CURRENT STUDIES IN PITAYA VIRAL DISEASES
Pitaya (Hylocereus spp.), also known as pitahaya or dragon fruit, is a perennial climbing epiphytic plant in the family of Cactaceae. This cactus plant is native to the forests of Mexico, Central and South America and is becoming a high-value tropical fruit crop in Taiwan. Commercially cultivated pitayas are mainly propagated from cuttings. If mother plants are infected with viruses, diseases can be spread easily. In 2001, the research team of Professor Ruey-Fen Liou at National Taiwan University (NTU) published the first paper of pitaya-infecting virus, Cactus virus X (CVX) which is a member of the genus Potexvirus, in Taiwan. They purified a CVX-Hu isolate, prepared its antiserum and developed the DAS-ELISA for CVX detection. In addition, they cloned and completely sequenced the CVX-Hu isolate. Liao et al. at Taiwan Agriculture Research Institute performed a pitaya disease survey and the result indicated that CVX was widespread in the pitayas and the infection incidence ranged
50%~90% in different areas of Taiwan and Kinman.
In 2005, we isolated a new NTU strain of CVX from the pitayas grown in the experimental farm at NTU, and obtained the complete genomic sequence of CVX-NTU. During 2006 to 2008, one potexvirus, Zygocactus virus X (ZyVX), was isolated and identified by our laboratory in the pitaya samples collected from the Yanmingshan orchard. This is the first report of ZyVX in Taiwan and also the first record of pitaya-infecting ZyMV. In addition, another potexvirus was isolated from the same batch of pitaya samples, and it was identified to be a new species of the genus Potexvirus. Because this new potexvirus was originally found in pitaya and thus named as Pitaya virus X (PiVX). The results of phylogenetic analyses on the full-length genomes of CVX, ZyVX-P39, PiVX-P37 and other published potexviruses demonstrated that all Cactaceae-infecting potexviruses belong to the same cluster. To study the distribution of CVX, ZyVX and PiVX in the pitaya field, we developed a multiplex RT-PCR method for field survey. Pitayas sampled from several production areas in Taiwan were investigated for the infection rate and distribution of potexviruses. The field survey results showed that CVX was indeed widespread; ZyVX was not present in Taichung and Changhua; PiVX existed in Taipei, Ilan, Taitung and Changhua. If we can use this multiplex RT-PCR to screen pitaya propagation materials, a certification program for virus-free pitayas can be established to provide high quality pitaya seedlings for farmers.