ABSTRACT
The Asian soybean rust caused by the fungus Phakopsora pachyrhizi was cited for the first time in Argentina during the 2002-2003 growing season (3). During 2003-2004, the disease spread to other northern provinces and was also observed in north-central Santa Fe, the main producing soybean province of the country. Because the disease appeared at the end of the crop growing season (late March to early April) it had little or no impact on crop yields. The objectives of this study were to characterize morphologically and pathometrically the disease on soybean and check the presence of P. pachyrhizi on volunteer soybean plants that could eventually carry the disease to the next growing season. The study was conducted in the San Justo Department, Santa Fe Province (between 30 and 31°S latitude), where the presence of the soybean rust was molecularly confirmed by Sistema Nacional Vigilancia y Monitoreo (on-line publication at www.sinavimo.gov.ar ). Three field locations were sampled and identified as M1, M2, and M3. Transversal cuts of soybean leaves through rust lesions and histo-pathological staining were used for micromor-phologic characterization of the developmental stages of P. pachyrhizi. The disease incidence was estimated as the proportion of affected soybean plants and leaves. Average severity, expressed as the percentage of leaf area affected, including chlorosis, was measured on the terminal leaflet of leaves sampled from the lower one-third of the canopy. Three replicates of 10 plants, randomly chosen, were used. The number of uredinia per square centimeter and per lesion (symptomatic foliar area showing chlorosis and necrosis caused by the fungus) was measured on the undersides of the sampled leaflets at ×40 magnification (1). Typical signs and symptoms of P. pachyrhizi coexisted on soybean leaves with brown spot (Septoria glycines), downy mildew (Peronospora manshurica), anthracnose (Colletotrichum truncatum), and blight and leaf spot (Cercospora kikuchii) and also with bacteria (Pseudomonas and Xanthomonas spp.). Uredinia and telia of the P. pachyrhizi cycle were observed. Uredinia were also observed on soybean petioles. The average size of urediniospores (n = 60) was 23.3 × 16.6 µm. Telia were located adjacent to the uredinia. These telia were dark and crusty with four stacked layers of teliospores. Rust incidence in plants was 100% for the three fields, while the incidence in leaves was 100% for M1 and M2 and 60% for M3. Average disease severity was 50.3, 25.6, and 14.8% for M1, M2, and M3, respectively. The mean number of uredinia per square centimeter was 327, 179, and 177, for M1, M2, and M3, respectively. The number of uredinia per lesion ranged from 1 to 6. P. pachyrhizi was also found on volunteer soybean plants that emerged shortly after harvest. On 40 leaflets, the foliar incidence was 25%, showing one to two lesions with one to two uredinios per leaflet (2). The volunteer soybean plants could constitute a potential early source of inoculum. References: (1) M. Marcchetti et al. Phytopathology. 66:461, 1976. (2) R. Pioli et al. La roya asiática en Santa. Fe, Arg. XII Cong. AAPRESID, II Sem. Internac. Soja, Arg. 283-290, 2004. (3) R. L. Rossi. Plant Dis. 87:102, 2003.
ABSTRACT
Fusarium graminearum, a pathogen of wheat and corn, was reported recently as a saprophytic fungus colonizing soybean (Glycine max L. Merr.) fruits and seeds at R7 in Argentina (2). To evaluate the capacity of F. graminearum obtained from stem and seeds of symptomatic soybean plants that cause disease on soybean seedlings, isolates were obtained during the 2001 to 2002 growing season from: (i) the basal one-third of stems from field-grown soybean plants, collected at R5, with light brown external and internal discoloration and leaves with interveinal chlorosis; and (ii) soybean seeds with pink tegument. The pathogen was isolated on potato glucose agar acidified with 0.2% lactic acid (PGAA). Isolates were identified as F. graminearum on the basis of growth rate and pigmentation of colonies on PGAA, lack of microconidia (1), and morphology and size of typical macroconidia in sporodochia developed on Spezieller Nährstoffarmer Agar (3). Isolates of F. graminearum, CE135 and CE136 (from wheat) and CE137 (from corn) deposited in the Centro de Referencia en Micología (CEREMIC), Fac. Farmacia y Bioquímica, UNR, Argentina, were used as references in identifying the soybean isolates. Plants (14-day-old) were inoculated separately with stem and seed isolates in the greenhouse at 26 ± 2 and 20 ± 2°C day/night temperature by inserting a piece of mycelium into a wound made with a scalpel in the hypocotyl. A completely randomized block design (RCB) was utilized with four replicate pots with four plants per pot. Plants wounded but without mycelium served as controls. This test was conducted twice (experiments 1 and 2). Another test was completed by burying a thin layer of wheat caryopsis colonized by fungal mycelium of the stem isolate CE170 in the soil of pots. Plants in pots with soil without inoculum served as controls (4). The experiment was conducted twice (experiments 3 and 4) in an RCB with five replications, four plants per replication. The progress of symptoms in experiments 1 and 2 were stem with light brown discoloration around the inoculation point that extended progressively along the stem, interveinal chlorosis or loss of turgence of unifoliate leaves, and interveinal chlorosis of trifoliate leaves followed by plant wilting and death. Twenty-one days after inoculation, average percentages of dead plants (%DP) was 42 and 21% for stem and seed isolates, respectively. For experiments 3 and 4, %DP was 56%, 45 days after emergence. These plants had roots with light brown, necrotic areas. Control plants remained healthy. The pathogen was reisolated from the stem (100%) and root (57%) tissues of symptomatic plants but not from similar tissues of control plants. To our knowledge, this is the first report of a pathogenic relationship between F. graminearum and soybean. References: (1) P. E. Nelson et al. Fusarium species: An Illustrated Manual for Identification. The Pennsylvania State University Press, University Park, PA, 1983. (2) R.N. Pioli et al. Fitopatología 35(2):111, 2000. (3) B. A. Summerell et al. Plant Dis. 87:117, 2003. (4) C. E. Windels. Fusarium. Pages 115-128 in: Methods for Research on Soilborne Phytopathogenic Fungi. L. L. Singleton, J. D. Mihail, and C. M. Rush, eds. The American Phytopathological Society, St. Paul, MN, 1992.
ABSTRACT
The first report of soybean stem canker (SSC) caused by Diaporthe phaseolorum var. caulivora in South America was published in 2001, and was based on an isolate obtained in 1999 at Oliveros, Santa Fe (32°33'S, 60°51'W), Argentina (2). During the 2001 to 2002 growing season, isolates of D. phaseolorum var. caulivora were obtained from stems of field-grown soybeans (Glycine max L.) exhibiting SSC symptoms. Isolates were collected in three localities of the main soybean-producing region of Argentina: Marcos Juárez, Córdoba (32°66'S, 62°10'W); Salto, Buenos Aires (34°20'S, 60°33'W); and Diego de Alvear, Santa Fe (34°21'S, 62°10'W), and disease incidence in the fields was 10 to 60%, 5 to 15%, and 10 to 20%, respectively. The pathogen was isolated on potato glucose agar acidified with 0.2% lactic acid cultured in the dark at 25 ± 1°C. White colonies with compact and tufted mycelium were produced and turned yellow and light tan after 6 days. Appressed and fluffy mycelia were observed in old cultures. Stromata (2 mm diameter) were produced but pycnidia were not detected. After 20 days in culture at 25 ± 1°C under a 12-h light and 12-h dark regime, clustered perithecia developed on stem segments. For each isolate, 10 perithecia, 90 asci, and 30 bicellular, biguttulate ascospores were measured. Averages of asci length and width were 28.3 ± 2.3 and 5.9 ± 0.7 µm, respectively. Averages of ascospores mean length and width were 8.4 ± 0.6 and 2.5 ± 0.4 µm, respectively. These measures were similar to the measures obtained previously (2). Based on these features, the new isolates were classified as D. phaseolorum var. caulivora (Athow & Caldwell). Clustered perithecia, smaller asci and ascospores, and the development of fluffy mycelia with age were the main characteristics that distinguished D. phaseolorum var. caulivora from D. phaseolorum var. meridionalis (1). Pathogenicity trials were performed on cvs. Tracy M, Crockett, Hutchenson, and RA 702 in the greenhouse by placing a small amount of mycelium in soybean seedling hypocotyls wounds made with a scalpel. The pathogen was reisolated from stem portions of the symptomatic plants. Control plants remained healthy. The results reported here show that D. phaseolorum var. caulivora is widely disseminated in the main soybean-producing region of Argentina, where it coexists with D. phaseolorum var. meridionalis (2). The coexistence of both varieties indicates pathogen variability in the region is higher than previously recognized. References: (1) R. N. Pioli et al. Plant Dis. 83:1071, 1999. (2) R. N. Pioli et al. Plant Dis. 85:95, 2001.
ABSTRACT
A soybean stem canker (SSC) outbreak caused by Diaporthe phaseolorum (Cooke & Ellis) Sacc. var. meridionalis Fernández was reported in Santa Fe, Argentina, in 1997 (3). In 1999 an isolate, which was morphologically distinct from D. phaseolorum var. meridionalis, was obtained from stems of field-grown soybean plants exhibiting SSC symptoms, at Oliveros, Santa Fe, Argentina (Lat. 32° 33'S, Lon. 60° 51'W). Disease incidence was 76% in the field where samples were collected. The pathogen was isolated in darkness at 25°C on potatoglucose agar acidified with 0.2% lactic acid (3). The isolate produced white colonies with compact and tufted mycelium that changed to yellow and light tan with age. Stromata and pycnidia were not produced. After 35 days in culture, clustered perithecia were frequently observed on stem segments. Fifty asci, five from each of 10 perithecia, and bicellular, biguttulated ascospores were measured. Ascus mean length was 26.9 ± 2.5 µm and width was 5.3 ± 0.5 µm; ascospore mean length was 8.3 ± 0.6 µm and width was 2.6 ± 0.1µm. Based on these features, the new isolate was classified as D. phaseolorum var. caulivora Athou & Caldwell (1). To further compare the new isolate with previous identified ones, a principal component analysis (PCA, SAS Systems) was performed using seven isolates of D. phaseolorum var. meridionalis, three isolates of D. phaseolorum var. sojae, and two isolates of Phomopsis longicolla. Seventeen morphological characters, all related with the color and texture of the colonies, the presence and shape of the pycnidia and conidia, the presence and type of stromata and perithecia, and the length of the asci, were compared. According to the PCA analysis, the principal characters that discriminated SSC producing isolates (D. phaseolorum var. meridionalis and D. phaseolorum var. caulivora) from non-SSC producing ones (D. phaseolorum var. sojae and P. longicolla) were the development of perithecia (r = 0.98) and low frequency stromata (r = 0.98) in D. phaseolorum var. meridionalis and D. phaseolorum var. caulivora isolates. The principal components that discriminated SSC producing isolates were the more compact and tufted aspect of the mycelia (r = 0.95) and the shorter length of the asci (r = 0.83) in D. phaseolorum var. caulivora compared with D. phaseolorum var. meridionalis. Pathogenicity trials were performed under greenhouse conditions by inoculating D. phaseolorum var. caulivora mycelia in hypocotyls of soybean seedlings by the toothpick method (2). Typical SSC symptoms were observed on susceptible plants and the pathogen was re-isolated and identified from stem portions of the first internode above the inoculation point. Pathogenicity trials were repeated twice with similar results. This is the first report of D. phaseolorum var. caulivora in Argentina and, as far as we know, in all of South America. References: (1) F. A. Fernández et al. 1999. Stem canker. Pages 32-35 in: Compendium of Soybean Diseases, 4th ed. APS Press, St. Paul, MN. (2) B. L. Keeling. Phytopathology 72:807-809, 1982. (3) R. N. Pioli et al. Plant Dis. 81:1215, 1997.
ABSTRACT
The objective of this study was to characterize the pathogenicity of several local isolates of Diaporthe phaseolorum (Cooke & Ellis) Sacc. var. meridionalis Fernández and its anamorph, Phomopsis phaseoli (Desmaz.) Sacc. meridionalis Morgan-Jones, the causal agent of southern stem canker of soybean (Glycine max (L.) Merr.), in soybean lines carrying major resistance genes. Soybean plants with typical stem canker symptoms were collected during the 1996 to 1997 and 1997 to 1998 growing seasons in the central and southern areas of Santa Fe Province, Argentina. The pathogen was isolated from the internal tissues of infected stems, cultured on potato glucose agar acidified with 0.2% lactic acid (APGA), amended with streptomycin at 100 mg/liter, and maintained in the dark at 25 ± 1°C. Isolates were characterized based on the morphology of colonies, perithecia, and pycnidia and measurement of asci, bicellular, biguttulate ascospores, and alpha conidia (1). Soybean cultivars used to assay pathogenicity included Tracy M (Rdc1 and Rdc2 genes), Isoline I (Tracy Misoline with only the Rdc1 gene), Isoline II (Tracy M isoline with only the Rdc2 gene), Crockett (Rdc3 gene), Hutchinson (Rdc4 gene), and RA 702 (susceptible cultivar). Hypocotyls of 14-day-old seedlings grown in the greenhouse were inoculated by the toothpick method. Four replicates of nine seedlings each were used. Seedlings punctured with sterile toothpicks served as controls. The experiment was repeated twice with similar results. The D. phaseolorum var. meridionalis isolates assayed and their collection locations were Dpm1 (Malabrigo), Dpm2 (Los Molinos), Dpm3 (San Justo), Dpm5 (Oliveros), Dpm6 (San Jerónimo), and Dpm7 (Clarke). Twenty-eight days after inoculation, stem canker reactions were measured as the percentage of dead plants. The pathogen was reisolated from stems of randomly chosen symptomatic plants on day 14 after inoculation. These plants were included in the calculation of the percentage of dead plants. In control plants, lesions were not detected, and mycelial growth did not occur from stem portions plated on APGA. Tracy M and RA 702 had 0 to 7% dead plants and 70 to 95% dead plants, respectively, with all assayed isolates. Cultivars with single resistance genes reacted differently to various isolates. Isolates Dpm1 and Dpm3 caused little or no stem canker (<10% dead plants) on all cultivars with resistance genes. Isolates Dpm2 and Dpm6 killed 56 and 52%, respectively, of Isoline II (Rdc2 gene) plants. Isolates Dpm2 and Dpm7 killed 25% of cv. Hutchinson (Rdc4 gene) and Isoline I (Rdc1 gene) plants, respectively. Isolate Dpm5 killed <12% of plants with genes Rdc1, Rdc2, or Rdc3. The reaction of isolate Dpm5 with Hutchinson (Rdc4 gene) was not evaluated. The pathogenic diversity of these isolates of D. phaseolorum var. meridionalis may have been induced by the wide diffusion of resistant host cultivars (2). References: (1) F. A. Fernández and R. T. Hanlin. Mycologia 88:425, 1996. (2) A. W. Zhang et al. Phytopathology 88:1306, 1998.
