The perfect stage of powdery mildew of Beta vulgaris found in Michigan

Authors: Hanson, Linda; McGrath, Jon

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2011
Publication Date: 4/1/2011
Citation: Hanson, L.E., McGrath, J.M. 2011. The perfect stage of powdery mildew of Beta vulgaris found in Michigan. Plant Disease. 95(4):494.

Interpretive Summary: Beet powdery mildew, caused by the fungus Erysiphe polygoni, is a disease that can affect sugar beet, Swiss chard, and table beet. It has been common in many sugar beet growing areas of the United States since the major epidemic in the United States in 1974. The disease has been largely associated with the asexual stage of the pathogen. However, the sexual stage has started to be found, first in Western states such as Idaho and Colorado, and most recently it was reported in North Dakota. In 2010, sugar beets and Swiss chard with powedery mildew were observed in four locations in Michigan. At two of these locations, evidence of the sexual stage of the fungus was found in October. These results appear to indicate a spread of the perfect stage eastward since its initial finding in the United States. The sexual stage can be a concern since it may allow more rapid development of new strains to overcome host resistance or spread of fungicide resistance in the population. To our knowledge, this is the first report of the sexual stage of beet powdery mildew on sugar beet or Swiss chard in Michigan.

Technical Abstract: Powdery mildew (Erysiphe polygoni [synonym E. betae]) affects several different crops in the genus Beta, 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 on beet in the United States in 1974. The disease had been largely associated with the asexual stage of the pathogen in the United States until the perfect stage was found widely, first in western states such as Idaho and Colorado, and then in more Midwestern states such as Nebraska, and most recently in North Dakota. Similar to the situation in North Dakota, powdery mildew has not been a major problem in the Michigan growing area. However, it does appear sporadically, particularly on beets that have not been sprayed to control other foliar diseases such as Cercospora leaf spot. In 2010, powdery mildew infection on sugar beets was observed in late August in a field in the Saginaw Valley of Michigan. In early October sugar beet and Swiss chard plants with heavy powdery mildew infection also were observed at three location in the East Lansing, MI area. On both the Saginaw Valley beets and the Swiss chard in the East Lansing area, ascomata were observed on a few leaves in mid-October. No ascomata were found on beets at the other two fields in the East Lansing area. Ascomata were primarily dark brown to black when located and varied in size from 70-100 micrometers in diameter. Asci contained 1 to 4 generally hyaline ascospores similar to those described in other growing regions on sugar beet (2, 3, 4). No ascomata had been detected on powdery mildew infected beets from either the Saginaw Valley or the East Lansing area the previous two years, although beets had been observed with powdery mildew infection. These results appear to indicate a spread of the perfect stage east from it’s initial finding in the United States. To our knowledge this is the first report of the perfect stage of powdery mildew of sugar beet or Swiss chard in Michigan.