Photinia serrulata Franch. & Sav. (syn. P. serratifolia (Desf.) Kalkman), called Chinese photinia, is native to China, Taiwan, Japan, Indonesia, the Philippines, and India. The plants are evergreen shrubs to small trees belonging in the Rosaceae, and are widely cultivated throughout the world for ornamental purposes. Since 2005, severe powdery mildew infection has been observed on this plant in the Chengyang District of Qingdao City in Shandong Province, China. Powdery mildew colonies were circular to irregular white patches on both sides of the leaves and on young stems. As the disease progressed, white mycelial growth covered the entire shoot portion, causing leaf distortion. Voucher specimens (n= 7) were deposited in the herbarium of Qingdao Agricultural University, China. Hyphae were flexuous to straight, branched, septate, 4 to 6 μm wide, and had nipple-shaped appressoria. Conidiophores arising from the upper part of the hyphae were 110 to 185 × 9 to 12 μm and produced two to six immature conidia in chains with a crenate outline. Foot cells of conidiophores were straight, 30 to 40 μm long, and cylindric to somewhat attenuated toward the base. Conidia were hyaline, ellipsoid-ovoid, 22 to 32 × 13.5 to 20 μm (length/width ratio = 1.5 to 1.9), and had distinct fibrosin bodies. No chasmothecia were observed. The structures and measurements were compatible with those of the anamorphic state of Podosphaera leucotricha (Ellis & Everh.) E.S. Salmon as described by Braun (2). To confirm the identification, the complete internal transcribed spacer (ITS) region of the rDNA was amplified with nested PCR (4) and sequenced. The resulting sequence of 562 bp was deposited in GenBank (Accession No. JQ999954). A GenBank BLAST search of this sequence revealed 100% identity with that of seven isolates of P. leucotricha on rosaceous plants. Pathogenicity was confirmed through inoculation tests by gently pressing diseased leaves onto young leaves of three asymptomatic, potted 3-year-old photinia plants. Three non-inoculated plants were used for a control treatment. Plants were maintained in a greenhouse at 22 ± 2°C. Inoculated leaves developed symptoms after 5 days, whereas the control plants remained symptomless. The fungus present on the inoculated leaves was morphologically identical to that observed on the original diseased leaves, fulfilling Koch's postulates. The powdery mildew infections of P. serrulata associated with P. leucotricha have been recorded in New Zealand, Ukraine, Italy, and the United States (1,3). To our knowledge, P. leucotricha on P. serrulata has not been reported in Asia except for a record of a Podosphaera sp. on P. serratifolia in Japan (3). Since this ornamental shrub is native to China and nearby countries, occurrence of powdery mildew in China poses a potential threat to the health of photinia in other places.
References: (1) E. Baldacci. Rev. Appl. Mycol. 16:358, 1937. (2) U. Braun. Beih. Nova Hedw. 89:1, 1987. (3) D. F. Farr and A. Y. Rossman. Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from , May 14, 2012. (4) Y. Matsuda et al. Phytopathology 95:1137, 2005.
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