During July to November 2000, alfalfa (Medicago sativa L.) plants with powdery mildew symptoms were observed in commercial hay and seed fields, research plots, and a greenhouse in southwestern Idaho (Canyon County) and eastern Oregon (Malheur County). Affected leaves and stems showed white, effuse, dense, amphigenous mycelium bearing Oidium conidia, often with necrotic, brown blotches or streaks. In July, symptoms and signs were found mostly on the lower stems and leaves, but by October, all aerial parts of the plants were involved. Based on location of the mycelium and the conidiophore and conidial characteristics, the fungus was identified as Erysiphe pisi (J. C. David, personal communication). Cleistothecia were not observed. Natural incidence of the disease, which varied from less than 5% infected plants in July to more than 90% infected plants in November, was observed in commercial fields of cvs. Pioneer 53V08 (hay), Sparta (hay), and DS 907 (seed); in research (seed production) plots on cvs. Alfagraze, Anchor, Barrier, Beaver, Cut'N'Graze, Rambler, and Rangelander; and in the greenhouse on cvs. Aggressor, Archer II, Nemagone, Pecos, and Robust. Greenhouse inoculations by dusting with conidia from infected alfalfa plants (cv. Pioneer 53V08) demonstrated the susceptibility of alfalfa cvs. Affinity, Amerigraze, Innovator, and Salado and pea (Pisum sativum L.) cvs. Aladdin, Badger, Bolero, Cabree, Dwarf Gray Sugar, Early Perfection, Encore, Lazor, Maestro, Melting Sugar, Novella, Pursur, Somerwood, Spring, Stampede, and Sugar Ann. Red clover (Trifolium pratense L.) cvs. Redland, Redland II, and Premier showed positive but limited symptoms (smaller colonies, scant mycelium, and brown necrosis) following inoculation. Uninoculated control plants remained free from powdery mildew. Previous reports of powdery mildew on alfalfa in the United States (2) have attributed it to E. polygoni (Massachusetts and Wyoming) or Oidium sp. (Hawaii). Recently, powdery mildew caused by E. pisi was reported from alfalfa in research nurseries and greenhouses in Wisconsin (1). To our knowledge, this is the first report of occurrence of powdery mildew of alfalfa caused by E. pisi in the northwestern U.S., and may constitute a potential new problem for hay and seed production in this region.
References: (1) J. I. Edmunds et al. Plant Dis. 82:832, 1998. (2) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. APS Press, St. Paul, MN.