Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: October 17, 2014
Citation: Samac, D.A. 2014. Powdery mildew. In: Samac, D. A., Rhodes, L. H., and Lamp, W. O., editors. Compendium of Alfalfa Diseases and Pests. 3rd edition. St. Paul, MN: APS Press. p. 16. Technical Abstract: Powdery mildew is found sporadically on alfalfa and annual medic species. It has been reported on commercial hay and seed fields in Europe, the People's Republic of China, Egypt, and the U.S. It is a common foliar disease in greenhouse-grown plants. Severe infections can cause defoliation and reduce forage and seed production. Symptoms: Leaves and stems are covered with a white, diffuse to dense amphigenous mycelium bearing conidia singly or in short chains. Lower leaves are usually the first to show symptoms. Under favorable conditions the fungus spreads to all aerial portions of the plant. Leaves may develop brown necrotic blotches or streaks. Necrotic leaves drop prematurely. Causal Organism: Erysiphe pisi is an obligate biotrophic fungus that is found worldwide. It is a major pathogen of pea and also causes disease on vetch, lupine, and lentil. Host specialization of individual strains has been observed. Conidia of E. pisi (31-38 X 17-21 um) are ellipsoid to elongate. Cleistothecia have not been observed on alfalfa. Disease Cycle and Epidemiology: The fungus overwinters on crop debris or alternate hosts. Conidia germinate on leaf surfaces in the absence of free water, but a humid atmosphere stimulates germination. The germ tube forms an appressorium that directly penetrates the epidermis and forms a haustorium in an epidermal cell. Hyphae grow from the conidium, penetrate epidermal cells at intervals, and form small white or gray spots. The spots grow and coalesce, completely covering the leaf surface. Secondary spread to uninfected leaves occurs from conidia produced by the mycelium on the leaf surface. Management: No information on management of the disease in the field is available. Disease in the greenhouse can be managed by application of a systemic fungicide. Resistance has been observed in certain accessions of annual medics.