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First Report of Leaf Spot Caused by Bipolaris setariae on Cassava in China

Cassava (Manihot esculenta Crantz) is an important food crop in tropical regions of China. Seventy percent of the cassava output is used for starch and ethanol production and it has become the base of food and bioenergy industries. In July 2009, a new leaf spot disease was found on cv. HuaNan205 from a cassava plantation in Danzhou, Hainan Province. Disease occurred on 50% or less of the plants. Initial symptoms were elliptical, chlorotic, and water-immersion lesions of 2 to 4 mm in diameter. These lesions became dry and yellow due to the progress of the disease. A brown halo was around the lesions, and in wet conditions, a dark gray mildew often appeared in the middle of the lesion. Diseased leaves turned yellow and the plants eventually became defoliated. The pathogen was isolated and pathogenicity was established by following Koch's postulates. Young, healthy, and fully expanded green leaves of Cassava cv. HuaNan205 were surface sterilized and then inoculated by spraying them with a suspension of conidia (1 × 10⁵ conidia per ml) of the isolate. Sterile water was used as a control. The leaves were kept in a humid chamber at 28°C for 4 days, at which time similar symptoms to those described above were observed on the leaves. The pathogen was reisolated from inoculated leaves. Microscopic examination showed the conidiophores were fasciculate and brown, septate and straight, and the basal cell was enlarged and hemispherical. Well-developed conidia were long-obclavate, obtuse at both ends, straight, brown, with five to eight transverse septa, and measured 49.7 to 117.1 × 13.3 to 17.2 μm. Genomic DNA of this isolate was extracted with a cetyltrimethylammoniumbromide protocol, and amplification and sequencing of the internal transcribed spacer (ITS) region of rDNA was performed with procedures outlined by Cooke et al. (2). The sequence of the region was deposited in GenBank (Accession No. GU290228). Comparison of the sequences available in the GenBank database revealed that the current ITS sequence differs by three base pairs from two Bipolaris setariae isolates (EF452444 and FJ606786). Morphological identification and sequence analysis of ITS rDNA showed that the pathogen was B. setariae. B. setariae is one of the most important pathogens of lawn grass, gramineous crops, and other plants (1,3). However, no leaf spot disease caused by B. setariae has been recorded previously on cassava in China or elsewhere.

References: (1) P. Busey. Crop Sci. 43:1899, 2003. (2) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) H. D. Wells and W. W. Hanna. Phytopathology 78:1179, 1988.