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Powdery Mildew Caused by an Oidium sp. in Alfalfa in Wisconsin

In July 1996 and August 1997, dense amphigenous mycelia bearing Oidium conidia infected the lower stems and leaves of alfalfa (Medicago sativa L.) plants in research nurseries in south-central Wisconsin, causing the defoliation of the affected leaves. In the greenhouse, the disease spread rapidly from transplanted field plants to adjacent flats of alfalfa seedlings. A teleomorphic state of the fungus was not observed. Morphological characteristics of the oidia were similar to those of Erysiphe pisi (1). Conidia were ellipsoid-cylindrical and closely resembled those shown from Pisum sativum L. (2). Dry (2) conidia averaged 14.6 × 41.4 μm (range: 13.0 to 15.6 × 39 to 46.8 μm). Conidia shaken from mildewed alfalfa plants onto plants of garden pea (Pisum sativum) cv. Early Perfection and red clover (Trifolium pratense L.) cv. Lakeland infected both hosts. However, powdery mildew developed sooner and was much more severe in pea than in red clover. Noninoculated control plants remained free of powdery mildew. This is the first report of powdery mildew in alfalfa in Wisconsin fields. In extensive pathogenicity studies in greenhouses in Wisconsin (2), conidia from at least one isolate from red clover infected Medicago falcata L. and M. sativa L. var. glutinosa Urban, but not alfalfa cv. Vernal.

References: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980. (2) J. R. Stavely and E. W. Hanson. Phytopathology 56:309, 1966.