西瓜細菌性果斑病之研究

Abstract

西瓜細菌性果斑病是本省新病害，嚴重發生於南部及中部的一些西瓜 田。在 田間，果斑病徵常出現於噴灌及陰雨期後，在西瓜果實的上表面 開始出現小而油 膩的水浸狀斑點，隨後擴展成暗綠色水浸狀邊緣不規則 的水浸狀區域，後期病斑 呈黃褐色，果皮常崩裂並有琥珀色的菌泥滲出 ，在破裂處易引起其他腐生菌的侵 入而致使果實腐爛；在成株葉部上病 徵不明顯初期病斑為淡褐色略帶紅棕色，在 葉片下表面並時常沿葉的中 肋擴展，後期為暗灰色壞疽病斑。在幼苗期，可在子 葉上形成水浸狀的 病斑，隨後變成壞疽斑，常有黃色暈環；在葉上也能形成相似 病斑，胚 軸上如出現病斑，常造成幼苗枯萎。由罹病果實及葉片分離的病菌，經 生理生化特性測定及接種試驗結果，證實該病害是由Acidovorax avenae subsp. citrulli引起。西瓜果斑病菌在田間除危害西瓜及洋香瓜外，也 能接種感染越瓜 、胡瓜、苦瓜、冬瓜、蒲瓜、南瓜和絲瓜等葫蘆科植物 ，且由洋香瓜來源的果斑 病菌在西瓜上也能引起相同的病徵。一般而言 ，在幼苗上，西瓜來源的菌株對西 瓜及洋香瓜的致病力相似，但由洋香 瓜來源的菌株對洋香瓜的致病力均高於對西 瓜的致病力。西瓜果斑病菌 以噴霧接種在西瓜果實表面，果齡愈小具有較高的發 病程度，且以套袋 處理者發病程度也較嚴重。田間罹病西瓜果實之種子帶菌率可 達54.3 ~ 72.6%。利用長出試驗檢測西瓜16個品種各一批種子時，除一個品種外 ， 均帶有果斑病菌，有些種子帶菌率可達20%以上。利用幼苗噴霧接種及種 子浸 菌接種，顯示西瓜品種間之感病性具有顯著差異。種子以六種方法 的滅菌處理中 ，1%稀鹽酸處理的效果最好，0.1M熱酸性硫酸鋅溶液 (38℃, pH 2.8) 處理的效 果次之，而溫湯處理(50℃, 20 min)則無效。

The bacterial fruit blotch of watermelon, a newly found disease, occurred in some fields of southern and central parts of Taiwan. In the field, the disease appeared after overhead irrigation or a rainy period. The most pronounced symptom of the disease was the appearence of water-soaked blotches on the upper surface of fruit. The blotch may expand to cover most area of the upper surface, however, the lesions rarely extended into the flesh of the fruit. Water-soaked spots which became dark brown and then necrotic also formed on cotyledons and true leaves of the seedlings. Lesions on leaves of mature plants were generally inconspicuous, light brown to reddish brown in color and often spread along the midrib of the leaves. Based on physiological and biochemical characteristics and inoculation tests, the causal organism was identified as Acidovorax avenae subsp. citrulli. In the field, the bacterium affected watermelon and muskmelon. Artificial inoculation showed that the bacterium also was pathogenic to orie ntal pickling melon, cucumber, bitter gourd, sponge gourd, bottle gourd, pumpkin and wax gourd. Generally,strains of A. avenae subsp. citrulli from watermelon were similarly virulent to watermelon and muskmelon, however, the strains from muskmelon were more virulent to muskmelon than to watermelon. The disease was more severe on younger fruits ( age at 2-3 days after fruit setting ) than on older fruits ( age at 5-7 days or 7-14 days after fruit setting ) when spray inoculated with the pathogen; the diseas e severity was also greater on inoculated fruits covered with plastic bags than those without covering. Seeds collected from the diseased fruits were found to carry the pathogen with the infestation rate of 54.3 - 72.6%. Seed assays by the grow-out test revealed that 15 of 16 cultivars of watermelon tested ( one seed lot for each cultivar ) were infested with Acidovorax avenae subsp. citrulli, and with some cultivars, more than 20% of seeds were infested. The 15 test cultivars of watermelon differed in thei r susceptibility to the bacterium when evaluated on seedlings by spray inoculation and seed-soaking inoculation methods. Among six seed treatments , treatment with 1% HCl was most effective in reducing the disease incidence, followed by treatment with 0.1M acidified ZnSO4 ( 38℃, pH 2.8 ), whereas hot water treatment ( 50℃, 20 min ) was not effective.