聖誕紅細菌性葉斑病之診斷與防治

Abstract

聖誕紅在全世界或在台灣皆為重要的盆花作物，台灣主要供應內銷市場，花農於每年6~8月以扦插繁殖，至9月中旬苞片開始轉紅，於聖誕節前出售；市場以3吋與5吋盆的規格為主，年產約130萬盆。聖誕紅的細菌性葉斑病是近年來聖誕紅栽培產業上遭遇的重大難題，在2005年首次於台中市東勢區被報導，鑑定其病原菌為Xanthomonas axonopodis pv. poinsettiicola (Xap)，造成葉部褐色病斑、周圍有明顯黃暈圈，當病斑擴大後會形成壞疽，最終使葉片枯死而落葉。此病害在夏天好發，正值扦插繁殖期，感染幼株造成嚴重落葉，因而必須重新扦插種植，導致植株生長延遲及盆花產量下降，倘若趕不及聖誕節前出售將造成農民巨大損失。有鑑於此，建立病害診斷方法及擬定防治策略為本研究之重點目標。本研究初步利用Xan-D選擇性培養基培養病原菌，若為Xanthomonas屬的病原菌將會在此培養基上出現水亮、上凸、由黃色變綠色的菌落，周圍有透明圈和霧圈，並讓培養基從橘黃色轉變為綠色。另外以NCBI上的Xap序列為參考，包括estA、hrpF、gumD、gyrB、rpoD等基因序列，自行設計出專一性的引子對，其中rpoDⅡ-b引子對可用PCR法進行Xap的快速鑑別。藥劑防治策略方面，選擇盤尼西林 (penicillin)、鏈土黴素 (streptomycin + oxytetracycline)、幾丁聚醣 (chitosan) 與奈米級二氧化鈦 (titanium dioxide) 等防治資材進行預防與治療試驗，並挑選目前市場上主要的七個聖誕紅品種在溫室做測試。在治療試驗的結果方面，結果顯示施用鏈土黴素1500倍稀釋液效果較佳；預防試驗方面，發現若在病菌感染前3~4天施用幾丁聚醣，則有呈現保護的效果。綜觀整個防治試驗結果，聖誕紅品種影響病害嚴重的程度較大，感病品種不論在事先預防用藥或事後噴藥，防治效果都不彰。此外，病原菌的第一次感染也是影響病害後續發展的主要因素，若可以針對颱風後帶來的傷口和發病高峰，做出預防性的施藥 (如使用幾丁聚醣)，及做好防風設施以減少傷口產生，又如以滴灌取代撒水系統等，都將有助於防止本病害的嚴重發生。

Poinsettia (Euphorbia pulcherrima) is an important ornamental plant in the world as well as in Taiwan. It is mainly cultivated for the domestic flower market with the size of 3 and 5-inch pots in Taiwan, and approximately 1.5 million pots of poinsettia are produced every year. They were propagated by cutting from June to August, and their color-turning of red bracts initiates in mid-September ready for the sale during Christmas holidays. The newly surfaced disease, leaf spot of poinsettia, caused by Xanthomonas axonopodis pv. poinsettiicola (Xap) was first discovered in the Dongshih Township, Taichung in November 2005. It showed brown spots surrounded by yellow haloes on the leaves. The spots enlarged rapidly, coalesced into necrosis and finally lead to early defoliation. The disease often happened in the summer and retarded the vegetative growth period, which significantly reduced the poinsettia production and lost the best timing for selling. This study was dedicated to develop efficiently diagnostic and control measures for this disease to provide assistance for the poinsettia industry. The selective medium, Xan-D, was used for the culture and differentiation of Xanthomonas identified by the wet-shiny, convex and yellow-green colonies surround with milky and clear zones. For the further molecular detection, the specific primers of Xap were also designed for PCR based on the published Xap genomic sequences including estA, hrpF, gumD, gyrB and rpoD genes. The primer pair rpoDⅡ-b showed the better specificity to Xap. In the control trials, several chemicals such as penicillin, streptomycin + oxytetracycline, chitosan and nanoscale titanium dioxide were used for either prevention or therapy tests in the greenhouse using totally 7 important cultivars of poinsettia. The results of therapy tests showed that the streptomycin + oxytetracycline is most effective for this disease control especially in the initial stage of disease development. The results of prevention tests showed that the chitosan had good protective effect when it was used before Xap infection. Various cultivars of poinsettia showed different susceptibilities for Xap, and all of tried control measures seemed to be not effective in those cultivars categorized into high susceptibility. The application of adequate bactericides or protectants (ex. chitosan), wind-proof facilities and dipping irrigation system (instead of spraying) might be recommended for the growers to improve the control of this disease.