柑桔木蝨傳播黃龍病之生態研究

Abstract

黃龍病為柑桔重要病害，病原為Candidatus Liberibacter asiaticus，普遍發生為害台灣各柑桔品種，可經由嫁接或柑桔木蝨(Diaphorina citri)媒介傳播，植株受感染後無法根治，因此黃龍病的防治首重預防，可採行之方法為種植無病健康苗及避免帶毒木蝨危害而造成再感染。寄主植物之調查發現黃龍病寄主種類有柑橘屬、金柑屬、枳殼屬和山柑仔屬植物，柑桔木蝨寄主種類有柑橘屬、金柑屬、枳殼屬、山柑仔屬、月桔屬、黃皮屬及木蘋果屬植物。柑桔木蝨的發生受柑桔新芽數量的影響，一年有三個高峰期，春芽期最高，夏芽期次之，秋芽期最低。木蝨的遷飛現象發生於3~5月與成蟲羽化後族群密度的高低有關。為了解決健康苗種植田間後再感染的嚴重問題，本試驗由探討木蝨的媒介傳播特性及黃龍病的田間傳播生態發現，柑桔木蝨為不經卵傳播之終生帶毒的媒介昆蟲，主要在若蟲期獲毒，病株上繁殖的木蝨將來較可能成為帶毒蟲，荒廢柑園則是孕育帶毒木蝨的最佳場所。田間黃龍病主要靠木蝨成蟲媒介傳播，單隻木蝨即可造成感染，但木蝨的傳播效率低，帶毒木蝨傳毒成功機率僅12.9%，需有大量的木蝨才可能造成感染。柑桔木蝨帶毒蟲率會隨季節而變動，3月及8月是一年當中木蝨帶毒蟲率較高的時期，帶毒蟲率變動的原因受到溫度、新芽數量及植株內病原濃度的影響。黃龍病的感染與木蝨族群密度、木蝨遷飛數量及木蝨帶毒蟲率高低有關，主要發生於3~5月春芽期。黃龍病的防治可由清除荒廢柑園、挖除病株及芽期施藥等工作抑制木蝨族群密度與減少感染源。黃龍病的病勢發展曲線符合推理增長之數學模型，初期傳播速率極為緩慢，當超過某一臨界點後病害會快速增加而造成流行。黃龍病的管理可由減少感染源及降低傳播速率著手，管理原則為控制病勢的發展在大發生的臨界點之下，然後配合各種防治措施使病害數量逐漸減少來維持柑桔產能。

Huanglongbing (HLB) caused by Candidatus Liberibacter asiaticus is the most destructive disease of citrus in Taiwan. It affects most citrus species, hybrids, and cultivars. Pathogens can be transmitted by grafting and by insect vectors. Once the healthy seedlings are planted out in the field, control of huanglongbing depends on control of the psyllid vector. Huanglongbing affected 4 genera of the Rutaceae, namely Citrus, Poncirus, Fortunella and Severinia. Diaphorina citri feeds and breeds on the entire group of horticultural Citrus, with additional hosts in seven different genera belonging to the Rutaceae, namely Citrus, Poncirus, Fortunella, Severiniam, Murraya, Clausena and Feronia. Psyllids in citrus orchards showed that the population of adults had three peaks per year which were coincided with the periods of citrus flushing at spring, summer and fall. The number of adults was most abundance in the first population peak. Flight phenology of D. citri occurred from March to May when adults that emerge from nymphs and psyllid populations are highest. Citrus psyllids acquire the huanglongbing bacterium primarily in the nymphal stage, and adult psyllids arising from nymphs that feeding on diseased shoots are easy to become vectors. A single adult of D. citri can transmit HLB. Citrus psyllids are not highly efficient vectors of huanglongbing in Taiwan, and epidemics occur only when high vector populations and an extensive inoculum reservoir are present. The trend of natural infectivity during the year indicated a high proportion of D. citri collected in March and August were PCR-positive for the pathogen. The efficiency of huanglongbing transmission is related to high vector population, percentage of psyllids that carrying pathogens and vector dispersal activity. Thus, flight activity and seasonal rhythms of D. citri populations would be very important considerations for management. The epidemic pattern follows the typical sigmoidal curve. The disease is cumulative and increases with time, depending on the abundance of the vector and the availability of infected plants. It starts slowly, increases gradually year after year until it reaches the exponential stage and only then the epidemic increases rapidly. Control strategy is to prevent disease damage from exceeding that level where profit or required yield is significantly diminished. Reduce the initial inoculum and reduce the rate of infection, two major strategies can be used for managing HLB disease epidemics, and we can organize our plant disease control tactics under one or more of these overall strategies.