蘭嶼鐵莧(Acalypha caturus Blume)又稱圓葉仔,英名為Lutao Copperleaf,為大戟科 (Euphorbiaceae)鐵莧屬 (Acalypha)之常綠小灌木,分布於菲律賓與蘇門答臘等地,台灣主要產於台東地區之都蘭、綠島與蘭嶼。蘭嶼鐵莧為綠島早期飼養梅花鹿之重要飼草,蘭嶼的雅美族人也取其嫩葉餵養羊與鹿。2009年本研究室於台東地區發現田間蘭嶼鐵莧之葉片具有黃綠嵌紋病徵,田間病害發率為35%。蘭嶼鐵莧葉片呈現嵌紋之罹病株經以棉蚜(Aphis gossypii Glover)媒介傳播分離可獲得一種球形病毒分離株(KC-1),病毒粒子直徑約21-24 nm,此病毒經由棉蚜感染蘭嶼鐵莧葉片可造成嵌紋及新葉葉脈黃化或嵌紋病徵,與田間罹病株之病徵相似。KC-1病毒株除蚜媒傳播外無法經由機械與種媒傳播;測試桃蚜與棉蚜二種蚜蟲媒介傳播KC-1病毒株,結果顯示僅棉蚜可傳播。棉蚜以持續型方式傳播KC-1病毒株,其最短獲毒取食停留時間為12小時,傳毒率為19%,最短接種取食停留時間為30分鐘,傳毒率為11%,單一隻棉蚜即可有效傳播病毒,傳毒率為14%,若蟲的傳毒效率較成蟲好,最長保毒期限為13天,病毒可經孤雌蚜繼代傳播,目前已知可持續到F3世代。寄主範圍測定9科62(品)種供試植物,僅大戟科與茄科共2科18(品)種植物可被感染,分別出現局部性黃斑及系統性黃斑或嵌紋之病徵。電顯觀察KC-1病毒株感染之蘭嶼鐵莧病葉葉脈組織切片,可於韌皮部細胞內觀察到細胞質凝聚病變與病毒粒子結晶陣列聚集。以100 g菸草病葉為材料經高低速交替離心及蔗糖密度梯度離心純化,可獲得收量約0.33 mg純化病毒。依據上述KC-1病毒株之粒子形態、蚜媒傳播特性與組織病理學,推定本研究之病毒為韌皮部限侷型病毒,可能屬於Luteoviridae科,此為台灣地區首次有關蘭嶼鐵莧嵌紋病之研究報導亦為世界之首例。
Lutao copperleaf (Acalypha caturus Blume) is an evergreen small shrub, which belongs to the family Euphorbiaceae, Acalypha genus. Lutao copperleaf is widely grown in Philippines and Sumatra. In Taiwan, it is distributed at Dulan, Lyudao, and Lanyu of Taitung regions. In early days, Lutao copperleaf was important forage for feeding formosan sika deers in Lyudao. Also, Yami tribe people get the leaves to feed sheep and deers in Lanyu. In 2009, an undescribed virus disease, yellow and green mosaic on the leaves of Lutao copperleaf was found in Taitung, Taiwan. The field survey revealed that the rate of incidence was about 35%. An isometric of KC-1 virus isolate of 21-24 nm in diameter was isolated from Lutao copperleaf showing mosaic on leaves through means of Aphis gossypii transmission. KC-1 virus isolate infected Lutao copperleaf induced mosaic and systemic vein yellow or mosaic symptom on leaves which are similar to the field diseased copperleaf. The KC-1 virus isolate cannot be transmitted by mechanical inoculation and seed transmission except the A. gossypii transmission. The results of testing KC-1 virus isolate of aphid vector transmission by Myzus persicae and A. gossypii showed that only A. gossypii could transmit. A. gossypii could continuously spread KC-1 virus isolate. Its minimal duration of acquisition and inoculation access periods by A. gossypii were 12 hours and 30 minutes, respectively; its transmission efficiency was 19% and 11%, respectively. Single aphid could effectively transmit KC-1 virus isolate with the transmission efficiency of 14%. The nymphs of aphids showed higher transmission efficiency than the adult aphids. The A. gossypii could retain KC-1 virus isolate for 13 days post virus acquisition. The virus could passage to progeny viviparae of A. gossypii, and can continue pass to the F3 generation. The results of the host range test indicated that the virus could only infect 2 families, 18 plant species belonging to Euphorbiaceae and Solanaceace among 62 plant species in 9 families tested. The ultrathin section of a vein of Lutao copperleaf infected with KC-1 virus isolate showed crystalline arrays of virus particles and cytoplasma granulated in phloem cell. The purified virus preparation was obtained with a yield of 0.33 mg/100 g from infected tobacco leaves. In conclusion, the virus particle morphology, aphid-borne transmission characteristics, and histopathology of the KC-1 virus isolate on the Lutao copperleaf showed that the presumption of KC-1 virus of this study was a phloem-limited virus, and might belong to Luteoviridae. This is the first report on Lutao copperleaf mosaic disease in the world.
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