Identificación morfológica y molecular de los hongos Kabatiella zeae y Exserohilum turcicum, patógenos de maíz (Zea mays). Caracterización de las estrategias patogénicas y de sobrevivencia como un aporte al conocimiento de sus ciclos biológicos
Fecha
2018
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Resumen
En Argentina, el desarrollo y adopción de nuevas tecnologías productivas agrícolas, como la
incorporación masiva de sistemas de labranza conservacionista, son factores fundamentales
en la aparición de nuevas enfermedades que afectan a los cultivos extensivos, entre ellos a
maíz (Zea mays L.). Estos sistemas permiten la presencia de rastrojo en superficie y de
plantas voluntarias, los que en general son sustratos de sobrevivencia de patógenos
necrotróficos. Por otro lado, las siembras tardías de maíz estabilizan el rendimiento, reducen
los costos de producción, sin embargo las condiciones climáticas favorecen la emergencia y
re-emergencia de enfermedades, las cuales causan pérdidas relevantes que pueden afectar
la economía y seguridad alimentaria de un país. El estudio de aspectos morfológicos,
genéticos y epidemiológicos de Kabatiella zeae y Exserohilum turcicum, hongos causales de
la mancha ocular (MO) y tizón foliar (TF) del maíz respectivamente, permitirán conocer
aspectos poco estudiados de ambos ciclos biológicos como una condición básica para el
manejo integrado de ambas enfermedades. Las giras de prospección de enfermedades por
la región núcleo maicera (Córdoba, Buenos Aires y Santa Fe) y la provincia de Entre Ríos
confirmaron la presencia de las dos enfermedades, con distinta prevalencia e intensidad. La
MO (K. zeae) resultó una enfermedad de frecuencia relativa y severidad leve a moderada;
en siembras de primera y tardías. Por el contrario, el TF (E. turcicum) se caracterizó por ser
una enfermedad prevalente y de incidencia alta en lotes de producción e híbridos más
sembrados; la severidad alcanzó valores altos en híbridos susceptibles. En la mayoría de los
ciclos agrícolas, las siembras tardías de diciembre y enero fueron favorables para la
ocurrencia de ambas enfermedades foliares, principalmente del TF. La descripción de los
síntomas y signos de K. zeae en maíz y de E. turcicum en maíz, sorgo (Sorghum bicolor) y
sorgo de Alepo (S. halepense) contribuyeron a un conocimiento detallado de la MO y TF en
infecciones naturales de campo. Los ensayos comparativos de rendimientos (ECR) de maíz
que determinan la estabilidad agronómica de los híbridos en diferentes ambientes de las
regiones maiceras, permitieron diferenciar el comportamiento de los materiales frente a las
dos enfermedades. Se registraron híbridos susceptibles a MO y resistentes a TF y
viceversa, así como susceptibles o resistentes a ambas enfermedades. Además, la
cuantificación de ambas enfermedades con escalas diagramáticas específicas bajo un
protocolo de evaluación que consideró un número mínimo de surcos y de plantas, con
evaluaciones en la hoja de la espiga (HE), hoja inmediata inferior (HE-1) y hoja inmediata
superior (HE+1), fue una instancia superadora a la simple estimación visual. Por otro lado, la
utilización de diversos métodos experimentales permitió aislar, multiplicar y conservar K.
zeae y E. turcicum para su posterior utilización en los estudios de variabilidad morfológica y
genética, pruebas de patogenicidad y mantenimiento de aislados de diversos orígenes
geográficos por más de 600 días. El hongo K. zeae a diferencia de E. turcicum, mostró una
marcada variabilidad cultural, y ambos, una gran diversidad genética. Estos patógenos no se
observaron en semillas por lo cual es necesario continuar con otro tipo de estudios
específicos y hasta el presente no constituyen una estrategia preferencial de sobrevivencia.
Sin embargo, se comprobó y confirmó que el rastrojo, las plantas voluntarias de maíz y el
sorgo de Alepo constituyen una fuente de inóculo primario. La contribución al conocimiento
del ciclo biológico de ambos patógenos es de gran importancia epidemiológica y permitirá
delinear técnicas de manejo con un abordaje integral a nivel de agroecosistema.
The development and adoption of new technologies in agricultural production in Argentina, like the massive incorporation of conservation tillage systems, are key factors in the emergence of new diseases that affect crops, such as corn (Zea mays L.). These systems allow the presence of stubble on the surface and volunteer plants, usual substrates for the survival of necrotrophic pathogens. Late sowings of corn stabilize yield, reduce production costs, but the climatic conditions favor the emergence of diseases which cause losses and may affect the economy of a country and its food security. Study of morphological, genetic and epidemiological aspects of the fungi Kabatiella zeae and Exserohilum turcicum, causal agents of eyespot and northern corn leaf blight (NCLB), respectively, will contribute to the knowledge of the biological cycles of both pathogens, necessary information for their integrated management. Disease surveys in the main corn production area (Córdoba, Buenos Aires y Santa Fe) and in Entre Ríos province confirmed the presence of both diseases with different prevalence and intensity. Eyespot was a disease of relative frequency and low to moderate severity; in contrast, NCLB showed high prevalence and incidence in production fields and most planted hybrids, respectively; severity reached high levels in susceptible hybrids. In most crop seasons, late plantings (December or January) were highly favorable for both foliar diseases, mainly for NCLB. Description of symptoms and signs of K. zeae in corn and E. turcicum in corn, sorghum (Sorghum bicolor) and johnsongrass (S. halepense) contributed to a more detailed knowledge of both diseases in field under natural infections. Comparative trials yields allowed not only to establish the stability of the hybrids in different environments, but also to differentiate responses to diseases. Some were susceptible to eyespot and resistant to NCLB and viceversa, or susceptible or resistant to both diseases. In addition, quantification of both diseases with specific diagrammatic scales under an assessment protocol which takes into account a minimum number of rows and plants as the assessment ear leaf, immediate lower and upper ear leaf, represented an improvement to using just a simple visual estimation. Different experimental methods do it possible to isolate, increase and preserve K. zeae and E. turcicum for later use of morphological and genetic variability studies, pathogenicity tests and maintenance of isolates from diverse geographic origins for more than 600 days. K. zeae, unlike E. turcicum, showed high levels of cultural variability, and both, high levels of diversity. Both fungi were not observed in the seeds for which we must continue the specific studies and they would not constitute a preferred strategy of survival. However, it was confirmed that stubble, volunteer plants of corn, and jhonsongrass plants constitute the source of primary inoculum. The contribution to the knowledge of the biological cycle of both pathogens is of great epidemiological importance and allows to define management techniques with a comprehensive approach at the agroecosystem level.
The development and adoption of new technologies in agricultural production in Argentina, like the massive incorporation of conservation tillage systems, are key factors in the emergence of new diseases that affect crops, such as corn (Zea mays L.). These systems allow the presence of stubble on the surface and volunteer plants, usual substrates for the survival of necrotrophic pathogens. Late sowings of corn stabilize yield, reduce production costs, but the climatic conditions favor the emergence of diseases which cause losses and may affect the economy of a country and its food security. Study of morphological, genetic and epidemiological aspects of the fungi Kabatiella zeae and Exserohilum turcicum, causal agents of eyespot and northern corn leaf blight (NCLB), respectively, will contribute to the knowledge of the biological cycles of both pathogens, necessary information for their integrated management. Disease surveys in the main corn production area (Córdoba, Buenos Aires y Santa Fe) and in Entre Ríos province confirmed the presence of both diseases with different prevalence and intensity. Eyespot was a disease of relative frequency and low to moderate severity; in contrast, NCLB showed high prevalence and incidence in production fields and most planted hybrids, respectively; severity reached high levels in susceptible hybrids. In most crop seasons, late plantings (December or January) were highly favorable for both foliar diseases, mainly for NCLB. Description of symptoms and signs of K. zeae in corn and E. turcicum in corn, sorghum (Sorghum bicolor) and johnsongrass (S. halepense) contributed to a more detailed knowledge of both diseases in field under natural infections. Comparative trials yields allowed not only to establish the stability of the hybrids in different environments, but also to differentiate responses to diseases. Some were susceptible to eyespot and resistant to NCLB and viceversa, or susceptible or resistant to both diseases. In addition, quantification of both diseases with specific diagrammatic scales under an assessment protocol which takes into account a minimum number of rows and plants as the assessment ear leaf, immediate lower and upper ear leaf, represented an improvement to using just a simple visual estimation. Different experimental methods do it possible to isolate, increase and preserve K. zeae and E. turcicum for later use of morphological and genetic variability studies, pathogenicity tests and maintenance of isolates from diverse geographic origins for more than 600 days. K. zeae, unlike E. turcicum, showed high levels of cultural variability, and both, high levels of diversity. Both fungi were not observed in the seeds for which we must continue the specific studies and they would not constitute a preferred strategy of survival. However, it was confirmed that stubble, volunteer plants of corn, and jhonsongrass plants constitute the source of primary inoculum. The contribution to the knowledge of the biological cycle of both pathogens is of great epidemiological importance and allows to define management techniques with a comprehensive approach at the agroecosystem level.
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Palabras clave
Hongos, Kabatiella zeae, Setosphaeria turcica, Exserohilum turcicum, Maíz, Patogenicidad