柑桔黃龍病之研究及健康管理

Abstract

柑桔黃龍病(Huanglongbing=HLB, greening)於1943 年由華南首次發現報告，於1947 年在南非發生以greening 病名發表。在台灣光復後6 年，於1951 年首次發現此病，經松本教授認定為新病，並命名為立枯病(Likubin)，到1956 年此病蔓延本島各地嚴重發生，松本教授等開始其病因研究，証實為經嫁接可傳播之似病毒病害，此病於1961 年正式在美國Florida 召開第二屆IOCV 國際會議由松本教授發表。病原菌以電顯切片觀察發現係韌皮部侷限，存活於篩管中不能培養之特異細菌，屬Candidatus Liberibacter asiaticus，作者等以病原性及毒性之生物鑑定發現有四種系統(strain)存在於台灣，並常與萎縮病毒(CTV)及破葉病毒(CTLV)混合感染。此病造成病株葉片出現黃化斑駁(yellow mottling)病徵，樹勢衰弱終至枯萎。在分子診斷試劑開發上，經病菌基因DNA 選殖，獲得病菌特異性DNA 片段，而研發為一非放射性標誌之核酸探針(DNA probe)，用於點漬雜配法。之後又經定序設計引子，建立PCR 測定法，成為快速準確檢疫技術。另再創製碘液簡易檢疫法。此系統性病害之防治一般採用綜合防治之健康管理法，包括無病柑苗之生產與種植、傳染源之撲滅及媒介昆蟲之防驅以防止病害之傳播蔓延。建立無病柑桔種苗體系為首要防治對策。頂梢微體嫁接改進法獲得之無病柑桔原種及無病原種網室之建立，可供無病芽/穗繁殖株之生產而建立其苗圃，由合格網室苗圃業者以快速繁苗法生產無病柑苗，供柑農種植。從媒介柑桔木蝨之寄主範圍中發現野生烏柑仔(Severinia buxifolia)為黃龍病菌之中間寄主，帶病病株成為傳染源，田間衛生以病株及中間寄主之隨時掘除為要。流行病學研究顯示由柑桔木蝨週年密度動態、帶毒率之頻度、飛來蟲量頻度及柑株感染率與潛伏期等監測資料判定嘉義地區柳橙之感染高峰在2 月底至3 月間，此期為柑桔木蝨之防驅關鍵期。對罹病大柑株在發病初期(病葉比率占樹冠一半以下)以加壓注射器將1000 ppm四環黴素注射病株三次，具有藥療效果。按驗證法規並進行無病苗之生產與推廣為黃龍病防治之首要對策。 Citrus Huanglongbing (HLB) locally called “Likubin” was fist identified in Taiwan in 1951, and became seriously epidemic over the island in 1956. Thenafter, Matsumoto and his colleagues started etiological study on the disease and clarified the transmissible nature as a virus-like disease and finally found phloem-limited fastidious bacteria with pleomorphic morphology to be associated with HLB. Nowadays, the tropical form of psyllid-borne HLB pathogen has been nominated as Candidatus Liberibacter asiaticus. So far, the authors identified four strains of HLB pathogen in Taiwan. The greening disease commonly with mix-infection of citrus tristeza and/or tatter leaf viruses, causes sever yellow mottling and decline, and ultimately death of citrus tree. These systemic diseases are generally controlled by the integrated control measures including cultivation of pathogen-free seedlings, elimination of inoculum sources and prevention of secondary spread by controlling vector insects. Establishment of pathogen-free nursery system is primarily important for preventing prevalence of these diseases. The pathogen-free nursery system has been properly performed through improved techniques of shoot-tip micrografting for obtaining virus/HLB-free citrus foundation stocks, and then production of healthy citrus seedlings within insect-proof screen house under health certificate regulation in Taiwan. The precise and rapid indexing techniques are indispensable for management of pathogen-free nursery system and health management of citrus tree in the field. Production and cultivation of pathogen-free and high-quality nursery trees are fundamentally important component of HLB management. A healthy citrus orchard planted with pathogen-free seedling may outlive the grower. Healthy citrus trees have a great potential for sustainable high yield over many decades, provided appropriate horticultural and disease management practices are followed.