Z.-M. Cao, and
P.-Q. Li, College of Forestry, Northwest A & F University, Taicheng Road, Yangling, Shaanxi 712100, China; and
C. Liang, College of Agronomy and Plant Protection, Qingdao Agricultural University, Qingdao 266109, China
Prunus hypoleuca (≡ Maddenia hypoleuca), a native plant in China, grows in the Qinling Mountains that lie at the intersection of several forest regions in north, central, and southwest China. In October 2013, P. hypoleuca suffering from heavy powdery mildew infections was found with approximately 75% of the plants affected. The powdery mildew at first appeared as circular to irregular white patches, which subsequently showed abundant hyphal growth on both sides of leaves, leading to the withering of the leaves. A voucher specimen was maintained in the Mycological Herbarium of Northwest A & F University (Accession No. HMNWAFU-CF 2013166). Hyphal appressoria were nipple-shaped or nearly absent. Conidiophores were cylindrical, measured 83 to 110 × 10 to 12.5 μm, and produced two to five immature conidia in chains with a crenate outline. Foot-cells of conidiophores were straight, cylindrical, and 28 to 62 × 7 to 10 μm. Conidia were hyaline, ellipsoid to ovate, and measured 20 to 32 × 14 to 21 μm (length/width ratio 1.4:1.8). Chasmothecia were scattered or gregarious, depressed globose, and 65 to 112 μm in diameter. Appendages, arising from the upper half of the chasmothecia, usually had two to four dichotomous branches, and were one to three and a half times as long as the chasmothecial diameter. A single ascus in a chasmothecium was subglobose or broadly ellipsoid-ovoid, measured 66 to 86 × 47 to 76 μm and contained six to eight ascospores. The ascospores were ellipsoid-ovoid and 15 to 27 × 12 to 18 μm. The fungus was identified as Podosphaera tridactyla based on its anamorph and teleomorph characteristics (1,2). To confirm the identification, 28S rDNA and the ITS region were amplified. The ITS5/P3 and then PM5/ITS4 primers were used to amplify the ITS region by nested PCR. The primers LSU1/LSU2 were used to amplify the 28S rDNA, and the cloned fragments were sequenced. The 28S rDNA and ITS region sequences were deposited in GenBank (Accession Nos. KJ879240 and KM213121). A GenBank BLAST search of two sequences revealed 99% identity with P. tridactyla infecting Prunus salicina Lindl. in Korea (3). Based on ITS and a 28S rDNA phylogenetic tree, the two sequences retrieved from the Chinese specimen clustered within a strongly supported clade (bootstrap value = 100%) with P. tridactyla (JQ517296 and AB022393, respectively). Cladistic trees were constructed using the neighbor-joining method with the Kimura two-parameter substitution model in MEGA 5.0. Branch robustness was assessed via bootstrap analysis with 1,000 replicates. Phylogenetic analysis data were in agreement with morphological characters (3). To our knowledge, this is the first report of powdery mildew caused by P. tridactyla on P. hypoleuca. While Koch's postulates have not been carried out because of the biotrophic nature of the pathogen, the present report serves as a novel resource in order to improve the understanding of the etiology and epidemiology of the powdery mildew (P. tridactyla) on P. hypoleuca. The occurrence of P. tridactyla, a common powdery mildew on Prunus s. lat., supports recently published results of phylogenetic analyses of the Prunus complex, indicating that Maddenia must be reduced to synonymy with Prunus (4).
References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) U. Braun and R. T. A. Cook. Taxonomic Manual of the Erysiphales (Powdery Mildews), CBS Biodiversity Series No. 11. CBS, Utrecht, Netherlands, 2012. (3) S. C. Lee et al. Res. Plant Dis. 18:49, 2012. (4) J. Wen and W. T. Shi. PhytoKeys 17(2):39, 2012.