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Powdery mildew showing up late in a sugar beet field in south central Montana

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By Oscar Perez-Hernandez and Jack Meyer

A sugar beet field with the majority of plants showing a dusty appearance on leaves was identified in south central Montana, near Billings, on September 9. Plants exhibited the symptoms both on the upper and underside of leaves at moderate to high levels of severity (Fig 1). Close observation of the symptoms suggested they corresponded to the disease known as powdery mildew, which is caused by the fungus Erysiphe polygoni. Leaf samples were taken to an MSU lab for microscopic observation and confirmation of the pathogen identity. Scattered, whitish mycelial mats were observed on both leaf surfaces containing abundant, cylindrical to elliptical conidia (asexual spores of the fungus) that originated singly from straight, short conidiophores (Figs. 2). No chasmothecia (sexual stage of the fungus) were present in the observed leaves. Occurrence of the disease in this field coincides with a period of suitable weather conditions for disease development in this location, namely: warm, dry weather, and large diurnal temperature changes towards end of August-beginning of September (Fig. 3). At this stage, since the crop in the scouted field is near harvest, a fungicide treatment is not justified. In situations where the disease is detected early in the season, a fungicide treatment would be recommended upon detection to reduce a potential rapid disease increase. Presence of the disease in this location constitutes a source of inoculum for healthy neighboring fields, though according to communication with the Yellowstone agriculture extension agent and area agronomists, no other fields with similar symptoms have been observed as yet. Disease presence in this location suggests future imminent risk for seasonal incursions of pathogen spores into the area from afar overwintering sites.

For questions and further discussion please contact Dr. Oscar Perez-Hernandez, MSU Extension Row Crop Pathologist (; 406-994-4091)