Potato purple top (PPT) disease has caused severe economic losses in some potato (Solanum tuberosum) growing areas of Mexico. Two distinct phytoplasma strains belonging to the aster yellows and peanut witches'-broom groups (16SrI and 16SrII groups) have been associated with PPT disease in several regions of Mexico. However, there has been no previous large-scale survey in the main potato growing areas in Mexico to analyze the diversity and geographical distribution of phytoplasmas. Potato samples were collected between 2003 and 2006 and were analyzed by nested polymerase chain reaction assays. On the basis of results from nucleotide sequence comparisons and virtual restriction fragment length polymorphism analysis of 16S rDNA, four different phytoplasma groups were detected in potato growing areas in Mexico. The aster yellows group (16SrI) ‘Candidatus Phytoplasma asteris’ was distributed in all potato growing areas, whereas peanut witches'-broom group (16SrII) ‘Candidatus Phytoplasma aurantifolia’ was detected in Guanajuato and Sinaloa, X-disease group (16SrIII) was detected in Coahuila and Guanajuato, and the Mexican periwinkle virescence (16SrXIII) was only detected in Sinaloa. Phytoplasmas from X-disease and Mexican periwinkle virescence groups were detected in potato samples for the first time in Mexico.

Pepper (Capsicum annuum) and tomato (Lycopersicon esculentum) are important vegetable crops in Mexico. Recently, symptoms associated with phytoplasma diseases such as witches'-broom (shoot proliferation) and little leaf were observed in pepper and tomato fields in central and northwestern Mexico. DNA extracted from symptomatic and asymptomatic plants was used in nested polymerase chain reaction (PCR) assays with primers amplifying 16S rDNA sequences for phytoplasmas. Twenty-four percent of pepper and 49% of tomato samples yielded a nested rDNA product of 1.25 kb. Restriction fragment length polymorphism profiles and sequencing of PCR products allowed classification of the detected phytoplasmas with the aster yellows group (16SrI). Both phytoplasmas, pepper little leaf (PeLL) and tomato little leaf (ToLL), could be included as new members of the aster yellows group because HaeIII and TaqI restriction enzymes discriminated among these phytoplasmas and members of other 16SrI subgroups. PeLL and ToLL phytoplasma sequences were deposited and compared with those in GenBank, and the maximum identity was found with several isolates of ‘Candidatus Phytoplasma asteris’. The highest identity (99%) has been observed with tomatillo little leaf phytoplasma and ash witches'-broom phytoplasma. This is the first report of ‘Ca. Phytoplasma asteris’ associated with pepper and tomato diseases in the Mexican states of Guanajuato and Sinaloa.

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A number of potential sources of general and specific resistance to southern corn rust were identified from 1,890 plant introduction accessions that were screened for reactions to Puccinia polysora race 9. Resistance appeared to differ among four accessions on which uredinia were not observed in initial screenings. Resistance to P. polysora in PI 186215 (Argentine inbred 2-687) was a chlorotic fleck, hypersensitive reaction that was conditioned by a single, dominant gene that was allelic with or very closely linked to the Rpp9 gene based on tests of allelism. All but 3 of 2,357 testcross progeny, (inbred 2-687 × Rpp9) × PS were resistant. Resistance in Ames 19016 (Va59) was effective in F1 progeny and appeared to be dominant and simply inherited; however, this resistance appeared to be a slow-rusting or incomplete resistance that was effective in adult plants but not in young seedlings. Severity of southern rust was less than 10% on resistant progeny from crosses with Va59 compared with severity exceeding 70% on susceptible progeny. Resistance in plant introduction (PI) 186209 (Venezuelan flint) and NSL 75976 (IA DS61) were not effective in F1 hybrid combination and, thus, probably have limited value in commercial maize. Resistance in PI 186209 may be conditioned by a single recessive gene and resistance in NSL 75976 may be co-dominant.

Se estudió la macromorfología de cepas de Beauveria bassiana (B1), Metarhizium anisopliae (M1) e Isaria javanica (HPI-210) y se determinó su crecimiento radial a temperaturas de 25, 28, 30, 35 y 40 °C en medio PDA suplementado con polvos de Diatraea considerata, Spodoptera frugiperda y Galleria mellonella. Los hongos se inocularon adicionando 2 µl de una suspensión con 1 x 107 esporas/ml. Después de 15 días se midió el crecimiento radial de las cepas en mm/d. Para determinar la interacción entre las tres cepas con el polvo de los tres insectos más el control PDA y las cinco temperaturas, se usó un análisis factorial 3 x 4 x 5; los datos del crecimiento de los hongos fueron analizados mediante un ANOVA y una prueba de Tukey. Se encontraron diferencias estadísticas significativas en el crecimiento a diferente temperatura, respecto al control; el crecimiento óptimo en los tres hongos fue a 28 °C, la cepa B1 suplementada con polvo de D. considerata creció 2,76 ± 0,06 mm/d, M1 con G. mellonella 2,77 ± 0,02 mm/d, y HPI-210 con D. considerata 3,27 ± 0,06 mm/d. La cepa B1 creció 0,45 ± 0,03 mm/d, M1 1,22 ± 0,0 mm/d a 30 °C, mientras que HPI-210 no creció después de 28 °C. Estos resultados sugieren que las tres cepas fueron inducidas a crecer mejor por efecto de los medios suplementados con los insectos; a 28 °C, HPI-210 presentó el mayor crecimiento/d (F = 25,24, E8, P = 0,0001) seguido de M1 y B1, mientras que M1 con suplemento de S. frugiperda creció mejor a 30 °C (F = 38,33, E8, P = 0,0001). Estos resultados de crecimiento radial, desarrollo macromorfológico y tolerancia a la temperatura permiten seleccionar a las mejores cepas para evaluar su virulencia en insectos plaga.