In October 2018, soybean plants displaying elongated black to reddish-brown lesions on stems were observed in a field planted to the cv. BRS Serena in the locality of Puerto López (Meta, Colombia), with 20% incidence of diseased plants. Symptomatic stems were collected from five plants, and small pieces (∼5 mm2) were surface sterilized, plated on potato dextrose agar (PDA) and incubated for 2 weeks at 25°C in darkness. Three fungal isolates with similar morphology were obtained, i.e., by subculturing single hyphal tips, and their colonies on PDA were grayish-white, fluffy, with aerial mycelium, dark colored substrate mycelium, and produced circular black stroma. Pycnidia were globose, black, occurred as clusters, embedded in tissue, erumpent at maturity, with an elongated neck, and often had yellowish conidial cirrus extruding from the ostiole. Alpha conidia were observed for all isolates after 30 days growth on sterile soybean stem pieces (5 cm) on water agar, under 25ºC and 12 h light/12h darkness photoperiod. Alpha conidia (n = 50) measured 6.0 – 7.0 µm (6.4 ± 0.4 µm) × 2.0 – 3.0 µm (2.5± 0.4 µm), were aseptate, hyaline, smooth, ellipsoidal, often biguttulate, with subtruncate base. Beta conidia were not observed. Observed morphological characteristics of these isolates were similar to those reported in Diaporthe spp. by Udayanga et al. (2015). DNA from each fungal isolate was used to sequence the internal transcribed spacer region (ITS), and the translation elongation factor 1-α (TEF1) gene, using the primer pairs ITS5/ITS4 (White et al. 1990) and EF1-728F/EF1- 986R (Carbone & Kohn, 1999), respectively. Results from an NCBI-BLASTn, revealed that the ITS sequences of the three isolates (GenBank accessions MW566593 to MW566595) had 98% (581/584 bp) identity with D. miriciae strain BRIP 54736j (NR_147535.1), whereas the TEF1 sequences (GenBank accessions MW597410 to MW597412) had 97 to 100% (330-339/339 bp) identity with D. ueckerae strain FAU656 (KJ590747). The species Diaporthe miriciae R.G. Shivas, S.M. Thomps. & Y.P. Tan, and Diaporthe ueckerae Udayanga & Castl. are synonymous, with the latter taking the nomenclature priority (Gao et al. 2016). According to a multilocus phylogenetic analysis, by maximum likelihood, the three isolates clustered together in a clade with reference type strains of D. ueckerae (Udayanga et al. 2015). Soybean plants cv. BRS Serena (growth stages V3 to V4) were used to verify the pathogenicity of each isolate using a toothpick inoculation method (Mena et al. 2020). A single toothpick colonized by D. ueckerae was inserted directly into the stem of each plant (10 plants per isolate) approximately 1 cm below the first trifoliate node. Noncolonized sterile toothpicks, inserted in 10 soybean plants served as the non-inoculated control. Plants were arbitrarily distributed inside a glasshouse, and incubated at high relative humidity (>90% HR). After 15 days, inoculated plants showed elongated reddish-brown necrosis at the inoculated sites, that were similar to symptoms observed in the field. Non-inoculated control plants were asymptomatic. Fungal cultures recovered from symptomatic stems were morphologically identical to the original isolates. This is the first report of soybean stem canker caused by D. ueckerae in Colombia. Due to the economic importance of this disease elsewhere (Backman et al. 1985; Mena et al. 2020), further research on disease management strategies to mitigate potential crop losses is warranted.
La antracnosis (Colletotrichum acutatum) reduce considerablemente el rendimiento de los cítricos. Los síntomas de la enfermedad se caracterizan por necrosis de los pétalos, caída prematura de los frutos y formación de cálices persistentes. Con el fin de determinar el efecto del patrón y de condiciones ambientales como la precipitación, la humedad relativa y la temperatura en el desarrollo de la antracnosis, se seleccionaron al azar plantas de lima ácida Tahití injertadas en seis portainjertos: Citrumelo, Sunky x English, Carrizo, Volkameriano, Kryder 15-3 y Cleopatra, establecidos en un huerto experimental en el Centro de Investigación La Libertad de la Corporación Colombiana Agropecuaria AGROSAVIA en Villavicencio, Meta. El experimento se llevó a cabo durante tres estaciones de floración, cada una con condiciones climáticas diferentes. Se evaluó la incidencia de la enfermedad y el porcentaje de cálices persistentes. Los resultados no evidenciaron diferencias significativas en la incidencia de la enfermedad y el número de cálices persistentes entre los patrones evaluados. Sin embargo, el efecto en la caída de los frutos, expresado en cálices persistentes, dependió de las condiciones climáticas que favorecieron el desarrollo del patógeno, siendo los meses lluviosos, la humedad relativa elevada y las temperaturas bajas factores predisponentes de la pérdida considerable de frutos a causa de la enfermedad en la lima ácida Tahití cultivada en el piedemonte llanero de Colombia.
Phytophthora sojae Kaufm. & Gerd., causante de la pudrición de raíces y tallo de la soja (Glycine max L.), se considera uno de los patógenos más limitantes del cultivo en Colombia. En este estudio se compararon tres métodos de inoculación de P. sojae basadas en micelio y oosporas (200-400 UFC mL-1) como fuente de inóculo, con el objetivo de validar un método de inoculación eficiente para selección masiva de germoplasma de soja por su respuesta a la inoculación del patógeno en condiciones de casa de malla. Se evaluaron 12 tratamientos en un diseño de bloques completos al azar con arreglo factorial (3 × 2 × 2): tres métodos de inoculación (inyección, palillos y discos de agar), dos variantes de inoculación (con y sin inóculo) y dos variedades (Williams y Corpoica Orinoquía 3), con tres repeticiones. Cada unidad experimental estuvo constituida por 10 a 15 plántulas. Considerando las variables analizadas en este estudio (período de incubación en días, porcentaje de mortalidad y tiempo invertido en minutos (min) para la inoculación de 10 plántulas), se observó que los tratamientos basados en palillos colonizados con P. sojae causaron 96.67 % de mortalidad de plántulas en un período de incubación de 8.08 días en promedio; presentaron además la mayor eficiencia en la inoculación de germoplasma de soja a gran escala, caracterizándose por su practicidad durante el proceso de inoculación. Los resultados permitieron la validación del método de palillos colonizados y su recomendación para ser aplicado en el programa de mejoramiento genético de soja de AGROSAVIA en Colombia.
La roya de la soya se reportó en Colombia por primera vez en 2004 y se considera la enfermedad más limitante para el cultivo. Dos especies de hongos del género Phakopsora son los agentes causales de la enfermedad. La roya asiática se atribuye a P. pachyrhizi y la roya americana, a P. meibomiae. En cultivos de soya de la Orinoquía colombiana, la especie predominante es P. pachyrhizi. Los daños severos de la enfermedad involucran la defoliación precoz de las plantas y generan pérdidas de rendimiento hasta de 90% dependiendo de las condiciones ambientales.
Soybean crops grown in the plains of eastern Colombia have been affected by the incidence of rust. This disease, still officially regarded as American soybean rust (caused by Phakopsora meibomiae) by the Colombian Department of Agriculture (ICA), causes serious damage to soybean growing areas. Symptoms of rust, such as reddish-brown lesions, have been observed since 2004 in the upper half of plants during vegetative stages. In 2005 and 2018, infected leaf tissues and uredinospores were collected from an experimental area and from commercial soybean fields. Once morphological identification and Koch’s postulates confirmed the presence of Phakopsora spp., molecular characterization was performed to identify the pathogen associated with the disease to species level. This was based on standard procedures using internal transcribed spacer (ITS) regions of rDNA and PCR amplification with specific primers for Phakopsora meibomiae and P. pachyrhizi. The obtained sequences were BLASTed against GenBank/NCBI data bank. Results indicated that P. pachyrhizi Sydow is in fact the causative agent of soybean rust in Colombia, considering the samples collected in 2005 and 2018. The ITS-rDNA sequences of P. pachyrhizi were deposited at GenBank under the accession numbers MK933723 to MK933731. This finding was reported to ICA, so that they could officially update the phytosanitary status of this soybean pathogen in Colombia.
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