A survey of seed potato tubers in Michigan seed production storage facilities was carried out during 2009 and 2010. Fusarium spp. associated with tuber dry rot symptoms were identified to species and tested for sensitivity to difenoconazole, fludioxonil, and thiabendazole. Symptomatic tubers (n = 370) were collected from a total of 51 seed lots, from which 228 isolates of Fusarium were recovered and identified to 11 species. Fusarium oxysporum was the most commonly isolated species (30.3%), followed by F. equiseti (19.3%). F. sambucinum and F. avenaceum were third most prevalent (each at 13.6%). Less prevalent species (each at 4 to 10%) included F. cerealis, F. solani, and F. acuminatum; and species present at ≤3% included F. sporotrichioides, F. torulosum, F. tricinctum, and F. graminearum. Representative isolates of all species were pathogenic when inoculated onto seed tubers (‘Dark Red Norland’). Isolates of F. sambucinum were the most virulent. All 228 isolates of Fusarium were sensitive to difenoconazole (effective fungicide concentration that caused 50% inhibition of mycelial growth [EC50] < 5 mg/liter). Insensitivity to fludioxonil (EC50 > 100 mg/liter) was detected only for F. sambucinum and F. oxysporum isolates at 8.9 and 20.4%, respectively. All isolates were sensitive to thiabendazole (EC50 < 5 mg/liter), except for those of F. sambucinum (EC50 > 100 mg/liter). Therefore, knowledge of what Fusarium spp. are present in seed potato storage facilities in Michigan may be important if using fludioxonil or thiabendazole for seed piece treatment but not when using difenoconazole.

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Migrations or introduction of new genotypes of Phytophthora infestans to a specific region imposes a different perspective for potato production. During 2009-2010, a late blight epidemic affected the Northeastern United States, which quickly spread through several states. The epidemic was characterized by the appearance of a new genotype of P. infestans designated US-22, which was isolated from tomato and potato. Potato tubers are an essential component of late blight epidemics where the pathogen cannot overwinter on Solanaceous plants. Six potato cultivars were inoculated with 12 isolates of P. infestans (five different genotypes), including isolates of the genotype US-22. Tuber blight development was characterized in terms of tissue darkening expressed as area under the disease progress curve values and lenticel infection. The responses indicated that US-8 was more aggressive than US-22, but US-22 isolates obtained from potato were more aggressive on potato than those acquired from tomato. Tuber periderm responses to infection were limited, yet US-8 isolates infected the periderm more often than US-22 isolates. There were significant differences among the cultivars tested but cv. Jacqueline Lee was the most resistant overall. Although isolates of P. infestans genotype US-22 were less aggressive in comparison with US-8 isolates, US-22 isolates still infected potato tubers and were as aggressive us US-8 isolates on some cultivars. Management of late blight caused by isolates of US-22 through host resistance may be feasible but imposes a different set of criteria for consideration from those that US-8 imposed.

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The interactions of different cultivars/Advance Breeding Lines (ABL) of potato with different genotypes of the potato late blight pathogen (Phytophthora infestans) at three storage temperatures on tuber late blight development were evaluated. The contribution of the medullar storage tissues was assessed rather than the periderm and outer cortical cell tissue. Tuber late blight severity measured as tuber darkening [mean Relative Average Reflectance Intensity [RARI (%)] generally increased with temperature. There was little difference in tuber late blight development between 7°C and 10°C treatments and in some combinations significantly more tissue darkening developed at 7 than at 10°C but little or no development occurred at 3°C. Resistance in tubers was observed only in Torridon andStirling and to some extent Jacqueline Lee, but the cultivar Missaukee had weak tuber resistance. The US-8 genotype isolates were the most aggressive in tubers in most years causing rapid and significantly more tuber damage than any other genotype of P. infestans and similar to the US-6, US-10 and US-14 isolates used in 2006.Resumen Se evaluaron las interacciones de diferentes variedades y líneas avanzadas (ABL) de papa con diferentes genotipos del patógeno del tizón tardío (Phytophthora infestans) a tres temperaturas de almacenamiento para el desarrollo del tizón tardío en tubérculo. Se analizó la contribución de los tejidos medulares de almacén en vez del peridermo y de tejido celular cortical más externo. La severidad del tizón tardío del tubérculo, medida como el oscurecimiento del tubérculo [media de la intensidad del promedio relativo de refractancia [RARI (%)], generalmente se incrementó con la temperatura. Hubo poca diferencia en el desarrollo del tizón tardío del tubérculo entre los tratamientos de 7 y 10°C, y en algunas combinaciones se desarrolló mayor oscurecimiento de tejido significativamente a 7 que a 10°C pero no se presentó a 3°C. La resistencia de los tubérculos se observó solo en Torridon y Stirling y hasta cierto punto en Jacqueline Lee, pero la variedad Missaukee tuvo débil resistencia de tubérculo. Los aislamientos del genotipo US-8 fueron los más agresivos en tubérculos en la mayoría de los años causando más daño en tubérculo rápida y significativamente que cualquier otro genotipo de P. infestans y similar a los aislamientos US-6, US-10, y US-14 usados en el 2006.