First Report of Powdery Mildew Caused by Blumeria graminis on Avena sativa in China

Oat (Avena sativa) is extensively planted as a fodder crop and most of the production is used for livestock feed in China (Yang et al. 2010). In March 2017, powdery mildew was observed on A. sativa in the Forage Germplasm Nursery (30.47785° N, 103.75878° E) in Xinjin County, Sichuan Province, China. Approximately 40% of plants (about 0.2 ha in total) were affected. The symptoms initially appeared as irregular white mycelial colonies on the adaxial leaf surface. Subsequently, the colonies increased in number and size, and later covered a large area of leaf and stem. In advanced stages of infection, white patches turned into sunken reddish-brown lesions on leaf surfaces and eventually caused leaf death. Hyphae were flexuous, septate, branched, and 2.6 to 7.6 μm (n = 20) wide. The well-developed appressoria on the hyphae were observed with diameter between 3.2 and 6.3 μm (n = 10). Conidiophores were unbranched and cylindrical with swollen bases, measuring 6.3 to 13.2 × 23.4 to 67.5 μm (n = 20), and borne vertically on hyphae. Each conidiophore produced 3 to 7 conidia (mostly 6 conidia) (n = 20) in a chain. The conidia were single celled, colorless, oval in shape, measuring 11.2 to 17.5 × 20.2 to 43.6 μm (n = 50). Cleistothecia were not observed. Based on morphological characteristics, the fungus was identified as Blumeria graminis (DC.) Speer. To further identify the species of B. graminis, the internal transcribed spacer (ITS) of 560 bp was amplified from conidial DNA using ITS1 and ITS4 primers (White et al. 1990). The ITS sequence was deposited in GenBank (accession no. MF033904). The ITS sequence showed 100% homogeneity with B. graminis on A. sativa in Swizerland (AF011284) (Saenz and Taylor 1999). For pathogenicity test, conidial suspension (2 × 105 conidia/ml) was prepared from infected leaf samples and sprayed on 10 healthy plants (6 weeks old) of A. sativa (cv. Molasses). Another set of 10 plants were sprayed with sterilized distilled water to serve as control. Potted oat plants were individually covered with transparent polyethylene bags to maintain high relative humidity and placed in a greenhouse at 16 to 26°C. The plastic bags were removed after 3 days. Fifteen days later, typical symptoms of powdery mildew developed on all the inoculated plants except control plants. Microscopic and ITS analysis showed that the fungus on the inoculated plants is identical to naturally occurring powdery mildew infection on plants. Powdery mildew is common in northwestern Europe and South America and results in significant losses (5 to 10%) of oat in western Europe (Chowdhury et al. 2014). In China, oat is an important forage crop and present information will be helpful in understanding the range of pathogen that can infect this plant. To our knowledge, this is the first report of powdery mildew caused by B. graminis on A. sativa in China. © 2017, American Phytopathological Society. All rights reserved.