Pear (Pyrus spp.) is cultivated worldwide and it is one of the economically important fruit crops in Taiwan with ≈8300 ha of cultural areas, which account for cash value of 3.6 billion NT dollars in 2008, just next to citrus, pineapple, and mango production. The good taste of Japanese pear (P. serotina Rehder) fruits has made it a very popular fruit in Taiwan. However, the Japanese pear cultivars required high-chilling temperature and produced no fruits in the low elevation area of Taiwan. The farmers invented unique top-working cultivation (Lin et al., 1990) to produce high-quality pear fruits by grafting flower bud-containing pear scions imported from Japan onto the local dominant low-chilling required cultivar, P. pyrifolia var. Hengshen, during November and January every year. The major pear production area was located in central Taiwan where 71.9% and 19.1% of the total yields were produced from Taichung County and Miaoli County, respectively. The popular Japanese pear cultivars of Shinko, Kosui, Hosui, and Seiki are sold in Taiwan and exported to international markets, including Hong Kong, Singapore, Malaysia, and Vietnam.
Pear is a natural host for several graft-transmissible pathogens of economical importance (Motoshima et al., 1983; Nemeth, 1986; Pfaeltzer, 1962). Viral diseases had not been reported in Taiwan until 2003 (Jan et al., 2003). In our previous works (Jan et al., 2003), Apple stem grooving virus (ASGV) and Apple chlorotic leaf spot virus (ACLSV) were detected on the pear leaves showing virus-like symptoms by reverse transcription–polymerase chain reaction (RT-PCR). However, the virus isolation and identification including back-inoculation to pear plants were not accomplished.
ACLSV is one of the most economically important graft-transmissible pathogens in fruit trees worldwide (Nemeth, 1986; Yoshikawa, 2001). ACLSV belongs to Trichovirus in the family Flexiviridae (Martelli et al., 1994, 2007) and it is made of filamentous particles of ≈640 to 760 × 12 nm in size. ACLSV can be transmitted by mechanical inoculation and grafting, but the natural dissemination in woody plant species is limited as a result of ineffective vectors (Martelli et al., 2007; Yoshikawa, 2001). ACLSV causes severe yield loss of many crop species. It infects various rosaceous fruit trees (Nemeth, 1986; Yoshikawa, 2001), including almond (Spiegel et al., 2005) and quince (Mathioudakis et al., 2007; Rana et al., 2007). ACLSV induces symptoms of deformation, reduced size, chlorotic leaf spots, and ring pattern mosaic on leaves of the susceptible cultivars of apple and pear (Nemec, 1967; Yoshikawa, 2001). It also causes apple russetting and top-working diseases (Yanase et al., 1979). Several virulent isolates can cause symptoms of bark split and pseudopox in some plums or peaches and graft incompatibility in apricots or cherries (Nemeth, 1986; Yoshikawa, 2001).
Recently, a pear plant showing symptoms of small leaves and leaf distortion in Taiwan has been identified to be caused by a pear isolate of ASGV (Wu et al., 2010). Another pear disease, consisting of chlorotic spots on the leaves of the pear cultivar Hengshen, had been observed in the orchards in central Taiwan. The etiology of this chlorotic spot disease of pear has not been clarified. The objective of this study was to identify and characterize the causal agent of this pear disease. We report here the isolation, serological and molecular characterizations of the causal agent, ACLSV, of the chlorotic spot disease of pear in Taiwan. We also performed sequence analyses by using the amino acid sequences of coat protein (CP) genes of this virus isolate and those of other ACLSV isolates available in GenBank to gain the information on the molecular variability and phylogenetic relationship among ACLSV populations.
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Yanase, H. Yamaguchi, A. Mink, G.J. Sawamura, K. 1979 Back transmission ofAnn. Phytopathological Soc. Jpn. Apple chlorotic leaf spot virus(type strain) to apple and production of apple topworking disease symptoms in Maruba kaido ( Malus prunifoliaBorkh. var. ringoAsami) 45 369 374