Authors
Q. Du,
S. F. Zhao,
C. L. Wu, and
L. Kong, The Key Laboratory of Prevention and Control for Oasis Crop Disease, Shihezi University, Shihezi, Xinjiang, 832003, China; and
P. Zhang, Institute of Agriculture Science and Technology of Division No. 2 of Xinjiang Production and Construction Group, Korla, Xinjiang, 841000, China
Ziziphus jujuba Mill. (Rhamanaceae), commonly known as Chinese jujube, is a plant that is native to China and widely distributed in China and South Korea. Chinese jujube is a traditional Chinese medicine and food that has been used for thousands of years. From spring 2011 to spring 2012, Chinese jujube trees (<4 years old) from commercial orchards in Kashi and Akzo developed symptoms of severe wilting, chlorosis, and stunting. The taproot and lateral roots exhibited black, sunken, necrotic lesions, and progression of these symptoms eventually caused mortality of infected trees. In April 2011 and 2012, 15 trees with root rot symptoms were collected from different orchards, and a fungus was recovered consistently from symptomatic rhizome samples that had been surface-sterilized with 0.1% mercuric chloride and plated onto potato dextrose agar (PDA). Thirteen isolates were obtained. Colony growth on PDA was initially white but gradually turned grayish yellow-brown, with the lower side of the plate turning a deep yellow-brown. Colony texture was felty and slightly raised with no concentric growth zones. Pycnidia formed abundantly after 30 days, and each exuded a dull orange cirrhus containing conidia. The conidiomatal stromata were immersed in the bark, erumpent, discoid, convex to conical, and 1.53 ± 0.13 mm in diameter. Discs were dark brown or grey, nearly flat, circular, and up to 0.6 mm in diameter, with one to eight lateral ostioles. Ostioles were brown, 40 to 80 mm in diameter, and located above the disc surface. Locules were globose, with four to eight in a stroma, undivided, and did not share common walls. Conidia were hyaline, eguttulate, and allantoid, with sizes of 12 to 20 × 1.0 to 1.5 μm. The agar culture and conidiomatal stromata morphology were in accordance with descriptions of Cytospora sacculus (1,4). PCR amplification of the internal transcribed (ITS) spacer region of rDNA (using primers ITS1/4), combined with amplification of the β-tubulin gene (using primers BT1a/BT1b) (2,3), followed by sequencing of the amplified DNA, led to identification of isolate 6T-17 as Valsa ceratosperma (anamorph C. sacculus). The sequence of the ITS1-5.8s-ITS2 region of rDNA (GenBank Accession No. JX560175) matched the ITS sequence of V. ceratosperma Accession AB369475 with 99% similarity, and the β-tubulin gene sequence (KC840674) resulted in a 99% match with that of V. ceratosperma Accession EU219136. Mycelial plugs of a culture of each fungal isolate on PDA were inoculated onto 10 rhizomes of 2-month-old seedlings of Z. jujube, and maintained in a greenhouse at 25 to 30°C. Ten rhizomes of 2-month-old seedlings of Z. jujuba were inoculated with non-colonized PDA plugs as the control treatment. After 2 months, all inoculated seedlings showed chlorotic leaves and root rot symptoms similar to those observed in the original commercial orchards. No symptoms were observed on the control plants. The pathogen was reisolated successfully from symptomatic plants but not from the control plants, fulfilling Koch's postulates. To our knowledge, this is the first report of C. sacculus causing a root rot of Z. jujuba trees in China.
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