First Report of Powdery Mildew on Pyrus calleryana Caused by Podosphaera leucotricha

Callery pear (Pyrus calleryana Decne.) is native to Asia and its varieties are planted as ornamentals in urban areas worldwide. It is also used as a source of resistance to fireblight in some breeding programs. In April 2007, symptoms of powdery mildew infection were observed on the foliage of almost every P. calleryana cv. Chanticleer tree planted along a 1.5-km road in Budapest, Hungary. These trees were planted 5 to 6 years ago and were the first callery pears used as ornamentals in Hungary. Powdery mildew infections were also detected on P. calleryana trees planted in other parts of the city. White powdery mildew mycelium appeared on the lower and sometimes upper leaf surfaces, especially on young shoots, and caused chlorotic spots on the upper leaf surfaces and severe distortions of leaves. The spread of the infection was monitored between April and August of 2007 in several sample sites. More than 100 trees that were examined became heavily infected by May 2007. Powdery mildew conidiophores were typical of the genus Oidium subgen. Fibroidium, the anamorph of the teleomorph genus Podosphaera (2). Conidia developed in chains, contained fibrosin bodies, germinated at one of their ends with germ tubes terminating in unlobed appressoria, and measured 16 to 27 × 10 to 15 μm. Hyphal appressoria were nipple shaped or inconspicuous. The teleomorph was not found. To precisely identify the pathogen, DNA was extracted from conidia collected with a sterile brush from a single leaf using a Qiagen DNeasy Plant Kit (Hilden, Germany), and the internal transcribed spacer (ITS) sequence of the ribosomal DNA was amplified and determined as described by Szentiványi et al. (3). The ITS sequence, deposited in GenBank under Accession No. EU148597, was identical to those determined in Podosphaera leucotricha (Ell. & Ev.) Salmon collected from apple in Australia (GenBank Accession No. AF073353) and Canada (GenBank Accession No. AY157844) and also from pear in Canada (GenBank Accession No. AY157845). Thus, the pathogen was identified as Podosphaera leucotricha on the basis of the host genus, morphology of the anamorph, and ITS sequence. Specimens were deposited under Accession No. BPI878262 at the U.S. National Fungus Collection. To our knowledge, Podosphaera leucotricha has not been reported on P. calleryana in any parts of the world so far. An Oidium sp. infecting this plant in Australia was listed by Amano (1), but the exact identity of that fungus is not known. Thus, this is the first report of an identified powdery mildew fungus on P. calleryana. References: (1) K. Amano. Host Range and Geographical Distribution of the Powdery Mildew Fungi. Japan Scientific Societies Press, Tokyo, 1986. (2) U. Braun et al. Pages 13-55 in: The Powdery Mildews: A Comprehensive Treatise. R. R Bélanger et al., eds. American Phytopathological Society, St Paul, MN, 2002. (3) O. Szentiványi et al. Mycol. Res. 109:429, 2005.