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In August and September 2014, different avocado orchards (Persea americana Mill. Var. Hass) were sampled in four municipalities (Tancítaro, Uruapan, San Juan Nuevo Parangaricutiro and Peribán) of the “Franja aguacatera” in Michoacan State, Mexico. Samples were collected from avocado trees showing symptoms of the “Tristeza del aguacate” disease. Strains of Pythium sp. were obtained consistently from symptomatic tissue isolated in the selective medium V8®-Agar-PARPH. Morphological identification was made through taxonomic keys, and molecular identification was obtained by sequencing Internal Transcribed Spacer (ITS) gene. Koch's postulates were corroborated by pathogenicity tests carried out on P. americana seedlings with six true leaves, identifying the strain of P. sp. amazonianum as the causal agent of the “Tristeza del aguacate” disease. This is the first report that associates the “Tristeza del aguacate” with P. sp. amazonianum in Michoacan, Mexico. Resumen En los meses de agosto y septiembre del 2014 se muestrearon diferentes huertas de aguacate (Persea americana Mill. Var. Hass) en cuatro municipios (Tancítaro, Uruapan, San Juan Nuevo Parangaricutiro y Peribán) de la “Franja aguacatera” en el estado de Michoacán, México. Las muestras se recolectaron de árboles de aguacate que mostraban síntomas de la enfermedad “Tristeza del aguacate”. Se obtuvieron cepas de Pythium sp. consistentemente de tejido sintomático, aisladas en medio selectivo V8®-Agar-PARPH. La identificación morfológica se realizó mediante claves taxonómicas y la identificación molecular se obtuvo a través de ITS-PCR. Los postulados de Koch se corroboraron mediante pruebas de patogenicidad llevadas a cabo en plántulas de P. americana con seis hojas verdaderas, identificando la cepa de P. sp. amazonianum como agente causal de la “Tristeza del aguacate”. Este es el primer reporte que asocia la “Tristeza del aguacate” con P. sp. amazonianum en Michoacán, México.

In August and September 2014, different avocado orchards (Persea americana Mill. Var. Hass) were sampled in four municipalities (Tancítaro, Uruapan, San Juan Nuevo Parangaricutiro and Peribán) of the “Franja aguacatera” in Michoacan State, Mexico. Samples were collected from avocado trees showing symptoms of the “Tristeza del aguacate” disease. Strains of Pythium sp. were obtained consistently from symptomatic tissue isolated in the selective medium V8®-Agar-PARPH. Morphological identification was made through taxonomic keys, and molecular identification was obtained by sequencing Internal Transcribed Spacer (ITS) gene. Koch's postulates were corroborated by pathogenicity tests carried out on P. americana seedlings with six true leaves, identifying the strain of P. sp. amazonianum as the causal agent of the “Tristeza del aguacate” disease. This is the first report that associates the “Tristeza del aguacate” with P. sp. amazonianum in Michoacan, Mexico.