Powdery Mildew Caused by an Oidium sp. on Twenty-One Annual Medicago spp. in California

Powdery mildew was observed in plots of USDA Plant Introduction accessions of medic plants grown for seed increase at Riverside, CA, from 1995 to 1998. White, diffuse to dense, amphigenous mycelia bearing Oidium conidia appeared on shoots in April each year and remained active until the last plants matured in June. Invaded leaflets became necrotic and dropped prematurely, sometimes leaving a green petiole. However, mildew incidence was low in most plots, and symptomless plants occurred in all species. Conidia were ellipsoid-cylindrical and measured 15 to17 × 39 to 47 µm. A sexual state of the fungus was not observed. Morphological characteristics of the anamorph matched those of Erysiphe pisi (1). In growth chambers, conidia from medic plants caused severe mildew of garden pea (Pisum sativum L. 'Early Perfection'). Powdery mildew occurred in plots of Medicago ciliaris, M. constricta, M. coronata, M. disciformis, M. doliata, M. granadensis, M. intertexta, M. italica, M. lesinsii, M. littoralis, M. lupulina, M. minima, M. murex, M. noeana, M. orbicularis, M. rigidula, M. rotata, M. rugosa, M. scutellata, M. truncatula, and M. turbinata. We believe this is the first report of naturally occurring powdery mildew on these species in the United States. Reference: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980.