The occurrence and chemical control of downy mildew on Eustoma graizdtflorum Salisb

Abstract

1993年1月中旬於南投縣埔里鎮及仁愛鄉清境農場之簡易溫室內發現洋桔梗露菌病之發生，旋於屏東縣鹽埔鄉亦陸續發現本病之蹤跡，至3月中旬，幼苗及成株皆發病嚴重，病徵最初出現於嫩葉，葉片下表皮產生灰色黴狀物，並逐漸擴展，罹病部份葉片因而稍呈褪色現象，葉片並向下扭曲，隨著病勢進展，葉片扭曲現象增加，嚴重時葉片上產生淡褐色病斑，其上覆蓋病原菌之菌體，若不加防治則整株枯死。原菌Peronospora chlorae de Bary，胞囊柄雙叉分枝，由寄主組織表面抽出，胞囊呈橢圓形，大小為16-22×10-15µm，平均為19×12.5µm，著生於胞囊柄頂端；胞囊直接抽出發芽管發芽，於8-32℃溫度範圍內，發芽率無差異。自日本進口之種子在臺灣播種可明顯發現感染露菌病，七個品種之罹病率介於12.46～35.63％之間。田間藥效試驗進行期問各藥劑均未發現藥害產生，然藥效間顯示5％顯著差異，推薦35％腈硫克絕可濕性粉劑1200倍、80％福賽得可濕性粉劑800倍、64％甲鋅歐殺斯可濕性粉劑400倍、66.5％普拔克溶液800倍及35％本達樂可濕性粉劑2000倍為防治洋桔梗露菌病之藥劑；然35％腈硫克絕可濕性粉劑1200倍於苗期施用時，明顯抑制種子發芽及幼苗生長，故不宜施用於苗床期。 The downy mildew in the vinyl house both in Puli and Jenai area was found occurring on Texas bluebell (Eustoma grandiflorum Salisb.) in mid January, 1993. The disease was also found at Yenpu near Pintung city at the same time. The disease causes severe damage on both seedlings and adult plants. The small gray mycelial masses appeared on the lower surface of young leaves and disseminated to make a inconspicuous discolored spots. The leaves curled to the lower surface of leaves and became twisted and distorted. And the spots then changed to slight brown and covered with mycelial mass and spores. Finally the infected leaves turned to dark brown and the whole plants died in severe condition. The causal organism was Peronospora chlorae de Bary. The erect sporangiophores on lower leaf surface are dichotomously branched with sporangia borne on sharply pointed terminal branches. Sporangia are 16- 22 X 10-15µm (19 X 12.5 µm in average) and ellipital in shape. They germinated directly to form germ-tube at 8-32℃. The disease incidence was 12.46- 35.63% when seven varieties imported from Japan were surveyed during seedling bed. The primary inoculum of downy mildew was suspected to be coming from seeds. The fungicidal efficacy was various in protecting plants against the pathogen and no phytotoxicity was found when screening test of fungicides was conducted in the fields. Five kinds of fungicides, 35% cymoxanil +dithianon WP in 1:1200 dilution, 80% fosetyl-Al WP in 1:800 dilution, 64% propineb +oxadixyl WP in 1:400 dilution, 66.5% propamocarb hydrochloride S in 1:800 dilution and 35% bena]axyl WP in 1:2000 dilution showed the best efficacy in field trials. The cymoxanil + dithianon 35% WP was not recommended to use in the seedling beds for the reason that the germination of seeds was inhibited apparently after treatment