Sensibilidad y resistencia a fungicidas de Cercospora kikuchii, agente causal del tizón de la hoja y mancha púrpura de la semilla de la soja

En Argentina, diferentes especies de Cercospora causan el tizón de la hoja (TH) y la mancha púrpura de la semilla (MPS) en soja. Debido a que no hay variedades de soja resistentes disponibles en el mercado, el manejo de TH/MPS depende en gran medida de aplicaciones de fungicidas. En este estudio se determinó la sensibilidad de 62 cepas de Cercospora argentinas a fungicidas inhibidores de la desmetilación (DMI), carbamato de metilbenzimidazol (MBC), inhibidores de la quinona externa (QoI), inhibidores de la succinato deshidrogenasa (SDHI) y mancozeb. Todos los aislados fueron sensibles a difenoconazole, epoxiconazole, protioconazole, tebuconazole y cyproconazole (los valores de EC50 variaron de 0,006 a 2,4 µg mL-1). Por el contrario, la mitad (51%) de los aislados fueron sensibles (los valores de EC50 variaron de 0,003 a 0,2 µg mL-1) y la mitad (49%) altamente resistentes (EC50> 100 µg mL-1) a carbendazim. Todos los aislados fueron completamente resistentes a azoxystrobina, trifloxystrobina, pyraclostrobina e insensibles a boscalid, fluxapyroxad y pydiflumetofen (EC50 > 100 µg mL-1). La mutación G143A se detectó en el 82% de los aislados resistentes a QoI y la mutación E198A en el 97% de los aislados resistentes a carbendazim. No se detectaron mutaciones de resistencia aparentes en los genes que codifican para las subunidades B, C y D de la succinato deshidrogenasa. Mancozeb inhibió completamente el crecimiento micelial de los aislados evaluados a 100 µg mL-1. Todos los aislados argentinos de Cercospora fueron sensibles a los fungicidas DMI evaluados, pero se reporta por primera vez resistencia a los fungicidas QoI y MBC. Otro mecanismo(s) además de la modificación del sitio objeto de control del fungicida podrían ser responsable(s) de la resistencia de Cercospora a los fungicidas QoI y MBC. Además, debido a la reciente introducción de fungicidas SDHI en soja en Argentina, se especula que las especies de Cercospora que causan TH/MPS son insensibles (naturalmente resistentes) a este grupo de fungicidas. La insensibilidad debe confirmarse en condiciones de campo.

Diagrammatic scale for assessment the severity of Cercospora leaf blight on soybean (Glycine max) leaflets

Introduction. Cercospora leaf blight (CLB) caused by Cercospora kikuchii is an endemic soybean crop disease in Argentina. Objective. To develop and validate a scale to quantify the severity of CLB in leaflets of soybean (Glycine max) for the evaluation of foliar fungicides and cultivar trials. Materials and methods. A diagrammatic scale to assess the severity of CLB on soybean leaflets (Glycine max) was developed. The scale was based on the evaluation of 50 leaflets. The different severity levels of the leaflets were determined according to Weber-Fechner’s stimulus-response law. The proposed scale included severity levels of 1, 4.5, 17.5, 50, 82.2, 95, and 99 %. The validation of the scale was performed by 23 raters (3 experienced and 20 inexperienced) who assessed the severity of CLB on 50 leaflets with and without the use of the scale. Results. The results from the three experienced raters were more precise than those obtained by the inexperienced raters. The scale improved the visual interpretation to assess the severity of CLB for the inexperienced raters, who obtained estimated severity values close to the actual severity values. When the inexperienced raters used the proposed scale to estimate the severity of CLB on leaflets, they were accurate having determination coefficients (R2) between 0.74 and 0.91. According to the absolute errors, the greatest deviation was observed between -2.49 and 2.46. However, these deviations were not significantly different from the population average. To speed the process of cultivar evaluations we also propose the use of scales based on grades. Conclusions. The proposed scale to assess the severity of CLB on leaflets can be used for the assessment of foliar fungicide trials and soybean cultivars. The scale showed specificity for assessing the severity of Cercospora leaf blight (CFB) in soybean leaflets.

Emergência de sementes de soja com diferentes porcentagens de infestação de mancha púrpura

A soja apresenta susceptibilidade às diversas doenças em campo, como é o caso da mancha púrpura em sementes causada pelo fungo Cercospora Kikuchii, sendo responsável por severas reduções no rendimento e na qualidade de sementes. Objetivou-se com este trabalho avaliar a emergência e o vigor de sementes de soja (RK7813) em lotes com diferentes porcentagens de infestação de mancha púrpura. Foi utilizado delineamento em blocos casualizados com cinco tratamentos (0%, 25%, 50%, 75% e 100% de mancha púrpura nas sementes) e quatro repetições. As variáveis estudadas foram: a porcentagem de emergência (%E), a velocidade de emergência (VE), o índice de velocidade de emergência (IVE), a matéria fresca (MV) e a matéria seca (MS) das plântulas. As sementes de soja não sofreram prejuízos, com relação à germinação e acúmulo de matéria fresca e seca, com a infestação pelo Cercospora kikuchii.