Asian pear (Pyrus pyrifolia Nakai), also known as Japanese or Korean pear, is widely cultivated in East Asia. On September 2011, irregularly shaped necrotic lesions were observed on leaves of cv. Shinheung growing in an orchard in Gangneung City, Korea. At 40× magnification under a microscope, the white to cream colored propagules were epiphyllous, conical, scattered to aggregated, and composed of stroma-like bases, globose to subglobose, 55 to 100 μm wide and 35 to 75 μm high with filamentous and claviform hyphae. The filamentous hyphae were cylindrical, 125 to 425 × 3.5 to 6 μm, 2- to 8-septate, and obtuse to subobtuse at the apex. The claviform hyphae were clavate to cylindrical, 35 to 125 × 5 to 12.5 μm, aseptate to 3-septate, and obtuse at the apex. The fungus was isolated from leaf lesions and cultured on potato dextrose agar (PDA). The colonies consisted of thin mycelia colored whitish at first and then pale brown on PDA. Sclerotia were produced on PDA after 2 weeks incubation at 15°C, but conidia were not observed in culture. An isolate from KUS-F26196 was deposited in the Korean Agricultural Culture Collection (Accession No. 25 KACC46693). These morphological and cultural characteristics were consistent with Mycopappus alni (Dearn. & Barthol.) Redhead & G.P. White (1,3,4). Fungal DNA was extracted with DNeasy Plant Mini DNA Extraction Kits (Qiagen Inc., Valencia, CA). The complete internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced. The resulting sequence product of 520 bp was deposited in GenBank (Accession No. JX458815). A BLAST search in GenBank revealed that the sequence was 99% similar to M. alni (AB254190, AB254177, AB254189). To determine the pathogenicity of the fungus, propagules were detached from lesions on the naturally infected leaves using fine needles. Each propagule was transferred individually onto five places of six detached healthy leaves. Control treatment comprised placing small agar blocks onto five places of six detached healthy leaves. The plants were incubated in a humid chamber at RH 100% and 18°C. Symptoms were observed after 2 days on all inoculated leaves. The pathogen was reisolated from lesions on the inoculated leaves, confirming Koch's postulates. No symptoms were observed on control leaves. The fungus has been associated with frosty mildew on Alnus spp., Betula spp., Crataegus spp., and Pyrus spp. in North America, Turkey, Russia, and Japan (1,2,4). To our knowledge, this is the first report of frosty mildew on P. pyrifolia caused by M. alni globally as well as in Korea. Since the infections may be limited to the mountainous area with low night temperature and high humidity, economic losses seem to be negligible. However, the disease could be a potential threat to the safe production of Korean pears in case of prolonged period of cool and moist weather.
References: (1) U. Braun et al. Mikologiya i Fitopatologiya 34(6):1, 2000. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology & Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, August 2, 2012. (3) S. A. Redhead and G. P. White. Can. J. Bot. 63:1429, 1985. (4) Y. Takahashi et al. Mycoscience 47:388, 2006.