril] is one of the most important fungal diseases of this crop worldwide. The recent introduction of Phakopsora pachyrhizi Syd. & P. Syd in the Americas represents a major threat to soybean production in the main growing regions, and significant losses have already been reported. P. pachyrhizi is extremely aggressive under favorable weather conditions, causing rapid plant defoliation. Epidemiological studies, under both controlled and natural environmental conditions, have been done for several decades with the aim of elucidating factors that affect the disease cycle as a basis for disease modeling. The recent spread of Asian soybean rust to major production regions in the world has promoted new development, testing and application of mathematical models to assess the risk and predict the disease. These efforts have included the integration of new data, epidemiological knowledge, statistical methods, and advances in computer simulation to develop models and systems with different spatial and temporal scales, objectives and audience. In this review, we present a comprehensive discussion on the models and systems that have been tested to predict and assess the risk of Asian soybean rust. Limitations, uncertainties and challenges for modelers are also discussed.

First Report of Asian Soybean Rust Caused by Phakopsora pachyrhizi from Mexico

Leaves of soybean (Glycine max (L.) Merr.; Fabaceae) cv. Huasteca 400, with conspicuous chlorotic spots and associated hypophyllous cinnamon-brown sori, were collected in commercial soybean plantings in Ébano and Tamuín in the state of San Luis de Potosí, Mexico on 26 October 2005. Uredinia, Malupa-type, are mostly hypophyllous, minute, pulverulent, cinnamon-brown, scattered or in groups, subepidermal becoming erumpent, cone like, surrounded by paraphyses; paraphyses are cylindric to clavate, 25 to 50 × 6 to 14 μm, colorless to yellow brownish with wall thickened at the apex. Urediniospores are obovoid to broadly ellipsoidal, measuring 18 to 37 × 15 to 24 μm, and have a minutely echinulate thin wall, hyaline to pale yellowish brown. This morphology is typical of Phakopsora pachyrhizi Syd. & P. Syd. and P. meibomiae (Arthur) Arthur. DNA was extracted from leaves containing sori with the PureLink Plant DNA Reagent (Invitrogen, Carlsbad, CA), and the identity of P. pachyrhizi was confirmed by the polymerase chain reaction protocol (1) with Ppa1/Ppa2 primers at the National Phytosanitary Reference Center of Mexico. The morphological and molecular diagnosis and presence of P. pachyrhizi in Mexico was officially communicated by the North American Plant Protection Organization (NAPPO) on 16 February 2006. Asian soybean rust was reported for the first time in North America in 2004 (2). To our knowledge, this the first report of P. pachyrhizi in Mexico. Voucher specimens have been placed in the USDA National Fungus Collection as BPI 871130, BPI 871131, and BPI 871132. Images and a complete description of Asian soybean rust can be viewed at http://nt.arsgrin.gov/taxadescriptions/factsheets/index.cfm?thisapp=Phakopsorapachyr hizi .

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

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.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Phaseolus spp. in the United States.

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-cm2 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.

Cytochrome b gene structure and consequences for resistance to Qo inhibitor fungicides in plant pathogens

The cytochrome b (cyt b) gene structure was characterized for different agronomically important plant pathogens, such as Puccinia recondita f sp tritici (Erikss) CO Johnston, P graminis f sp tritici Erikss and Hennings, P striiformis f sp tritici Erikss, P coronata f sp avenae P Syd & Syd, P hordei GH Otth, P recondita f sp secalis Roberge, P sorghi Schwein, P horiana Henn, Uromyces appendiculatus (Pers) Unger, Phakopsora pachyrhizi Syd & P Syd, Hemileia vastatrix Berk & Broome, Alternaria solani Sorauer, A alternata (Fr) Keissl and Plasmopara viticola (Berk & Curt) Berlese & de Toni. The sequenced fragment included the two hot spot regions in which mutations conferring resistance to QoI fungicides may occur. The cyt b gene structure of these pathogens was compared with that of other species from public databases, including the strobilurin-producing fungus Mycena galopoda (Pers) P Kumm, Saccharomyces cerevisiae Meyer ex Hansen, Venturia inaequalis (Cooke) Winter and Mycosphaerella fijiensis Morelet. In all rust species, as well as in A solani, resistance to QoI fungicides caused by the mutation G143A has never been reported. A type I intron was observed directly after the codon for glycine at position 143 in these species. This intron was absent in pathogens such as A alternata, Blumeria graminis (DC) Speer, Pyricularia grisea Sacc, Mycosphaerella graminicola (Fuckel) J Schröt, M fijiensis, V inaequalis and P viticola, in which resistance to QoI fungicides has occurred and the glycine is replaced by alanine at position 143 in the resistant genotype. The present authors predict that a nucleotide substitution in codon 143 would prevent splicing of the intron, leading to a deficient cytochrome b, which is lethal. As a consequence, the evolution of resistance to QoI fungicides based on G143A is not likely to evolve in pathogens carrying an intron directly after this codon.

First Report of Soybean Rust in South Africa.

In February 2001, rust caused by Phakopsora pachyrhizi Syd. was detected for the first time on soybean (Glycine max (L.) Merr.) near Vryheid in northern KwaZulu-Natal, South Africa. As the season progressed, the disease was also observed in other parts of the province, and epidemic levels were reached in the Karkloof, Cedara, Howick, and Greytown production regions. In affected areas, infection foci gradually increased in size and caused premature yellowing and defoliation of soybean crops, usually after the flowering stage. Typical rust symptoms (3) were produced predominantly on the lower surface of soybean leaves. Soybean rust subsequently spread to Amsterdam and Ermelo in the Highveld region of South Africa. Following emergency registration of triazole compounds, fungicides were commonly used to control soybean rust, especially in the more humid eastern production areas. Available yield data suggested a reduction in kernel mass between 4 and 23%, depending on the cultivar and host growth stage at the time of infection. Urediniospores from the original collection (isolate PREM 57280, Plant Protection Research Institute, Pretoria, South Africa) were 23 to 33 × 15 to 22 μm, indicating that spore dimensions fell within the known range for P. pachyrhizi (3). To confirm pathogenicity, 10 to 15 plants of each of the South African soybean cvs. Pan 589, Pan 780, Pan 854, Octa, and Prima were inoculated with isolate PREM 57280. Primary leaves were sprayed with a suspension of spores in light mineral oil (approximately 1 mg of spores per ml) before incubating plants in the dark in a dew chamber for 16 h. Large, sporulating uredinia, producing typical soybean rust urediniospores, developed on all inoculated plants. Classical and real-time fluorescent polymerase chain reaction assays as well as sequence analysis of the internal transcribed spacer regions verified the identity of isolate PREM 57280 as P. pachyrhizi (2). Since the disease is known to occur in Zimbabwe, Mozambique, and several other African countries (1,3,4), inoculum was most likely introduced by air currents from countries to the north of South Africa. It is highly probable that soybean rust will successfully overwinter in South Africa based on experience in other southern African countries.

Teliospore Germination of Soybean Rust Fungus (Phakopsora pachyrhizi Syd.)

AbstractTeliospores of the soybean rust fungus (Phakopsora pachyrhizi Syd.) germinated after treatment with 10—12 wetting and drying cycles at room temperature. Spores swelled, vacuolated and germinated with a slightly curved basidium bearing 1—4 sterigmata with basidiospores. High germination rates were observed when telia were stored at 5 °C for 5—6 months before breaking dormancy.

Degradation of Uredospores of the Soybean Rust Fungus (Phakopsora pachyrhizi Syd.) by Cell‐Free Culture Filtrates of the Mycoparasite Verticillium Psalliotae Treschow

Abstract Verticillium psalliotae isolates Taiw and Thai C are effective parasites of the soybean rust fungus. Cell‐free culture filtrates of these fungi, prepared after growth on autoclaved uredospores, contained β‐1,3‐glucanase, chitinase and protease activities and caused degradations, when rust spores were treated with them for 24 or 72 h. During these lytic processes carbohydrates, amino compounds and N‐acetylhexosamine were released. The carbohydrate fraction was composed of mannitol, arabitol, trehalose, glucose and unidentified substances showing low Rf‐values during thin layer chromatography. The amino compounds consisted of 10 amino acids (leucine and/or isoleucine, phenylalanine, glutamic acid, valine, arginine, asparagine, glutamine, serine, aspartic acid, histidine) and 5—7 substances which could not be identified. Verticillium lecanii isolate Konz is a weak parasite of soybean rust. During growth on uredospores the fungus produced culture filtrates without chitinase activity and with low total activities of β‐1,3‐glucanase and protease. Compared with V. psalliotae, culture filtrates of V. lecanii exerted lower lytic activities on soybean rust uredospores. The results are consistent with the aspect that the rapid growth of V. psalliotae on soybean rust fungus is primarily based on the secretion of lytic enzymes which make nutrients available to the mycoparasite.

Secretion of Extracellular Enzymes by Verticillium psalliotae Treschow and Verticillium lecanii (Zimm.) Viégas During Growth on Uredospores of the Soybean Rust Fungus (Phakopsora pachyrhizi Syd.) in Liquid Cultures

Two isolates of the mycoparasite Verticillium psalliotae grew rapidly in liquid cultures on autoclaved uredospores of the soybean rust fungus (Phakopsora pachyrhizi Syd.) as sole carbon source and secreted β-l,3-glucanase, chitinase, and protease activities into the medium. One isolate of Verticillium lecanii grew slowly, failed to produce measurable chitinase activity and secreted lower specific activities of β-l,3-glucanase and protease, compared with V. psalliotae. The tested isolates of V. psalliotae and V. lecanii produced comparable levels of lipolytic activity. Amylolytic activity was secreted by V. lecanii but not by V. psalliotae. The isolates of V. psalliotae and V. lecanii used in our experiments differed clearly in protein and protease pattern, determined by electrophoresis on polyacrylamide gels. The results indicate that the rapid growth of V. psalliotae on autoclaved uredospores in liquid culture and on uredosori is probably based primarily on nutrients made available to the mycoparasite by activities of β-1,3-glucanases, chitinases and proteases. Zusammenfassung Zwei Isolate des Mycoparasiten Vertkillium psalliotae wuchsen rasch in Flussigkulturen auf autoklavierten Uredosporen des Sojabohnenrostpilzes (Phakopsora pachyrhizi Syd.) als aUeintger Kohlenstoffquelle und schieden β-l,3-Glucanase-, Chitinase- und Proteaseaktivitaten in das Medium aus. Ein Isolat von Vertidllium lecanii wuchs dagegen langsam, produzierte keine mesbare Chitinaseaktivitat und schied im Vergleich zu V. psalliotae geringere spezifische Aktivitaten von β-l,3-Glucanase und Protease aus. Die getesteten Isolate von V. psalliotae und V. lecanii bildeten vergleichbare Mengen lipolytischer Aktivitat. Amylaseaktivitat wurde von V. lecanii ausgeschieden, jedoch nicht von V. psalliotae. Die von uns verwendeten Isolate von V. psalliotae und V. lecanii unterschieden sich deutlich im Muster extrazellularer Proteine und Proteasen, das durch Elektrophorese an Polyacrylamidgelen bestimmt wurde. Die Ergebnisse zeigen, das das rasche Wachstum von V. psalliotae auf autoklavierten Uredosporen des Sojabohnenrostpilzes in Flussigkulturen und auf Uredosporenlagem primar auf den Nahrstoffen beruhen konnte, die dem Mycoparasiten durch β-1,3-Glucanase-, Chitinase- und Proteasenaktivitaten verfugbar gemacht werden.

Defense Reactions in Host and Nonhost Plants Against the Soybean Rust Fungus (Phakopsora pachyrhizi Syd.)

AbstractGrowth and development of two races of the soybean rust fungus (Phakopsora pachyrhizi Syd.) were compared on host and nonhost plants. Both groups had several lines of defense, each of which could stop a part of attacking uredospores. Germ tubes and appressoria were produced equally well on hosts and nonhosts. A reduced formation of penetration hyphae contributed to the resistance of nonhosts and resistant host genotypes. In the epidermal cells of wheat and barley leaves, lower frequencies of penetration hyphae coincided with the production of papillae‐like structures which were not penetrated. The last line of defense of all nonhosts was localized in the epidermal cell where the growth of the penetration hyphae was arrested definitively. The colony development in these species was suppressed completely. In highly resistant host genotypes the last defense reaction occurred later as a hypersensitive cell collapse which interrupted the growth of the rust colony.

Identification of a Fourth Major Gene Conferring Resistance to Soybean Rust1

Studies were conducted in containment facilities at Frederick, MD to determine the genetic basis of resistance to Phakopsora pachyrhizi Syd. (incitant of soybean rust) carried by the soybean [Glycine max (L.) Merr.] line PI 459025. Previous studies showed that soybean genotypes PI 200492, PI 230970, and PI 462312 each carried a single dominant gene conferring resistance to a specific soybean rust isolate. Genotype PI 200492 gave a near‐immune reaction to rust isolate India 73‐1 and a susceptible reaction to Taiwan 72‐1. Genotype PI 462312 gave a resistant reaction to the India rust isolate and a susceptible reaction to the Taiwan isolate. Genotype PI 230970 and F4 lines having PI 230970 as a parent gave resistant reactions to both rust isolates. Genotype PI 200492, PI 230970, and PI 462312 gave a susceptible reaction when inoculated with rust isolate Taiwan 80‐2. Line PI 459025 was identified as resistant to Taiwan 80‐2 as well as Taiwan 72‐1 and India 73‐1. Line PI 459025 was crossed with each of the three previously identified sources of resistance. The F1 plants, F2 populations, and selected F3 lines were inoculated with each of the three rust isolates to determine their reaction. For each plant evaluated, a leaflet of a single trifoliolate leaf was inoculated with a different rust isolate. The results showed that PI 459025 carried a single dominant gene for resistance to all three rust isolates and that this gene was at a different locus from the three previously identified genes conferring resistance to specific rust isolates. The genotype assigned for rust resistance of PI 459025 is rpp1 rpp1 rpp2 rpp2 rpp3 rpp3 Rpp4 Rpp4.

A set of differential Glycine hosts for the identification of races of Phakopsora pachyrhizi Syd.

Variation in the responses of a wide range of accessions of four native Australian species of Glycine (viz. G. canescens, G. clandestina, G. tabacina and G. tomentella) to infection by eight Australian isolates of Phakopsora pachyrhizi was analyzed.

Selection of soybean cultivar and sowing date as a strategy for avoidance of rust (Phakopsora pachyrhizi Syd.) losses in coastal New South Wales

Critical outbreaks of rust on soybeans in coastal New South Wales may occur erratically from year to year, and are most likely to cause defoliation in the latter part of the season. Selection of cultivar and sowing date was investigated as a means of reducing rust losses. Seasonal effects on agronomic and phenological performance were determined for five susceptible soybean cultivars (Ruse, Forrest, Bragg, Ransom and Fitzroy), sown early or late in each of three seasons, with fungicide applications to control rust on half the plots. In unsprayed treatments, rust infestation occurred late in two seasons but reduced yield in only one season, and only in the latest maturing cultivar, Fitzroy, for which yield at both sowing dates was reduced by an average of 19.4%. Seed size was smaller in the sprayed plants of all cultivars in the one season when rust was detected. Rusting had no effect on time to pod maturity in any cultivar. In two seasons, cultivar x sowing date yield responses could be related to the severity and timing of moisture stress. Later sowing dates produced yields greater than or equivalent to those of early dates when significant moisture stress occurred before mid-February. Seed size was affected by sowing date but the differences were not consistent with yield response. Averaged over the three seasons, later sowings (late December-early January) led to higher yields in the absence of rust, but late maturing cultivars (later than Bragg) would be required to ensure that mature plant heights were adequate to reduce harvesting losses. The merits and problems of resistant cultivars are discussed and it is concluded that, in an environment where rust and moisture stress occur intermittently, selection of adapted later maturing cultivars and later sowing dates, appropriate to the locality, is the best short term strategy.

Relationships Among Three Genes Conferring Specific Resistance to Rust in Soybeans1

This study was conducted in containment facilities to further elucidate the genetic basis of specific resistance to Phakopsora pachyrhizi Syd. (incitant of soybean rust) among three soybean [Glycine max (L.) Merr.] lines previously reported to each have a single dominant gene conferring resistance. F4 lines, D79‐9690 and D79‐9697, each having rust resistance derived from PI 230970, were crossed with PI 462312 (‘Ankur’) and PI 200492, respectively. The cross PI 462312 ✕ PI 200492 also was made. F2 populations and selected F3 lines were evaluated for their reactions to two soybean rust isolates, Taiwan‐72‐1 and India‐73‐1. For each plant tested, a leaflet of a single trifoliolate leaf was inoculated with the Taiwan rust isolate and another leaflet was inoculated with the India isolate. PI 230970 and the two breeding lines D79‐9690 and D79‐9697 gave a resistant reaction to both isolates. PI 462312 gave a resistant reaction to the India isolate and PI 200492 an immune reaction to that isolate, but both were susceptible to the isolate from Taiwan. Our results confirmed previously published reports that each soybean line carried a single dominant gene for resistance and demonstrated that each gene was at a different locus. The genotypes for rust reaction assigned to the three parental types were: PI 200492, Rpp1 Rpp1 rpp2 rpp2 rpp3 rpp3; PI 230970, rpp1 rpp1 Rpp2 Rpp2 rpp3 rpp3; and PI 462312, rpp1 rpp1 rpp2 rpp2 Rpp3 Rpp3

Histological studies of the pre-penetration development and penetration of soybeans by rust, Phakopsora pachyrhizi Syd

Comparisons were made between susceptible, resistant and highly resistant soybean lines of pre-penetration development and penetration of uredospores of rust, Phakopsora pachyrhizi, at intervals after inoculation. Differences between lines were found in the percentage of uredospores which germinated on leaves, and smaller differences were found between lines in appressorium formation and penetration from germinated uredospores. The differences between lines were not related to infection type.

Resistance to Soybean Rust and Mode of Inheritance1

A study was conducted in containment facilities to determine the reaction to Phakopsora pachyrhizi Syd. (soybean rust) isolates from widely separated geographic areas on seven soybean [Glycine max (L.) Merr.] strains rated moderately resistant in field planting in Taiwan. Additional studies were conducted to determine the mode of inheritance of resistance to several rust isolates. Soybean strains PI 230970 and PI 230971 were resistant when inoculated with P. pachyrhizi isolates Australia‐72‐1, India‐73‐1, Philippines‐77‐1, and Taiwan‐72‐1. The other five soybean strains, although rated moderately resistant in field planting in Taiwan, were rated susceptible in these studies. All were rated resistant to an isolate with low virulence from Puerto Rico.Individual trifoliolate leaves of soybean plants of an F2 population of ‘Centennial’ × PI 230970 were inoculated with rust isolates from three different geographic areas. Plants rated as resistant to one isolate were resistant to all isolates. Similarly, plants rated susceptible to one isolate were susceptible to all isolates. A ratio of 92 resistant to 31 susceptible was obtained. Twenty‐five F2 plants from the cross D75‐10169 × PI 230971 gave 19 resistant and six susceptible plants after inoculation with the Taiwan isolate. The results suggest a single dominant gene for resistance to soybean rust.

Histological studies of resistance to soybean rust, Phakopsora pachyrhizi Syd

Histological comparisons were made of susceptible, resistant, and immune responses of soybean to infection by rust, Phakopsora pachyrhizi, at intervals after inoculation. In the susceptible host there was extensive hyphal growth and vigorously sporulating uredia developed. The immune hosts showed no macroscopic symptoms of infection, but at the microscopic level exhibited a hypersensitive response which was limited to a few cells. In the resistant host the hypersensitive response was not as limited, fungal development and associated host cell necrosis were more extensive, and symptoms of infection were visible macroscopically.

Phakopsora pachyrhizi Syd: the causal agent of Soybean rust.

Phakopsora pachyrhizi Syd: the causal agent of Soybean rust.