Conyza bonariensis as an alternative host for Colletotrichum species in Argentina.

AIMS: This study investigated the diversity of Colletotrichum isolates recovered from Conyza bonariensis leaves through the use of morphological characteristics, growth rate, carbon sources utilization and phylogenetic analysis. METHODS AND RESULTS: In all, 30 Colletotrichum isolates recovered from C. bonariensis leaves showing symptoms of disease were included in the present study. Based on the analysis of morphology and sequences, the isolates were distributed into six Colletotrichum species complexes. The concatenated alignment of GAPDH and ITS sequences showed that 20 out of 30 isolates were included in four species complexes which comprise the most important pathogens causing anthracnose in soybean or anthracnose and stalk rot in maize: C. truncatum, C. orchidearum, C. gloeosporioides and C. graminicola. The remaining 10 isolates were included in the C. boninense and C. destructivum species complexes or could not be assigned to any complex with the available information. CONCLUSION: Weeds belonging to genus Conyza are host to soybean and maize potential pathogenic species of Colletotrichum and could have a role as inoculum reservoir for cross contamination in the agroecosystem. SIGNIFICANCE AND IMPACT OF THE STUDY: The combined use of morphological, kinetics and physiological parameters of growth and phylogenetic analysis in Colletotrichum isolates from Conyza leaves allowed the detection of species complexes previously not identified in Argentina.

First Molecular Detection of Ramularia Leaf Spot (Ramularia collo-cygni) in Seeds and Leaves of Barley in Argentina.

The fungus Ramularia collo-cygni B. Sutton & J. M. Waller (Rcc) was identified as the causal agent of this emerging disease on barley (Hordeum vulgare L.) based on symptoms and signs on leaves and attributes of the fungus. The common name given to the disease was "necrotic sprinkling." This disease was found for the first time on barley in fields of Buenos Aires Province, Argentina, in 2001, with severities ranging from 60 to 100% (2). During the spring of 2012, the disease spread throughout most barley growing areas of the Pampean region, affecting almost all varieties of barley in the Buenos Aires, Entre Ríos, and Santa Fe provinces. The disease showed typical symptoms of small, brown spots on leaves, sheaths, and awns, and caused rapid loss of green leaf area and significant economic damage. The diagnosis of this disease is difficult by conventional techniques and has caused some confusion. In order to obtain appropriate information about the incidence of this pathogen in the most important barley growing region of Argentina, as well as to confirm its presence on seed, 39 seed samples containing 200 seeds each and eight leaf samples were analyzed using a real-time PCR diagnostic test (4). Thirty-five of the 37 seed samples had Rcc DNA levels above the minimum detection level (0.13 pg DNA). Ramularia has been described as a seedborne fungus previously (1) and seed infection could hasten the spread of the fungus to new geographically distinct areas. Moreover, leaf samples were microscopically assessed for conidophore presence and then ground for DNA extraction. Rcc DNA was detected by PCR in all eight leaf samples. To our knowledge, this is the first report about molecular detection of R. collo-cygni in barley seed of harvested samples from Argentina. Seeds infected/infested may also lead to new strains of the fungus arriving in barley fields and provide a source of inoculum for future epidemics (3). References: (1) N. D. Havis et al. FEMS Microbiol. Lett. 256:217, 2006. (2) M. Khier et al. Salpicado necrótico, nueva enfermedad de la cebada en Argentina causada por Ramularia collo-cygni. Page 47 in: Resúmenes XI Jornadas Fitosanitarias Argentinas, 26-29 June 2002, Río Cuarto, Córdoba, 2002. (3) P. Matusinsky et al. J. Plant Pathol. 3:679, 2011. (4) J. M. G. Taylor et al. Lett. Appl. Microbiol. 50:493, 2010.

First Report of Ascochyta rabiei Causing Ascochyta Blight of Chickpea in Argentina.

In November 2011, lesions similar to those reported for Ascochyta blight (1) were observed on Cicer arietinum L. (chickpea) plants growing in three commercial fields located at Río Primero and Río Segundo (Cordoba Province) and Lobería (Buenos Aires Province), Argentina. Disease incidence (percentage of plants affected) was 100% in all fields surveyed. Plants showed leaves, petioles, stems, and pods with brown lesions. Symptoms on leaves and pods were circular to oval (2 to 14 mm) while in the stems the lesions were elongated (2 to 30 mm). Seeds appeared small and shriveled with brown discoloration. Morphology of the fungi was examined on infected tissues. Numerous black pycnidia measuring 94.6 to 217.9 µm (145.9 ± 28.8 µm), arranged in concentric rings, were observed within of all the lesions. Conidia were predominantly aseptate, straight, hyaline with blunt ends, and measured 9.3 to 12.9 (11.3 ± 1.12) × 3.3 to 5.0 µm (4.2 ± 0.51). Morphological characteristics of the pathogen were similar to those described for Ascochyta rabiei (Pass.) Labrousse (teleomorph Didymella rabiei (Kovacheski) v. Arx (= Mycosphaerella rabiei Kovacheski)) (2). Fungus from infected leaf tissues was isolated on potato dextrose agar. Pathogenicity tests were conducted on seedlings of the susceptible cultivar by spraying leaves of each of 100 seedling plants with 10 ml of a conidial suspension (2 × 104 conidia/ml) of the isolated pathogen with a handheld atomizer. Plants were covered with plastic bags and placed in a growing chamber at 20 to 25°C for 3 days. The plastic bags were removed and the plants were maintained in high humidity at the same temperature. Noninoculated plants were used as controls. After 5 days, all inoculated plants showed typical symptoms. Foliar and stem lesions symptoms were similar to those originally observed in the field. Control plants remained healthy. Koch's postulates were fulfilled by isolating A. rabiei from inoculated plants. The colonies and the morphology of conidia were the same as those of the original isolates. To our knowledge, this is the first report of A. rabiei infecting chickpeas in Argentina. The outbreak of Ascochyta blight in Argentina is of concern because of its severity and the possibility that the pathogen was introduced on seed. This report underscores the need for further research on effective management programs for Ascochyta blight. References: (1) B. Bayaa and W. Chen. Compendium of Chickpea and Lentil Diseases and Pests The American Phytopathological Society, St. Paul, MN, 2011. (2) E. Punithalingam and P. Holliday. Page 337 in: CMI Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1972.

Overwinter and Survival of Asian Soybean Rust Caused by Phakopsora pachyrhizi in Volunteer Soybean Plants in Entre Ríos Province, Argentina.

Asian soybean rust (ASR) caused by Phakopsora pachyrhizi was initially reported in the Province of Entre Ríos, Argentina in April 2004 (1). During the 2004-2005 growing season, ASR was observed in the main soybean-producing (Glycine max) region of the country (4) and it was observed on kudzu (Pueraria lobata) in Misiones and Santa Fe provinces (2). Of the environmental factors affecting rust survival over the winter, temperature is probably the most important one with no germination occurring below 7°C (3). The objectives of this study were to analyze the subfreezing daily air temperatures in the presence of new erumpent uredinia and the germination of P. pachyrhizi urediniospores. Ten sites with volunteer plants close to the meteorological station were found in the Paraná Research Station (31°51'S, 60°31'W). Weekly, from June 2004 through December 2005, sites were randomly sampled for volunteer plants (n = 15). The presence of the ASR was confirmed with a polymerase chain reaction (PCR) assay by SINAVIMO (4). The ASR incidence (ASRI) and erumpent uredinia incidence (EUI) was estimated as the proportion of affected plants. Uredinia were classified as: new erumpent with colorless spores; mature cinnamon with pale cinnamon-brown spores; and dead, empty, and dark without spores. The disease density was estimated as an average of the diseased leaflets according to the following scale: light (number of lesions 1 to 100), moderate (101 to 500), and heavy (>500). The ability of urediniospores from the erumpent pustules to germinate was tested during July 2004 and September 2005 on 1.5% of water agar and kept at 25 ± 2°C for 2 days. The subfreezing daily air (0.05 m height) temperature was registered. During the complete evaluation period, surviving plants from the Vc to R6 stages were observed. However, plants with ASR were only observed from June to July 2004 and May to September 2005. Locally, first planting dates begin in October. New uredinia were observed close to mature and dead uredinia on unifoliate and trifoliate leaflets, and petiols and stems in plants from the V2 to R5 stages. There were 13 days with below freezing temperatures from 1 June to 31 July 2004 (-0.1 to -7.4°C), and ASRI and EUI was 100%. The ASR mean density was light. The coldest temperature was -7.4°C on 11 July 2004, and thereafter, no uredinia were observed until the next growing season. From 25 April to 16 September 2005 there were 20 days with below freezing temperatures (-0.1 to -4.9°C). The ASRI and the EUI were 92.3 (76.9), 75.0 (58.3), 59.1 (32.6), 50.0 (40.9), and 36.7 (23.3)% in May, June, July, August, and September, respectively. The incidence of plants with a moderate to heavy disease level was 50.0, 41.7, 28.6, 29.5, and 10% respectively. Germination rate of urediniospores collected in July 2004 was 11% and 28% in September 2005. Low temperatures do not seem to be a limiting factor for the survival of P. pachyrhizi, and urediniospores could survive on volunteer plants until new soybean plants grow. Since another host is rare or absent in the region, volunteer soybean plants may provide a reservoir of inoculum for the next season. References: (1) A. N. Formento. Roya de la soja en Entre Ríos. INTA-EEA Paraná. On-line publication. INTA, 2004. (2) A. N. Formento and J. de Souza. INTA-EEA Paraná. Serie Extensión No. 32, 2004. (3) M. Marchetti et al. Phytopathology 66:461, 1975. (4) SINAVIMO. Sistema Nacional Argentino de Vigilancia y Monitoreo de Plagas, Roya de la soja. On-line publication. SENASA, 2004.