(28(1):35-44)The citrus variety improvement and virus-free nursery tree propagation program in Taiwan

為獲得優良無病毒之柑橘種苗，曾調查柑橘母樹，並先後經初選與複選之步驟，計獲外觀生長強健產量豐富之椪柑132 裸、桶柑50 裸、柳橙80 棵及晚崙西亞橙100 棵作為母樹。後逐年採取果實行品質分析，並採取接穗以芽接繁殖於二年生健全廣東檸檬幼苗上，復經每日以40℃ 照光16 小時及30℃ 無照光8 小時交替循環溫度之熱處理三個月或更長時間，由幼苗採取新生嫩穗（young shoot）作嫁接繁殖（shoot-tip Propagation )，育成苗木後再利用不同指示植物檢定，獲得無Likubin, Tristeza, Psorosis, Concave gum 及Exocortis 之椪柑及桶柑苗各11 株作為原種樹，分植隔離網室。以同法施於柳橙及晚崙西亞橙品種，雖經熱處理，但測定後，仍含有Exocortis 病，故未有該兩品種之原種樹。 The citrus virus and virus-like diseases have become the citrus industry serious problem in Taiwan. It needs to obtain virus-free citrus nursery trees for commercial cultivation. In 1974, this cooperative program was established by the Departments of Horti-culture and Plant Pathology of Taiwan Agricultural Research Institute and other related organizations. At the first year, we aimed at the survey of citrus virus diseases and selection of apparently healthy trees with best horticultural characters, high production, and good quality as the parent source trees. More than six hundred trees from different areas of 22 orchards were selected. Four major cultivated varieties, namely Ponkan ( Citrus poon-ensis Hort. ), Tankan ( Citrus tankan Hay. ), Liuchen and Valencia ( Citrus sinensis Osbeck. ) were included in this program. Heat therapy was used for eliminating certain viruses. Citrus budwoods collected from parent trees were bud-grafted on Rangpure lime rootstock. They were exposed to the alternate temperature of daytime ca. 40℃ for 16 hr. and nighttime 30℃for 8 hr. for 3 months or longer in the heat chamber. In the second year, young shoots cut from those treated plants were propagated and reindexed the existence of viruses after a period of growth. According to the analysis of horticultural characters and indexing results, over half of the trees which were considered to be prospective parent source trees were cancelled because of the inferior horticultural characters and the existence of virus diseases in the third year. The results indicated that tristeza virus existed in most of the symptomless trees, psorosis and concave gum virus in certain orange orchards. It was surprised that all of the selected orchards of Liuchen and Valencia orange were infected with exocortis. Since exocortis is tolerant to heat, we failed to get exocortis-free materials from the described two varieties. However, we have successfully obtained some of the Ponkan and Tankan seedlings which are free from citrus likubin, tristeza, psorosis, concave gum and exocortis