In February 2014, a severe disease on maize (Zea mays L.) broke out in the fields of central and southwestern Taiwan and caused yield losses in sweet corn production. Chlorotic spots first appeared at the base of infected leaves and later developed into systemic mottling. Diffused necrotic patches were also found on leaves or husks of the diseased plants. Moreover, severe rosetting and stunting accompanied by abnormalities in ear production were observed on mature plants. Eighteen leaf samples from symptomatic plants were collected and submitted to our Plant Diagnostic Clinic for virus diagnosis. All of the samples were first tested by reverse transcriptase (RT)-PCR to detect Maize stripe virus (MSpV) and by indirect ELISA to detect Maize dwarf mosaic virus (MDMV) or Sugarcane mosaic virus (SCMV), which were endemic to this area (1). Only 2 out of 18 samples were positive for MDMV, SCMV, or mixed infection of both viruses. Sap inoculation tests conducted on seedlings of sweet corn cv. Honey 236 indicated that the MDMV- and SCMV-negative samples still had an unknown pathogen causing original symptoms in the receptor plants. The isolate from Yunlin county reacted only with the antibody to Maize chlorotic mottle virus (MCMV) (AC Diagnostics, Fayetteville, AR) in ELISA. For further identification, the MCMV-specific primers (forward: MCMVg3514F-GGGAACAACCTGCTCCA; reverse MCMVg4014R-GGACACGGAGTACGAGA) were designed from the nucleotide sequence of MCMV coat protein (CP) gene. In RT-PCR using the AccuPower RT/PCR PreMix kit (Bioneer, Daejeon, Korea), an expected 500-bp DNA fragment was observed. This PCR product was cloned and its nucleotide sequence was determined by Mission Biotech Co., Taipei, Taiwan. BLAST analysis of the CP gene of the MCMV-Yunlin revealed the maximum nucleotide identities (99%) with Chinese Sichuan isolates (GenBank Accession No. JQ984270) and 98% identities to four Chinese Yunnan isolates (GU138674, JQ982468, JQ982469, and KF010583) and one Kenya isolate (JX286709), compared with 97% to Kansas isolate (X14736) and 96% to Nebraska isolate (EU358605). Subsequently, the complete nucleotide sequence of the viral genome (KJ782300) was determined from five overlapping DNA fragments obtained from independent RT-PCR amplification. The virus isolate was infectious to sweet corn cultivars Bai-long-wang, Devotion, SC-34, SC2015, and Zheng-zi-mi, on which similar symptoms were developed after mechanical inoculation. During the spring of 2014, a total of 224 sweet corn samples were collected from the epidemic areas of Taichung, Yunlin, Chiayi, and Kaohsiung counties. Samples (n= 161) reacted positive for MCMV in ELISA and/or RT-PCR. In the field survey, more than 20 adult thrips might be observed on an MCMV-infected plant. Two species of Frankliniella were found on maize plants: F. williamsi Hood and F. intonsa Trybom. Maize thrips (F. williamsi), an occasional pest of maize occurring during winter and spring in Taiwan, was characterized by its abdominal sternite II on which 1 or 2 discal setae of equal length with posteromarginal setae were borne (2). Samples with 1, 5, 10, and 30 F. williamsi collected in the field were tested by RT-PCR; MCMV was detectable not only in the pooled crushed bodies but also in a single maize thrips. This is the first report of MCMV occurrence on maize in Taiwan and of the virus transmitted by maize thrips.
References: (1) C. T. Chen et al. Taiwan Sugar 37(4):9, 1990. (2) C.-L. Wang et al. Zool. Stud. 49:824, 2010.
