柑橘黃龍病之發生生態及防治研究

Abstract

1951 年柑橘黃龍病於臺灣北部首次發生，病菌初期只危害臺灣主要柑橘品種如椪柑、桶柑及柳橙等，1971 年後開始感染柚類，並逐步擴及所有品種，顯示病菌系統近幾十年來產生演變。因此本研究以四種鑑別柑橘品種(椪柑、柳橙、文旦與檸檬) 進行生物檢定，鑑別出目前台灣田間至少有 4 種黃龍病菌系統：系統 I 僅對椪柑與柳橙具病原性且產生典型病徵，為原始系統；系統 Ⅱ 對四種鑑別品種皆具病原性與高毒性，並可快速於其內增殖；系統 Ⅲ 僅在椪柑與柳橙引起中度病徵，在文旦出現輕度病徵。系統 Ⅳ 僅感染椪柑及柳橙，但幾無病徵表現。目前田間以系統 Ⅱ 為最優勢族群。於田間採集疑似黃龍病樣品中，平均僅約 3 成感染黃龍病，且常與柑橘病毒複合感染。經接種試驗發現，受黃龍病菌與柑橘病毒複合感染之植株，其病徵與植株萎縮程度皆嚴重於僅受到黃龍病單一病原感染株。為瞭解黃龍病標準病徵，於無病毒紅江橙園區採集各種黃化材料，並依病徵分為六型，發現以第六型 (黃綠不均斑駁狀) 病徵感染比率最高(72.3 %)。於柑橘品種對黃龍病菌之抗感性研究顯示，椪柑、桶柑對黃龍病菌最感病；晚崙西亞次之；文旦、檸檬與金柑再次之。各砧木依抗感性可分為感病型、中度耐病、高度耐病及抗病型四類。另研究發病菌在植株的分佈情形極不平均且病菌於涼溫之春秋兩季含量較多。化學治療上，本試驗以Su 與Leu 1972 年起使用的四環黴素療法，加以改進試驗：首先設計一灌注裝置於室內分析四環黴素與青黴素藥效，以四環黴素效力較佳。之後改進田間灌注方式，以加壓灌注器在 80 磅下，灌注抗生素於田間病柚老柑株，各抗生素配方均可有效降低病株病級數，以四環黴素單劑配方較佳。以 1000 ppm 四環黴素灌注 3 次可達良好治 療效果，處理後樹勢恢復，可產生好品質柑果。此藥療法引起暫時性藥害包括新生葉細長、葉脈褐化、葉片焦枯與大量落葉等現象，但皆可於半年後恢復。 Huanglongbing ( HLB) first occurred in northern Taiwan in 1951. Previously the HLB pathogen infected main citrus cultivars in Taiwan such as Ponkan mandarins, Tankan tangors and Liucheng sweet oranges whereas it did not infect pummelos. However, HLB pathogen began to infect pummelos in 1970s implying that the evolution of the pathogenicity occurred in recent decades. This study was designed to differentiate the strains of HLB bacteria (HLBB) through the bioassays with four indicator hosts.Four different HLBB strains were identified based on the results of their pathogenicity and virulence on indicator plants. Strain I was only pathogenic on mandarins and sweet oranges by inducing typical HLB symptoms. Strain Ⅱ waspathogenic with high virulence on all differential cultivars with fast multiplication rates. Strain Ⅲ only caused intermediate symptoms on mandarins and sweet oranges and mild symptoms on pummelos. Strain Ⅳ was a mild strain which could only infect but caused no symptoms on mandarins and sweet oranges. Strain Ⅱ was prevalent in fields. In field surveys, about 30% of citrus samples exhibiting yellows symptoms were infected by HLB and were popularly mix-infected with citrus viruses. Plants with mix-infection exhibited more severe symptoms than those infected only by HLBB. In order to understand the relations between HLB and various yellows symptoms showing in trees, samples of 6 different symptomatic patterns were collected from virus-free sweet orange field for comparison. After HLB inspection, the highest incidence of HLB was found in type 6(72.3%).In the study of resistance/susceptibility of citrus cultivars to HLB disease, Ponkan and Tonkan were the most susceptible, Valenica sweet orange showed moderate in susceptibility, and Wentan, Eureka lemon and kumquat were tolerant cultivars. According the disease index, the rootstock cultivars were categorized into 4 types; they were susceptible, moderate tolerant, highly tolerant, and resistant type. In addition, the distribution of HLBB in infected trees was diverse and the seasonal dynamic of HLBB population was higher in spring and autumn. In chemotherapy, we improved the transfusion method based on Su and Leu’s 1972 report. An infusion device was designed for analyzing the effect of various antibiotics in which tetracycline was shown to be better than penicillin-G. In field trials, all antibiotic formulas injected to old diseased pummelo trees with an air-pressured injector set at 80 lbs. could reduce disease index of HLB. Three applications of 1000 ppm tetracycline resulted in the best therapeutic efficacy. After treatments, pummelo trees recovered and produced normal fruits with good quality. This improved therapeutic procedure caused temporary phytotoxicity including vein browning, slender leaves, leaf scorching, and defoliation, but the injured trees would recover after half of a year.