Horseweed (Conyza canadensis (L).Cronq., Asteraceae) is an invasive exotic weed in Turkey and a problematic native weed in the United States where glyphosate-resistant populations of the weed have developed (2). These characteristics make horseweed a target for biological control efforts. In September 2009, small, brown leaf spots were observed on leaves of C. canadensis in Taflan, Turkey (41°25.398′N, 36°08.352′E). Globose, dark-walled pycnidia were also observed in brown spots on leaves. Diseased tissue was surface disinfested and placed on moist filter paper in petri plates. A fungus designated 09-Y-TR1 was isolated from the diseased leaves. Single-spore isolations were grown on potato dextrose agar (PDA). Cultures on PDA formed dark green-to-black colonies. Pycnidia matured after 3 to 4 weeks when plates were incubated at 23°C with a 12-h photoperiod (black light and cool white fluorescent light). Pycnidia were separate, immersed, and dark brown with a single apical ostiole. Matured conidia were one to three septate, filiform, straight to slightly curved, rounded at the apex, smooth walled, hyaline, and 22 to 40 × 1.4 to 2.5 μm. Morphology was consistent with Septoria erigerontis Peck (3). Comparison of the internal transcribed spacer (ITS) 1 and 2 sequence with available sequences of vouchered S. erigerontis specimens (GenBank EF535638.1, AY489273.1; KACC 42355, CBS 109094) showed 447 of 450 and 446 of 450 identities, respectively. Nucleotide sequences for the ribosomal ITS regions (ITS 1 and 2, including 5.8S rDNA) were deposited in GenBank (GU952666). For pathogenicity tests conidia were harvested from 3-week-old cultures grown on PDA, by brushing the surface of the colonies with a small paint brush, suspended in sterile distilled water, and filtered through cheese cloth. Conidia were then diluted in sterile distilled water plus 0.1% polysorbate 20 to a concentration of 5 × 106 conidia/ml. Stems and leaves of seven 5-month-old seedlings were spray inoculated with 10 ml of this aqueous suspension per plant. Inoculated plants and three noninoculated plants were placed in a dew chamber at 23°C in darkness and continuous dew, and after 48 h, plants were moved to a greenhouse bench. Symptoms were observed 2 days after inoculation. Disease severity was evaluated 2 weeks after inoculation by a rating system with a scale of 0 to 6 based on percentage of plant tissue necrosis, in which 0 = no symptoms, 1 = 1 to 5%, 2 = 6 to 25%, 3 = 26 to 75%, 4 = 76 to 95%, 5 = >95%, and 6 = dead plant. The average disease rating on inoculated plants was 3.55. No disease was observed on noninoculated plants. S. erigerontis was reisolated from all inoculated plants. To our knowledge, this is the first report of leaf spot on horseweed caused by S. erigerontis in Turkey where the fungus may have potential as a classical biological control agent. S. erigerontis has also been reported on C. canadensis in Korea and Portugal (1). In the United States, S. erigerontis has been reported on horseweed in several states (1) and these isolates may have potential as biological control agents of horseweed, particularly glyphosate-resistant horseweed, in the United States.
References: (1) D. F. Farr et al. Fungal Databases. Systematic Mycology and Microbiology Laboratory, Online publication. ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, March 2010. (2) I. Heap. www.weedscience.org, 2006. (3) M. J. Priest. Fungi of Australia: Septoria. ABRS/CSIRO Publishing. Melbourne, 2006.
