Powdery mildew (Erysiphe polygoni DC [synonym E. betae {Vanha} Weltzien]) affects several different crops of Beta vulgaris, including sugar beet, Swiss chard, and table beet. The disease has been prevalent in many sugar beet-growing areas of the United States since the first major epidemic in beet in 1974 (3). Powdery mildew in the United States was primarily associated with the asexual stage of the pathogen until the perfect stage was found, first in western states such as Idaho and Colorado (2), then in more Midwestern states such as Nebraska, and most recently in North Dakota (1). Similar to North Dakota, powdery mildew has not been a major problem in the Michigan growing area. It does appear sporadically, particularly on sugar beets that have not been sprayed to control other foliar diseases. In 2010, powdery mildew infection on sugar beet was observed in late August in a field in the Saginaw Valley of Michigan. Plants were inspected periodically for the presence of the sexual stage. In early October, sugar beet and Swiss chard plants with heavy powdery mildew infection also were observed at the Michigan State University (MSU) Horticultural Demonstration Gardens in East Lansing and on sugar beet at the MSU Plant Pathology and Botany research farms. On both the Saginaw Valley sugar beet and Swiss chard on the MSU campus, ascomata were observed on a few leaves in mid-October. No ascomata were found on sugar beet at the other two locations. The majority of ascomata were dark brown to black when located, although a few light tan ascomata were observed on the Swiss chard. Ascomata varied from 70 to 100 μm in diameter. Asci contained one to four hyaline or golden yellow ascospores similar to those described in other growing regions on sugar beet (1,2). No ascomata had been detected on powdery mildew-infected sugar beet from either the Saginaw Valley or the MSU research farms the previous two years. These results appear to indicate a spread of the ability to form the perfect stage eastward from the western United States. This may be due to movement of one mating type because E. polygoni has been reported to be heterothallic on some crops (4). The presence of the perfect stage indicates that sexual recombination could occur in E. polygoni on Beta species in Michigan, creating the potential for more rapid development of new strains that might vary in fungicide sensitivity and response to host resistance.
References: (1) C. A. Bradley et al. Plant Dis. 91:470, 2007 (2) J. J. Gallian and L. E. Hanson. Plant Dis. 87:200, 2003. (3) E. G. Ruppel. Page 13 in: Compendium of Beet Disease and Insects. E. D. Whitney and J. E. Duffus, eds. The American Phytopathological Society, St. Paul, MN, 1986. (4) C. G. Smith. Trans. Br. Mycol. Soc. 55:355, 1970.
