In December 1996 and January 1997, powdery mildew was observed on potted poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) plants in Monterey County, CA. Mycelia were observed on stems, petioles, mature and immature leaves, and bracts. Severely diseased leaves became twisted and bent and senesced prematurely. The white mycelia were conspicuous, epiphytic, and amphigenous; hyphae measured 4.6 to 6.9 μm in diameter. Growth initially was in patches but eventually became effused. Appressoria were slightly lobed to lobed and sometimes opposite. Conidiophore foot cells were cylindrical, sometimes bent at the base, and slightly flexuous to flexuous. Foot cells measured 30.0 to 46.2 μm × 5.8 to 6.9 μm and were followed by one to two shorter cells. Conidia were cylindrical to slightly doliform and measured 25.4 to 32.3 μm × 11.6 to 18.5 μm. The length-to-width ratios of conidia generally were greater than 2.0. Conidia were produced singly, placing the fungus in the Pseudoidium-type powdery mildew group. Conidia germinated at the ends, and no fibrosin bodies were observed. Cleistothecia were not found. The fungus was identified as an Oidium species. Pathogenicity was demonstrated by gently pressing infected leaves having abundant sporulation onto leaves of potted poinsettia plants (cvs. Freedom Red, Peter Star Marble, and Nutcracker White), incubating the plants in a moist chamber for 48 h, and then maintaining plants in a greenhouse. After 12 to 14 days, powdery mildew colonies developed on the inoculated plants, and the pathogen was morphologically identical to the original isolates. Uninoculated control plants did not develop powdery mildew. This is the first report of powdery mildew on poinsettia in California. This fungus appears similar to Microsphaera euphorbiae but has longer, slightly flexuous foot cells that do not match the description for M. euphorbiae (1,2). An alternative identification would be Erysiphe euphorbiae; however, there are no available mitosporic descriptions for morphological comparisons (1,2). In the United States, powdery mildew of poinsettia previously has been reported in various states in the Pacific Northwest, Midwest, and Northeast.
References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN.