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First Report of an Ovary Smut of Italian Thistle Caused by a Microbotryum sp. in Greece

Italian thistle (Carduus pycnocephalus L.), family Asteraceae, is a common weed in Greece. It is also a problematic invasive weed in the western United States and a target of biological control efforts. In May 2005, smutted capitula of Italian thistle were found in an abandoned field in Halkiades, Greece. A total of 38 smutted plants, representing approximately 20% of those plants present, were found in a portion of the field that was lightly infested with Italian thistle. In most cases, capitula of all diseased flowers were smutted. In one or two cases, capitula on some branches of the plants were smutted, whereas capitula on other branches were healthy. Diseased capitula were noticeably more globose than healthy ovoid capitula, and diseased capitula did not open completely. When diseased capitula were split open, the ovaries in all florets within the capitula were filled with powdery masses of smut teliospores. Diseased capitula were collected, air dried, and sent to the quarantine facility of the Foreign Disease-Weed Science Research Unit (FDWSRU), USDA/ARS, Fort Detrick, MD. Teliospores within the capitula were extracted and observed microscopically. Teliospores of isolate DB05-014 were relatively uniform in shape and size, globose, 12.0 to 17.3 × 12.3 to 18.0 μm (mean 14.5 × 15.1 μm), violet tinted pale to medium yellowish-brown; wall reticulate appearing as coarse, radiate wings on the spore margin, 5 to 7 polyangular meshes per spore diameter, muri, 0.7 to 2.0 μm high in optical median view appearing as gradually narrowing blunt spines, 0.5 to 1 μm wide at their basis; in scanning electron microscopy (SEM), the meshes were subpolygonal, wall and interspaces were finely verruculose. Teliospores were more globose and slightly smaller than the description of Microbotryum cardui (A. A. Fischer Waldh.) Vánky (2), but the mean sizes were within the described range. When compared with teliospores of M. cardui on C. acanthoides, the numbers of polyangular meshes per spore diameter were within the range of the description using SEM, but the muri were about one-half of the height of those described. Nucleotide sequences for the internal transcribed spacers (ITS 1 and 2) and 5.8S ribosomal region (GenBank Accession No. AY280460) were aligned with sequences of other smut fungi using the BLAST algorithm of the National Center for Biotechnology Information. The closest alignment of DB05-014 was with M. scorzonerae (590 of 627 bp identities or 94% with 2% gaps). No sequences of M. cardui were available for comparison, but only M. cardui has been reported on Carduus spp. (1,2). Another smut reported on a Carduus sp. is Thecaphora trailii (1). DB05-014 is a likely variant of M. cardui from a previously unknown host. Italian thistle is an annual plant that reproduces solely by seeds (achenes). Because of the lack of seed production on smutted plants and the systemic nature of the disease, this fungus has great potential as a biological control agent for Italian thistle in the United States. A voucher specimen has been deposited with the U.S. National Fungus Collections (BPI 871812). To our knowledge this is the first report of a Microbotryum sp. parasitizing C. pycnocephalus.

References: (1) K. Vánky. European Smut Fungi. Gustav Fischer Verlag, Stuttgart, Germany, 1994. (2) K. Vánky and D. Berner. Mycotaxon 85:307, 2003.