关键词: 美国红枫, 褐斑病, 病原菌鉴定, 小孢拟盘多毛孢

Abstract: [Objective] A newly found brown spot disease widely infests leaves of Acer rubrum 'October Glory' growing in Poyang, Jiangxi, and the pathogen is unknown.In this study the pathogen was identified for the purpose of controlling and futher studying the disease. [Method] Occurrence period and incidence rate of the disease were investigated on the spot, and symptoms of the disease were observed and described. A total of 30 infested leaf samples at different developing stages were randomly collected. Pathogen isolation and purification from the samples were conducted on PDA medium using normal tissue isolation method. In order to fulfil Koch'postulates, pathogenicity test was done by artificially inoculating healthy leaves of A.rubrum 'October Glory' with spore suspension (1×10⁶·mL^-1) of two representative isolates PM-1 and PM-2 respectively, and then the pathogen was re-isolated from the newly produced lesions. The tested isolates were inoculated on the center of PDA plates and incubated in 25 ℃ and darkness conditions for their growth, and the colony characteristics of size, color, shape and texture were daily observed. Slice samples as to pathogen on naturally infected plants as well as on PDA plates were prepared respectively, and they were microscopically observed and measured to get the morphology and size of conidiomata and conidia of the pathogen. The pathogen species was identified based on comparing its cultural and morphological results with corresponding description in related literature. DNA of the pathogen was extracted and used as template to amplify the ITS-5.8S rDNA region by PCR with primer pair ITS1/ITS4, and the PCR products were directly sequenced. For pathogenic identification on molecular level, nucleotide sequence homology was searched in GenBank using BLAST software, and phylogenetic tree was constructed by the neighbor-joining method. [Result] The brown spot disease of A. rubrum 'October Glory' occurred only on the leaves. It initially appeared in mid-May, and its peak period was in mid-late June with more than 60% incidence of disease. The lesions were circular, oval or irregular, and yellow-brown with black-brown borders which were often surrounded by a yellowish halo. Later, several lesions might coalesce, and severely affected leaves were withered and fell off. Under humid conditions, black spots, i.e. the acervuli and conidia were produced on the lesions. Totally 25 fungal isolates with the same cultural characteristics were obtained, and their colonies were all fluffy, flat and white on front side and light yellow on back side. Pathogenicity test indicated that PM-1 and PM-2 produced the same symptoms as the naturally infected ones. The spore-bearing structures, acervuli, were 108~ 205 μm in diameter. They produced fusiform, straight to slightly curved conidia with size of (19.3-26.8)μm×(5.3-6.8)μm. Each conidium contained 5 cells with three median cells dark brown and 11.9~17.0 μm long. Apical cell was hyaline, conico-acuminate, 6.3~18.3 μm long, with 2-4 non-branched, colorless and transparent appendages. The basal cell was also hyaline and had a short handle with 1.8~7.5 μm in length. ITS-5.8S rDNA sequence of the pathogen was 606 nucleotide long,which shared 100% homology with Pestalotiopsis microspore in GenBank, and had the closest relationship with the fungus in the phylogenetic tree.[Conclusion] Based on morphological and molecular identification results, it is considered that the pathogen causing brown spot disease of Acer rubrum 'October Glory' in Poyang, Jiangxi is Pestalotiopsis microspora (Speg.) G.C. Zhao & N. Li,a fungal species which belonged to genus pestalotiopsis, a anamorph in Ascomycota. This is the first report of Pestalotiopsis microspore paratisizing on Acer rubrum 'October Glory' and causing brown spot disease.

Key words: Acer rubrum, brown spot disease, pathogen identification, Pestalotiopsis microspora