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First Report of Phytophthora palmivora Causing Root Rot of Cassava in China

Cassava (Manihot esculenta) is an economically important crop grown widely in South China. Seventy percent of the cassava grown is used for starch and ethanol production and it has become the foundation of local food and bioenergy systems. In November 2010, a new root rot disease was found on cv. HuaNan205 from a cassava plantation in Danzhou, Hainan Province. Disease occurred on 30% or less of the plants. Initially, the upper leaves wilted at noon and recovered in the evening. Eventually, infected plants no longer recovered and the whole plant wilted and died. Root rot symptoms consisting of irregular brown patches occurred on the tuberous roots. Symptomatic root rot tissue was cut into 1-cm pieces, washed in distilled water, and soaked in a solution of 1% sodium hypochlorite for 3 min. A subsection was cut from each sterilized piece, placed on a plate of V8 agar medium, and incubated at 28°C for 7 days. Pathogenicity was established by following Koch's postulates. In July 2011, 10 plants of cassava cv. HuaNan205 were selected from a disease-free plantation in Danzhou. The pathogen was cultivated on V8 agar at 28°C for 14 days. Four holes were established 15 cm from the base of the cassava plants. Five plants were inoculated with 100 mL of the mycelial suspension in each of the four spots and covered by soil. The other five plants were treated with sterile water as control. Plants were maintained for 4 months. All five of the inoculated plants wilted and two died, while the control plants grew normally. Symptoms similar to the original root lesions were observed on tuberous roots of inoculated plants, while only scars formed on tuberous roots of control plants. The pathogen was reisolated from the lesions of inoculated plants. Microscopic examination showed the sporangia as papillate and ovoid with the widest part close to the base. They were easily washed off and each detached sporangium contained a short pedicel 1.2 to 6.9 μm long, average 2.9 μm. Chlamydospores were readily observed on diseased roots and observed in pure cultures on V8 agar. Morphological characteristics of the specimen were similar to the descriptions for Phytophthora palmivora (2). Genomic DNA of this isolate was extracted with a cetyltrimethylammoniumbromide protocol (3) from mycelium and used as a template for amplification of the internal transcribed spacer (ITS) region of rDNA with primer pair ITS1/ITS4 (1). The sequence (GenBank Accession No. HE580279) exactly matched several sequences (e.g., GenBank Accession Nos. HQ237481.1, AY745750, and AY745751) of P. palmivora. To our knowledge, this is the first report of root rot caused by P. palmivora on cassava in China.

References: (1) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (3) J. R. Xu et al. Genetics 143:175, 1996.