Canker on Bark of Populus spp. Caused by Cytospora tritici, a New Disease in China
Authors: Zhang Q. T. , He M. , Zhang X. Y. , Lu Q. , Liang J. 
Published: April 2012

Species of Cytospora Ehrenb. and associated teleomorphs cause dieback and canker on over 85 species of angiosperm and gymnosperm plants throughout the world (2). Cytospora tritici Punith. was first observed on Triticum asetivum in Germany in 1980 but may also affect many hardwoods (3). During a survey of landscape trees in 2007, Populus spp. with cankers were found in Fushun, Baoxing, and Luding counties and Chengdu city in Sichuan Province. In these trees, bark canker pathogens discolored the sapwood. During damp weather, conidia were pushed out and formed orange spore horns. Conidiomatal stromata were immersed in bark, prominent, and 1.53 ± 0.33 mm in diameter (n = 10). Discs were white to grey, circular, oval, and 0.59 ± 0.14 mm in diameter (n = 10), with one ostiole per disc. Ostioles were dark grey. Locules were multi-chambered, chambers irregular. Conidia were lelongate-allantoid shaped, hyaline, aseptate, 5.04 ± 0.65 μm long (n = 50), and 1.22 ± 0.13 μm wide (n = 50). Fragments (5 × 5 mm2) of the junction of diseased and healthy tissues were surface sterilized with 1% NaOCl for 30 s and then rinsed twice in sterile distilled water. The pieces were placed on potato dextrose agar (PDA) plates and incubated at 25°C for 7 days. The obtained isolates were cultured on PDA at 25°C in diffuse fluorescent light for 30 days. Upon isolation, the mycelium grew at a rate of 3 to 5 mm per day at 25°C, forming pale white-to-pure white flat colonies. Conidiomata never formed on PDA. ITS1-5.8S-ITS2 sequences were amplified via PCR from genomic DNA obtained from mycelia using universal primers ITS1 and ITS4 (4). The amplification products showed 100% sequence homology with C. tritici isolate DQ243812 from the GenBank database. The ITS sequences were submitted to GenBank (Accession No. JQ277333 to JQ277336). Pathogenicity was confirmed by inoculating 20 disinfected (70% ethanol) Populus tomentosa cuttings. Cuttings were incubated at 25°C for 30 days. Another two cuttings were treated with water agar as controls. In 18 of the 20 cuttings, the cambium developed a brown color and appeared water soaked 15 days later, whereas controls did not develop any symptoms. C. tritici was reisolated from symptomatic tissues. To our knowledge, this is the first report of C. tritici in China causing canker on Populus spp. Cytospora canker is common in practically all countries where poplar are grown. Canker expansion increases when tree defenses are compromised, usually by seasonal dormancy but also by drought, cold injury of wood, sun scald of bark, flooding of root, hail, freezing, or other stress (1). Future spread of C. tritici to western China is considered highly likely.

References: (1) G. C. Adams et al. Stud. Mycol. 52:1, 2005. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ March 25, 2012. (3) E. Punithalingam. Nova Hedwigia 32:585, 1980. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, 1990.