In July 2009, a leaf blotch disease was observed on sorghum in Sakarya Province, Turkey. Disease incidence and severity were rated at 45% and 25 to 75%, respectively. Typical symptoms included elliptical, straw-colored, necrotic lesions with darker margins. The lesions eventually coalesced, resulting in drying of leaves. A fungus was isolated from the lesions on potato dextrose agar (PDA) incubated under near ultraviolet light for 12 h daily at 22°C for 2 weeks. Flexuous conidiophores of the fungus were solitary or produced in small groups. They were geniculate with numerous well-defined scars, medium to dark brown, ≥300 μm long, and 4 to 9 μm thick. Five to eight or more conidia were produced on the apices of the conidiophores. The conidia were straight, oblong or cylindrical, rounded at the ends, golden brown at maturity except for a small area just above the dark scar, pseudoseptate, and 20 to 31 × 7.5 to 12.5 μm (1). The causal fungus was identified as Bipolaris spicifera (Bain) Subram. (teleomorph Cochliobolus spicifer Nelson). The identification was confirmed with specific PCR primers (Bipol-1 F: 5′-CAGTTGCAATCAGCGTCAGT-3′, R: 5′-AAGACAAAAACGCCCAACAC-3′, Bipol-2 F: 5′-GTGTTGGGCGTTTTTGTCTT-3′, R: 5′-CCTACCTGATCCGAGGTCAA-3′, Bipol-3 F: 5′-GATGAAGAACGCAGCGAAAT-3′, R: 5′-AAGACAAAAACGCCCAACAC-3′). These primers were designed by the authors using Primer3 primer design software and sequences of B. spicifera found in GenBank. PCR products were amplified from nuclear DNA of the cultured fungus and were sequenced after cleaning with a Beckman 8000 CEQ DNA sequencer. Sequences amplified using Bipol-1 and Bipol-2 showed 99 to 100% similarity with B. spicifera sequences from GenBank. The DNA sequence amplified using Bipol-2 was deposited in GenBank (Accession No. HQ538774). B. spicifera has been reported previously as pathogenic in Africa, North and South America, Asia, Australia, Oceania, and the West Indies on Agrostis, Avena, Cymbopogon, Cynodon, Dactylis, Desmostachya, Eleusine, Holcus, Hordeum, Oryza, Panicum, Pennisetum, Phleum, Poa, Saccharum, Sorghum, Triticum, and Zea spp. (3). Pathogenicity tests were conducted on sorghum (Sorghum bicolor (L.) Moench) and Sorghum × sudangrass hybrid cultivars. From PDA cultures, conidia were collected in sterile distilled water with a concentration of 3% Tween 20. Twenty-five plants (five per pot) were inoculated by spraying the conidia (105 ml–1) onto 21-day-old plants, which were then maintained at 25 ± 2°C in a greenhouse following 48 h in a humid chamber. The test was repeated once. Control plants were sprayed with sterile distilled water. Typical symptoms (2) were obtained from all inoculated plants 7 days after inoculation. No symptoms developed on the control plants. The pathogen was reisolated from inoculated leaves to fulfill Koch's postulates. To our knowledge, this is the first report of B. spicifera on sorghum in Turkey.
References: (1) M. B. Ellis. Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Kew, Surrey, England, 1971. (2) H. Koo et al. Plant Pathol. J. 19:133, 2003. (3) A. Sivanesan. Mycologia Papers 158:1, 1987.