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First Report of Branch Canker Caused by Cytospora atrocirrhata on Populus sp. and Salix sp. in China

Cytospora Ehrenb. species and their related teleomorphs are common inhabitants on over 85 species of plants throughout the world, and some of these pathogens have been associated with stem canker and dieback diseases. In July to August of 2011, samples of Cytospora canker were collected from Populus and Salix trees in Aershan City of Xingan League (46.51° N, 120.21° E) and Genhe (50.54° N, 120.30° E) (Inner Mongolia, China), the northeast part of the Chinese mainland, where the forests were frequently stressed by drought and cold springs and seriously suffered from Cytospora canker outbreaks, causing over 150,000 infected trees to die in 1999 (4). Symptoms observed included discoloration of the inner bark, cambium, and sapwood and sunken lesions at the site of active canker growth. The discrete erumpent ostiolar beaks of condimata were visible on the bark. The red spiral tendrils exuded from fruiting bodies when the relative humidity rose above 80%. All isolates were deposited into the China Forestry Culture Collection Center, strain numbers CXY1401, CXY1402, and CXY1403. The colony of single spore isolates on PDA medium was white and conidiomata were produced on autoclaved leaves and segments of Populus tomentosa Carr. and Salix babylonica Linn. The cultural characteristics of the isolates were conidiomatal stromata immersed in bark, discrete, erumpent, leucotorsellioid, and 0.5 to 1.1 × 0.4 to 0.9 mm. Discs were light grey, nearly flat, circular to ovoid, and 0.4 to 0.5 mm diameter, with one central dark grey ostiole. Locules were multi-chambered, subdivided by invaginations into chambers with seperate walls. Conidia were hyaline, eguttulate, elongate-allantoid, aseptate, and 5.5 to 7.0 × 0.8 to 1.2 μm. The ribosomal ITS1-5.8S-ITS2 region was amplified with primers ITS1 and ITS4 from gDNA. BLAST alignments of the consensus sequences of the ITS1 and ITS2 amplicons (JX534242, JX534243, JX534244) revealed 99% identical to the analogous ‘Cytospora atrocirrhata Gvrit.’ sequences reported from Populus spp. and Salix spp. in Iran (EF447305 and EF447306) (2). Pathogenicity tests were carried out using mycelium discs of isolates placed on disinfected 2-year-old P. tomentosa twigs, while the control were inoculated with sterile potato dextrose agar (PDA) discs. Cuttings were incubated at 25°C for 30 days. For 16 of the 20 cuttings, symptoms of brown spot and inner bark discoloration were similar to those observed in the field. Controls did not develop any symptoms, and Koch's postulates were fulfilled with the reisolation of the pathogen from symptomatic tissues. C. atrocirrhata was first reported in the former Soviet Union in 1973 (3) and more recently in Iran (1). To our knowledge, this is the first report of branch canker caused by C. atrocirrhata on Populus sp., and Salix sp. in China. The result provides new information on the geographic distribution of C. atrocirrhata. The appearance of C. atrocirrhata in China seriously threatens the Populus and Salix species, which are widely cultivated for wood production in flat areas. Control measures are needed to prevent further spread of the fungus to new areas.

References: (1) K. B. Fotouhifar et al. Rostaniha. 8:129, 2007. (2) K. B. Fotouhifar et al. Mycol. 102:1369, 2010. (3) M. N. Gvritishvili. Miko. Fitopatol. 7:544, 1973. (4) C. L. Wu. China Forest Pest and Disease (in Chinese). 2:36, 1999.