Soybean-Macrophomina phaseolina-Specific Interactions and Identification of a Novel Source of Resistance

Charcoal rot, caused by the fungus Macrophomina phaseolina, is an economically important disease of soybean (Glycine max) worldwide. Objectives of the present research were to (i) study the genetic and pathogenic diversity in a collection of M. phaseolina isolates from Argentina and Paraguay and (ii) develop an improved in vitro phenotyping method to evaluate disease response of soybean genotypes to M. phaseolina isolates. Cluster analysis showed no clear association among simple sequence repeat profiles, year of collection, pathogenicity, and geographical origin of the isolates from Argentina and Paraguay. Subsequently, the response of four soybean genotypes against seven M. phaseolina isolates was evaluated in the field and the results were confirmed using the in vitro assay developed. This assay, which is based on root disease development on soybean seedlings, allowed the detection of a differential level of aggressiveness among the isolates on four soybean genotypes. The results suggest the existence of specific interactions among soybean genotypes and M. phaseolina isolates. In addition, cultivar Munasqa RR showed a superior response against M. phaseolina compared with DT 97-4290 (moderately resistant), thus becoming a novel source of resistance to charcoal rot.

Elicitor-Based Biostimulant PSP1 Protects Soybean Against Late Season Diseases in Field Trials

Currently, fungicide application in soybean production accounts for an important amount of global pesticide use, and it is therefore most desirable to find new healthier and more environmental friendly alternatives for the phytosanitary management in this crop. In this study, we present convincing evidence for effective induction of disease protection by the agricultural biostimulant PSP1, a formulation based on the plant-defense eliciting activity of the fungal protease AsES (Acremonium strictum elicitor subtilisin), in multiple field trials in Argentina. PSP1 was shown to combine well with commercial spray adjuvants, an insecticide, a herbicide and fungicides used in Argentinian soybean production without losing any defense-inducing activity, indicating an easy and efficient adaptability to conventional soybean production and disease management in the region. Results from multiple soybean field trials conducted with different elite genotypes at several locations during two consecutive growing seasons, showed that PSP1 is able to induce an enhanced pathogen defense which effectively reduced late season disease (LSD) development in field-grown soybean. This defense response seems to be broad-range as disease development was clearly reduced for at least three different fungi causing LSDs in soybean (Septoria glycines, Cercospora kikuchii and Cercospora sojina). It was noteworthy that application of PSP1 in soybean alone gave a similar protection against fungal diseases as compared to the commercial fungicides included in the field trials and that PSP1 applied together with a fungicide at reproductive stages enhanced disease protection and significantly increased grain yields. PSP1 is the first example of an elicitor-based strategy in order to efficiently control multiple fungal diseases under field conditions in the soybean crop. These results show the feasibility of using induced resistance products as complements or even full-good replacements to currently used chemical pesticides, fulfilling a role as important components of a more sustainable crop disease management system.

A Study of the Sugarcane Yellow Leaf Disease in Argentina

Yellow leaf disease, caused by Sugarcane yellow leaf virus (SCYLV), is widespread around the world but very little information is available on this viral disease in Argentina. Therefore, the aims of the study were to assess the presence of SCYLV, analyze its distribution in the main sugarcane production areas of Argentina, characterize the virus, and determine histological alterations caused by its presence. For this purpose, 148 sugarcane samples with and without symptoms were collected in 2011 and 2012 from the province of Tucumán. One additional sample was collected in Salta, a different geographical, agroecological, and producing region. Results showed that SCYLV is widely distributed in commercial varieties of sugarcane throughout Tucumán in both symptomatic and asymptomatic leaves. A low but statistically significant positive correlation with virus detection and disease symptoms was found. BRA-PER was the only genotype detected by reverse-transcription polymerase chain reaction and sequence analysis of the SCYLV capsid protein gene. SCYLV-positive samples showed high starch levels in bundle sheath cells, whereas the asymptomatic ones, probably in an early stage of infection, were found to contain more chloroplasts. Symptomatic noninfected samples presented crystal formation probably associated with phytoplasma infection.

First Report of Fusarium Rot Caused by Fusarium oxysporum on Lemon in Tucumán, Argentina

Fusarium rot is considered a minor disease of citrus fruits. Several Fusarium species have been associated with fruit decay, most commonly F. lateritium Nees, F. moniliforme J. Sheld., F. oxysporum Schltdl., and F. solani (Mart.) Sacc. (2,3). In the winters of 2007, 2009, 2010, and 2011, lemon [Citrus limon (L.) Burm. f.] fruit with white mycelium covering the peduncle were submitted to the Phytopathology Lab at the Estación Experimental Agroindustrial Obispo Colombres. All fruit samples from Tucumán, Argentina, were stored in boxes kept in packinghouse for more than 1 month. In 2007 only, light to dark brown flavedo around the peduncle was observed in less than 1% of the sample fruit received. No internal breakdown was visible. No change in rind color was observed in the samples received in remaining years. Abundant Fusarium sp. conidia were observed on the mycelium. Colonies with white to violet fluffy aerial mycelium developed on potato dextrose agar (PDA) and produced abundant ovoid or oblong microconidia (1.9 to 3.6 × 4.8 to 10.8 μm), usually unicellular, borne in false heads on short monophialides, and loculated slightly falcate macroconidia were mostly three to five septate (2.4 to 4.8 × 19.2 to 31.2 μm). Unbranched and branched-monophialidic conidiophores were observed. Simple or paired chlamydospores developed on synthetic nutrient agar (1 g KH₂PO₄, 1 g KNO₃, 0.5 g MgSO₄.7H₂O, 0.5 g KCl, 0.2 g sucrose, and 20 g agar/liter distilled water). On the basis of morphological and cultural criteria, 22 isolates were identified as F. oxysporum (4) designated as D1 to D22. Morfological identification was confirmed by PCR (1) using genomic DNA extracted from the mycelium of pure culture, and an amplified product of 70 bp, specific for the species F. oxysporum, was obtained. The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS4/ITS5 and secuenced. BLAST analysis of the 600 bp segment showed a 100% indentity with F. oxysporum, strains CCF 4362 and 1166 (GenBank Accession Nos. HE974454 and FR731133, respectively). Pathogenicity tests were conducted twice by inoculating 10 surface-disinfected wounded lemon fruit. A rind disc (5 mm in diameter and 1 mm deep) near the stem end was removed and a 5-mm-diameter agar disc of D2 isolate (grown at 25°C for 5 days on PDA) was attached to the wound replacing the rind disc. The inoculation site was covered with moistened cotton wool and the fruit were wrapped in plastic bags to prevent the inoculum from drying out. Ten control fruit were inoculated with uncultured PDA plugs (5 mm in diameter). All fruit were maintained in a growth chamber at 25°C under humid conditions. After 5 to 6 days, all inoculated fruit showed white aerial mycelium, initially on the inoculation site and then on the peduncle, similar to that observed on naturally infected fruit. After 20 days, two fruit developed stem end dry rot and showed peduncle fall but no internal breakdown was visible. Control fruit developed any symptom as described above. F. oxysporum was consistently reisolated from infected tissues, completing Koch's postulates. To our knowledge, this is the first report of Fusarium rot caused by F. oxysporum on lemon in Tucumán, Argentina. References: (1) V. Edel et al. Mycol. Res. 104:518, 2000. (2) H. S. Fawcett. Citrus Diseases and Their Control, 1936. (3) A. Z. Joffe and M. Schiffmann-Nadel. Fruits 27:117, 1972. (4) P. E. Nelson et al. Fusarium species: An Illustrated Manual for Identification, 1983.

Characterization of a variant of Xanthomonas citri subsp. citri that triggers a host-specific defense response

Citrus is an economically important fruit crop that is severely afflicted by Asiatic citrus bacterial canker (CBC), a disease caused by the phytopathogen Xanthomonas citri subsp. citri (X. citri). To gain insight into the molecular epidemiology of CBC, 42 Xanthomonas isolates were collected from a range of Citrus spp. across 17 different orchards in Tucumán, Argentina and subjected to molecular, biochemical, and pathogenicity tests. Analysis of genome-specific X. citri markers and DNA polymorphisms based on repetitive elements-based polymerase chain reaction showed that all 42 isolates belonged to X. citri. Interestingly, pathogenicity tests showed that one isolate, which shares >90% genetic similarity to the reference strain X. citri T, has host range specificity. This new variant of X. citri subsp. citri, named X. citri A(T), which is deficient in xanthan production, induces an atypical, noncankerous chlorotic phenotype in Citrus limon and C. paradisi and weak cankerous lesions in C. aurantifolia and C. clementina leaves. In C. limon, suppression of canker development is concomitant with an oxidative burst; xanthan is not implicated in the phenotype induced by this interaction, suggesting that other bacterial factors would be involved in triggering the defense response.

Genetic diversity among viruses associated with sugarcane mosaic disease in Tucumán, Argentina

Sugarcane leaves with mosaic symptoms were collected in 2006--07 in Tucumán (Argentina) and analyzed by reverse-transcriptase polymerase chain reaction (RT-PCR) restriction fragment length polymorphism (RFLP) and sequencing of a fragment of the Sugarcane mosaic virus (SCMV) and Sorghum mosaic virus (SrMV) coat protein (CP) genes. SCMV was detected in 96.6% of samples, with 41% showing the RFLP profile consistent with strain E. The remaining samples produced eight different profiles that did not match other known strains. SCMV distribution seemed to be more related to sugarcane genotype than to geographical origin, and sequence analyses of CP genes showed a greater genetic diversity compared with other studies. SrMV was detected in 63.2% of samples and most of these were also infected by SCMV, indicating that, unlike other countries and other Argentinean provinces, where high levels of co-infection are infrequent, co-existence is common in Tucumán. RFLP analysis showed the presence of SrMV strains M (68%) and I (14%), while co-infection between M and H strains was present in 18% of samples. Other SCMV subgroup members and the Sugarcane streak mosaic virus (SCSMV) were not detected. Our results also showed that sequencing is currently the only reliable method to assess SCMV and SrMV genetic diversity, because RT-PCR-RFLP may not be sufficiently discriminating.

First Report of Angular Leaf Spot Caused by Phaeoisariopsis griseola on Phaseolus coccineus in Argentina

Angular leaf spot (ALS), caused by Phaeoisariopsis griseola (Sacc.) Ferraris, is one of the most destructive and widespread problems of common bean (Phaseolus vulgaris L.) in Tucumán and other northwestern provinces of Argentina (4). Symptoms similar to those of ALS were observed during April 2005 on most plants of runner bean (P. coccineus L.) in an 80-ha field in Tafí del Valle, Tucumán (2,000 m above sea level). Leaf lesions were brown to gray, irregular to angular to circular, and 0.5 to 1 cm in diameter. Lesions on pods were oval to circular with reddish brown centers surrounded by darker brown borders. Conidia in vivo were curved cylindrical to obclavate with one to five septa and measured 25 to 60 × 3.5 to 7 μm. The conidiophores were 100 to 250 μm high and clustered together to form synnemata measuring 20 to 50 μm in diameter. The pathogen was isolated by placing conidia from diseased leaves onto potato dextrose agar (PDA) at pH 6. Colonies measuring 2 to 3 mm in diameter composed of dense, dark olive mycelium developed after incubation in the dark at 24 ± 2°C for 3 to 4 days. Pathogenicity of the isolate was tested with conidia obtained from the second subculture of 14-day-old colonies on PDA. Conidial suspensions of 2 × 10⁴ conidia per ml were sprayed onto the upper and lower surfaces of the first trifoliolate leaves of six runner bean plants, 18 days after planting. Inoculated and control plants (sprayed with distilled water) were placed in a growth chamber with a 12-h photoperiod at 24 ± 2°C and 95 to 100% relative humidity and 48 h later moved to the greenhouse. Disease symptoms were evaluated 18 days after inoculation. While control plants were healthy, all inoculated plants showed symptoms similar to those observed in the field. The fungus that was consistently reisolated from lesions in the inoculated plants was identified as Phaeoisariopsis griseola on the basis of fungal morphology (1), symptoms produced on leaves (3), and random amplified polymorphic DNA data with primer 5'-CAATCGCCGT-3' (2). Runner bean is a new crop in Tafí del Valle, which is a geographically isolated area. In a period of only 2 years, the area cultivated with beans increased approximately five-fold. Because of this, the presence of a pathogen like Phaeoisariopsis griseola, which causes considerable reduction in yield in most common bean-producing areas of Argentina, is of concern. To our knowledge, this is the first report of ALS occurring on P. coccineus in Argentina. This report may prompt the inclusion of regular testing of seeds for ALS in P. coccineus-production areas. A voucher culture has been deposited in the LPSC (Culture collection of the La Plata Spegazzini Institute) No. 844. References: (1) M. B. Ellis. Dematiaceous Hyphomycetes. CMI, Kew, Surrey, UK, 1971. (2) P. Guzmán et al. Plant Dis. 83:37, 1999. (3) A. W. Saettler. Pages 15-16 in: Compendium of Bean Diseases. R. Hall, ed, The American Phytopathological Society, St. Paul, 1991. (4) S. A. Stenglein et al. Pages 209-243 in: Advances in Applied Microbiology, Vol. 52. A. I. Laskin et al., eds, Academic Press, San Diego, 2003.

Detection of Soybean Rust caused by Phakopsora pachyrhizi in Northwestern Argentina

Asian soybean rust, caused by Phakopsora pachyrhizi, is regarded as one of the most destructive diseases of soybean (Glycine max (L.) Merr.). In Argentina, it was first detected in the province of Misiones in the northeast near Paraguay and Brazil during the 2001-02 growing season (2). The following season, it also was found in the neighboring province of Corrientes. However, it did not reach major soybean production areas in northern Argentina until the end of the 2003-04 season. During April 2004, as soybean crops were nearing maturity, the disease was found throughout the region of northwestern Argentina, which includes the provinces of Tucumán, Salta, Jujuy, Catamarca, and Santiago del Estero, where approximately 6% of the soybean crop of Argentina is produced. During February and March, the area had a severe drought and above average temperatures, but in April, rainfall was abundant, particularly during the first half of the month. Soybean rust was first observed on 16 April in several locations of the departments (counties) of Moreno and Jiménez in the province of Santiago del Estero, and the following week in the departments of Alberdi, Burruyacú, Cruz Alta, Famaillá, La Cocha, and Leales in the province of Tucumán, in the department of Santa Rosa in the province of Catamarca, and in the departments of Anta, Metán, Rosario de la Frontera, and San Martín in the province of Salta. In those fields where the disease was detected, nearly all plants showed symptoms. Affected crops were mostly in growth stages R7 to R8, except for a few fields that had been planted late and were in a late R5 stage. Yield losses as much as 28% and premature defoliation occurred in these fields only. Disease severity, measured as percentage of affected leaf area, ranged from 45 to 50% in untreated fields and 0.9 to 39% in fungicide-treated fields. Leaf lesions were reddish brown, irregularly shaped, and were more abundant on the abaxial surface. Under the dissecting microscope, uredinia were observed as erumpent pustules with a conspicuous central pore. Masses of urediniospores were expelled through the pore and covered the pustules. Urediniospores were hyaline to pale yellow-brown, sub globose to ovoid, with finely echinulate, hyaline walls, and an average size of 27.8 × 18.5 μm. Because there are two morphologically similar species of Phakopsora that infect soybean, P. pachyrhizi (the Asian species) and P. meibomiae (the New World species), a molecular differentiation was carried out using the polymerase chain reaction (PCR) assay described by Frederick et al. (1). DNA extracted from 37 samples from different locations was amplified with specific primers for both species of Phakopsora and specific primers for P. pachyrhizi and for P. meibomiae. Twenty-eight samples amplified with the two species primers and with the P. pachyrhizi primer. None of the samples amplified with the P. meibomiae primer. Specimens have been deposited at Instituto Miguel Lillo, Tucumán, Argentina. These results confirmed the presence of P. pachyrhizi in the provinces of Catamarca, Tucumán, Salta, and Santiago del Estero, Argentina. References: (1) R. D. Frederick et al. Phytopathology 92:217, 2002. (2) R. L. Rossi. Plant Dis. 87:102, 2003.

The foliar fungal endophytes of Citrus limon in Argentina

Fungal endophytes associated with Citrus limon (L.) Burm., were investigated. Culture of surface-sterilized leaves from two orchards of lemon trees in Tucumán, located in northwestern Argentina, revealed the presence of endophytic fungi in all leaves examined. The colonization frequencies were 69.7% and 72.3% at the two sampled sites, and a total of nine taxa were isolated. Eight of the taxa had relative frequencies of >2% in at least one sample. The number of isolates was significantly lower in the spring. The assemblage composition at each site and sampling time was similar. Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. in Penz., responsible for anthracnose, was consistently dominant. Another pathogenic fungus of Citrus spp., Guignardia citricarpa Kiely was also recovered as an endophyte. Scanning electron micrographs of cut uncultured and cultured leaf surfaces, seen in transverse section, showed intercellular fungal hyphae in parenchyma cells No penetration of host cells by either fungus was observed. This is the first report on fungal endophytes in leaves of lemon plants in Argentina.Key words: endophytic fungi, lemon, colonization, scanning electron microscopy.

Frogeye Leaf Spot of Soybean Caused by Cercospora sojina in Northwestern Argentina

Frogeye leaf spot of soybean (Glycine max (L.) Merr.), caused by Cercospora sojina Hara, was first detected during the 1997-98 growing season at low incidence and severity (<1% of the leaf diseased) levels in the provinces of Tucumán, Salta, Jujuy, Catamarca, and Santiago del Estero in northwestern Argentina. During the 1998-1999 growing season, disease incidence increased and disease severity grew to 10% of the leaf surface diseased on highly susceptible cultivars in a few locations. An outbreak of frogeye leaf spot occurred throughout northwestern Argentina during the 1999-2000 growing season. Frogeye leaf spot was severe on susceptible cultivars in the provinces of Salta, Santiago del Estero and Catamarca with the greatest intensity in the northeastern part of the Province of Tucumán. Symptoms on leaves were circular lesions that ranged in size from 1 to 5 mm, were reddish-brown to gray or tan, and were bordered by a narrow, reddish-brown to purple margin. Conidiophores and conidia of C. sojina developed on the abaxial leaf surface (1,2). Severely diseased leaves were desiccated and dropped during the R6 stage of growth. Lesions also developed on stems, pods, and seeds. Field surveys indicated that this disease reduced the yields of the highly susceptible cultivars Anta 82 RR, Coker 6738, and A 6445 RG by 48, 34, and 25%, respectively. C. sojina was cultured from diseased tissue on PDA acidified with 0.2% lactic acid and maintained on V-8 juice agar amended with streptomycin sulfate (100 mg/l). Conidia were elongated, dark, 38 to 62 × 5 to 9 μm, with 2 to 6 septa, and borne on dark conidiophores with 1 to 4 septa. Pathogenicity tests were conducted on seedlings of the susceptible cultivars A 6445 RG and Coker 6738 and on the resistant cultivars A 8000 RG and Shulka. Seedlings were inoculated at the V3 growth stage by spraying the leaves with a conidial suspension (4 × 10⁴ conidia/ml) using a hand-held atomizer. Control plants were sprayed with sterile distilled water. Plants were placed in a moist chamber at 26°C for 2 days and then transferred to a greenhouse bench where they were kept at 25 to 30°C. Symptoms identical to those observed in the field became visible after 7 to 10 days. Ratings were made 14 days after inoculation by estimating the percentage of leaf area affected using a standard area diagram. Lesions covered 60 to 65% of the leaf area of susceptible cultivars, but less than 2% on resistant cultivars. Control plants remained healthy. C. sojina was reisolated from lesions on leaves of susceptible plants. Above-average rainfall and high relative humidity in northwestern Argentina during the first three months of 2000 may have encouraged the severe outbreak of frogeye leaf spot of soybean. The outbreak was aggravated by the widespread use of notillage systems in the region and the large hectarage planted with susceptible cultivars. References: (1) S.G. Lehman J. Agric. Res. 36:811-833, 1928. (2) D. V. Philips and J. T. Yorinori. 1989. Frogeye leaf spot. Pages 19-21 in: Compendium of Soybean Diseases, 3rd ed. APS Press, St. Paul, MN.

First Report of Colletotrichum acutatum on Strawberry in Northwestern Argentina

Isolates were obtained from strawberry tissue with anthracnose symptoms from several locations near Tucumán, Argentina. Isolates were characterized using several criteria. Isolates produced fusiform conidia, tapered to a point at both ends, and averaged 13.5 × 4.9 μm. On potato dextrose agar, colonies produced a white cottony mycelial colony that turned orange in older cultures. Compared with Colletotrichum fragariae, the new isolates produced fewer appressoria. Pathogenicity tests were conducted on detached leaves and plants in the greenhouse and field. Detached immature leaves of cvs. Chandler, Fern, and Sweet Charlie were inoculated with a 20-μl droplet of an aqueous conidial suspension (10⁶ conidia per ml) placed on the adaxial surface. Control leaves were inoculated with sterile distilled water. Leaves were maintained under white light (2,000 lux, 12 h/day) at 26°C, and 100% relative humidity. Necrotic spots were visible 4 days after inoculation. Greenhouse and field plants were spray-inoculated and covered for 48 h. Disease symptoms were mainly observed on petioles and runners 9 days after inoculation. No lesions were observed on control detached leaves or plants. Koch's postulates were confirmed in all cases. Based on morphological and cultural characteristics, isolates were identified as C. acutatum Simmonds (1). This is the first report of C. acutatum causing strawberry anthracnose in northwestern Argentina. Reference: (1) B. Smith and L. L. Black. Plant Dis. 74:69, 1990.

Soybean Disease Loss Estimates for the Top 10 Soybean Producing Countries in 1994

Soybean disease loss estimates were compiled for the 1994 harvested crop from the 10 countries with the greatest soybean production. The objective was to document the major soybean disease problems in these countries and any recent changes in the severity of individual soybean diseases. Total yield losses caused by Heterodera glycines in these 10 countries were greater than those caused by any other disease. Next in order of importance were stem canker, brown spot, and charcoal rot. The total yield loss due to disease during 1994 in these countries was 14.99 million metric tons, valued at $3.31 billion. Methods used to estimate soybean disease losses were field surveys, plant disease diagnostic clinic samples, variety trial data, information from field workers and university extension staff, research plots, grower demonstrations, and private crop consultant reports. Yield loss estimates due to a particular disease varied by country. For example, yield losses due to rust were reported from China and Indonesia, but no losses due to this disease were reported from any of the remaining eight countries. Soybean disease control research and extension efforts are needed to provide more effective preventive and therapeutic disease management strategies and systems to producers.