First Report of Damping-Off of Durum Wheat Caused by Arthrinium sacchari in the Semi-Arid Saskatchewan Fields.

Durum wheat (Triticum durum Desf.) is an important crop in western Canada. In 2005, Arthrinium sacchari was frequently isolated from soil and durum wheat plants of the semi-arid fields of Swift Current, Saskatchewan, Canada (50°16'N, 107°44'W). The susceptibility of durum wheat to damping-off caused by this fungus was evaluated. To our knowledge, this is the first report of A. sacchari in North America (1) and the first mention of its association with durum wheat. DNA was extracted (MoBio Isolation Kit, Carlsbad, CA) from 2-week-old A. sacchari isolates (FBC.3, FBC.45, and FBC.143) grown on PDA. The internal transcribed spacer (ITS) of the rDNA was amplified from each isolate and sequenced (Plant Biotechnology Institute, Saskatoon, SK, Canada) and similarity analyses were performed using the BLAST search algorithm in GenBank. All three sequences (Accession Nos. EF076710, EF076711, and EF076712A) showed 99% similarity with A. sacchari (Accession No. AF393679). An in vitro assay was performed by placing 1-cm2 agar plugs containing mycelia of A. sacchari (FBC.3, FBC.45, and FBC.143) onto surface-sterilized durum. Surface-sterilized seeds inoculated in the same way with Fusarium graminearum or F. avenaceum were used as negative controls, and noninoculated surface-sterilized seeds were used as a positive control. A second in vitro assay involved inoculating the same isolates onto seeds placed in sterilized sandy soil. In both assays, 10 seeds per petri plate and three plates per treatment were used and plates were incubated at 21°C for 1 week in darkness. All experiments were performed twice. On PDA, preemergence damping-off was found in 60% of A. sacchari FBC.3, 55% of A. sacchari FBC.45 and FBC.143, 50% of F. avenaceum, and 58% of F. graminearum inoculated seeds. In sterilized soil, the incidence of preemergence damping-off ranged from 43 to 30%. Subsequent incubation over a period of 3 weeks resulted in 100% postemergence damping-off in A. sacchari FBC.45 and FBC.3 as well as in both Fusarium spp. inoculated controls, 60% postemergence damping-off in A. sacchari FBC.143, and no damping-off in the noninoculated control. Arthrinium and Fusarium spp. were reisolated only from symptomatic plants, satisfying Koch's postulates. In conclusion, durum wheat is highly susceptible to damping-off caused by A. sacchari, showing characteristic dark brown or violet lesions in infected tissues. A. sacchari was previously reported to be present in South America and eastern Asia. In China, it is considered an important mycotoxigenic species (2). Thus, infection of durum wheat crops with A. sacchari could pose a significant threat to North American wheat production. References: (1) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory. Online publication. ARS, USDA, 2006. (2) X. J. Liu et al. Acta Mycol. Sinica 7:221, 1988.