First Report of Powdery Mildew on Trident Maple Caused by Sawadaea nankinensis in Korea
Authors: Lee H. B. , Kim C. J. , Mun H. Y. , Hong J. P. , Glawe D. A. 
Published: November 2009

Trident maple (Acer buergerianum Miq.) is widely grown in Korea as an ornamental tree as well as for the art of bonsai. During 2008 and 2009, a powdery mildew was observed on trident maple plants at the campus of Chonnam National University, Gwangju, Korea. Further surveys revealed the disease to be widespread on this species in other areas including Jeonbuk and Chungnam provinces in Korea. White, superficial mycelia were observed on young shoots and leaves early in spring. Both macroconidia and microconidia were produced beginning in May and conidial production continued through the summer into September and October. Production of chasmothecia was observed starting in September and continued into October. Macroconidia were produced in chains that were sinuate in outline. Individual macroconidia were barrel shaped and 23.4 to 30.0 (26.6) × 15.6 to 21.1 (18.1) μm. Foot cells of macroconidial conidiophores were 26.7 to 110.7 (48) × 7.1 to 11.2 (8.8) μm with one to five following cells. Microconidia were broadly ellipsoidal to subglobose and 8.9 to 12.5 (10.5) × 4.3 to 5.8 (5.1) μm. Chasmothecia typically were formed on adaxial leaf surfaces and 193.2 to 238.1 (216.8) μm in diameter. Appendages bore uncinate to circinate apices and were 176.8 to 267.7 (211.5) × 4.3 to 8.0 (6.2) μm. From extracted genomic DNA, internal transcribed spacer (ITS) region inclusive of 5.8S rDNA was amplified with ITS1F (5′-CTTGGTCATTTAGAGGAAGT-3′) and LR5F (5′-GCTATCCTGAGGGAAAC-3′) primers. The causal fungus was determined to be Sawadaea nankinensis (F.L. Tai) S. Takam. & U. Braun (2) on the basis of morphological data and ITS rDNA sequences. A BLAST search of GenBank with an ITS sequence from this fungus determined that the five sequences exhibiting the highest max score values (1,811 to 2,004) were from S. nankinensis; these sequences produced max ident values from 94% to 99%. In contrast, max score and max ident values from sequences of other Sawadaea spp. were lower, including scores of 1,063 and 98% similarity for S. polyfida var. japonica, 915 and 97% for S. tulasnei, and 913 and 97% for S. bicornis. Pathogenicity tests were conducted on field-grown plants in two replicates. These plants were inoculated with a paintbrush to apply conidia (~5 × 106/ml) collected from powdery-mildew-infected leaves. Inoculated plants developed powdery mildew symptoms within 5 days of inoculation and resembled those observed on naturally infected plants. S. nankinensis (synonym Uncinula nankinensis) was first reported on A. buergerianum from China in 1930 (2). Recently, S. nankinensis (F.L. Tai) S. Takam & U. Braun was reported to occur on A. buergerianum in Japan (3). Until now, three Sawadaea spp. (S. bicornis (Wallr.) Homma, S. negundinis Homma, and S. tulasnei (Fuckel) Homma) have been reported to cause powdery mildew on A. ginnala, but only S. bicornis (= U. circinata Cooke & Peck) has been reported to cause powdery mildew on A. ginnala in Korea (1). However, no Sawadaea sp. previously was reported to cause powdery mildew on A. buergerianum. To our knowledge, this is the first report of powdery mildew on trident maple (A. buergerianum) caused by S. nankinensis in Korea.

References: (1) H. D. Shin. Erysiphaceae of Korea. National Institute of Agricultural Science and Technology, 2000. (2) F. L. Tai. Page 1517 in: Sylloge Fungorum Sinicorum. Science Press, Academia Sinica, Peking, 1979. (3) S. Takamatsu et al. Mycoscience 49:161, 2008.