A bacterial disease of maize, bacterial stalk and top rot, was found in the state of Morelos in February 2011, and in the state of Puebla in July 2013, Mexico. In both cases, the incidence of diseased plants was lower than 0.5%. The typical symptoms were a soft rot and darkening of the tissues affecting the stalk and the top of the plant, causing breaking of the stalk. The lesions progressed from the top to below nodes, leaf sheaths and blades, and rotten tissues emitted an unpleasant odor. Eleven diseased plants were collected, and bacterial colonies were isolated from fragments detached from the edges of symptomatic tissues after sterilization with a 0.5% solution of NaClO for 30 s, rinsing three times in sterile water. The sterilized fragments were macerated in drops of distilled sterile water for 10 min and the extract was streaked on King's medium B (agar 15 g, distilled water 1,000 ml, proteose peptone 20 g, K2HPO4 1.5 g, MgSO4·7H2O 1.5 g, glycerol 10 ml). Eight representative strains from Morelos and five from Puebla were selected for identification. All strains were gram-negative, grew at 37°C, showed pectynolitic activity on potato tubers, were positive for indole production, utilized arabinose, galactose, glucose, glycerol, lactose, mannose, melibiose, rafinose, ribose, and sucrose but did not produce acid from arabitol, adonitol, and keto-methyl-glucoside (3,4). Pathogenicity tests were conducted with each strain by inoculating with a syringe four 25-day-old maize seedlings with 107 CFU ml–1 bacterial cells in the leaf collar. Plants were incubated in the greenhouse at 30°C during the day and 24°C during the night with a 12-h photoperiod, and relative humidity of 93%. The reference strains Erwinia chrysanthemi pv zeae ATTC29942 and Dickeya zeae CFBP 2052 were used as positive controls in laboratory and greenhouses tests. Sterile water was used as negative control. Two days after inoculation, soft stalk rot symptoms developed that were identical to those observed in the field. No symptoms were observed on the negative controls. Diagnostic amplification of DNA by conventional PCR was carried out and yielded the expected amplicon size of 420 bp of the Dickeya-specific pel gene with the ADE primers set (2). PCR was used to amplify the 16S rRNA gene with the universal primers 27f and 1495r (5) for molecular identification of the 13 strains (GenBank Accession Nos. KJ438941, KJ438942, KJ438943, KJ438944, KJ438945, KJ438946, KJ438947, KJ438948, KJ438949, KJ438950, KJ438951, KJ438952, and KJ438953). The strains D. zeae CFBP 2052 and E. chrysanthemi pv. zeae ATCC 29942 were sequenced as positive controls. A BLAST search with the 13 16S rRNA gene sequences of 1.4 kb were 99% identical to the sequence of D. zeae CFBP 2052 (NR_041923). D. zeae can be a major disease of maize in tropical and subtropical countries. It is particularly severe under conditions of high temperature and high humidity, but it occurs sporadically. Control of the vector, Chilo partellus, can aid disease management (1). To our knowledge, this is the first report of D. zeae causing maize stalk rot in Mexico. References: (1) CABI. Crop Prot. Compend. CAB International, Wallingford, UK, 2014. (2) A. Nassar et al. Appl. Environ. Microbiol. 62:2228, 1996. (3) R. Samson et al. Int. J. Syst. Evol. Microbiol. 55:1415, 2005. (4) N. W. Schaad et al. Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. APS Press, St. Paul, MN, 2001. (5) W. G. Weisburg. J. Bacteriol. 173:697, 1991.

La detección de patógenos asociados a semilla requiere contar con muestras representativas y homogéneas para obtener resultados confiables. Esta investigación tuvo como objetivos evaluar si: 1) el nivel de infección ( 5, 10 y 15 %); 2) el incremento del número de muestra simples (10, 15, 20 y 25), utilizadas para formar muestras compuestas; y 3) el incremento y la reducción del tamaño de muestra usual de 10% (establecido en el CIMMYT para el diagnóstico de plagas y enfermedades) a 15 y 5%, afectan la capacidad predictiva de la prueba DAS-ELISA al virus del mosaico estriado de la cebada (BSMV, por sus siglas en ingles). A partir de lotes de semilla de trigo harinero (Triticum aestivum L.), uno naturalmente infectado y otro sano, se formaron 210 muestras simples (MS), de 20 g cada una, divididas en grupos iguales de 70 muestras MS con 5, 10 y 15% de nivel de infección (NI) para formar tamaños de muestra (TM) de 15, 10 y 5%, y muestras compuestas (MC) con 10, 15, 20 y 25 muestras MS por NI y TM. Los resultados indicaron que al reducir el nivel NI en muestras MS de 15% a 10 y 5% decreció significativamente (α= 0.05) en 21 y 31.5%, respectivamente, la sensibilidad de la prueba al virus BSMV. Similarmente, la reducción del número de muestras MS para formar muestras MC afectó significativamente (α= 0.05) la sensibilidad de la prueba al virus e incrementó la probabilidad de registrar falsos negativos. Aunque el incremento y la reducción del TM de 10% a 15 y 5% afectó significativamente (α= 0.05) la sensibilidad de la prueba al virus no se encontró una relación lineal entre el TM de la muestra MC con el valor de absorbancia. Este estudio evidencia la importancia del nivel de infección del virus BSMV en la semilla, del TM y del número de MS en muestras MC para la obtención de resultados confiables por la prueba DAS-ELISA.