瓜類露茵病生態及防治

Abstract

瓜類露菌病係由Pseudoperonospora cubensis所引起，在本省胡瓜、洋香瓜、哈蜜瓜及絲瓜等瓜類發生普遍而且嚴重。胡瓜或洋香瓜之摘葉，在15、20及25℃下接種後4~5天可形成病斑，在15及20℃下經5~6天，25℃下經6~8天可產胞，但在10、30及35℃不產生病斑，亦不產胞。病原菌胞子囊發芽以10及15℃最合適，6小時後發芽率達95.18%及97.52%，高溫發芽率降低。胞子囊釋放游走子之數目以7個最多，3及11個最少。胞子囊在2%水瓊脂上於30℃一天後再放回15℃，需經4小時始可發芽，發芽率為20.87%，24小時發芽率達63.11％。若在35℃一天後再放回15℃，則需經24小時始可發芽，發芽率僅7.01%，48小時後發芽率僅22.42％。若將病葉放在30℃一天後，再放回15℃，則其上之胞子囊需經4小時始可發芽，病葉放在35℃一天後，再放回15℃，則其上之胞子囊需經24小時始可發芽。胞子囊之發芽率隨相對濕度之增加而增加，在相對濕度55、81、95及100%時，其發芽率分別為29.85、66.30、80.02及97.53%。在水瓊脂培養基中瓊脂之含量亦影響胞子囊之發芽率，瓊脂含量2.0時發芽率在15℃24小時後高達99.30%，瓊脂含量3.0％時發芽率亦達96.91%，但含量至8.0%時則胞子囊不發芽。胞子囊在V-8汁濃度越高，發芽率越低，濃度達35%時胞子囊不發芽。胞子囊在土壤含水量功％，經24小時尚不發芽，4天後發芽率僅2.66%，其發芽率隨含水量之增加而增加，50％含水量時24小時後之發芽率為20.38%。胞子囊在30％含水量土壤中至少可存活14天。胞子囊製成胞子懸浮液後在15℃經6及24小時接種，植株均可發病，但存放48小時者不發病。 分析胡瓜罹病度增加和氣象因子之關係，顯示露菌病和發病前7天及8~14天之降雨量呈正相關，但和平均相對濕度呈負相關。 在藥劑篩選，洋香瓜播種後三週之植株(1)先行噴藥一天後，再接種胞子懸浮液(4×10 4/ml)，經10天後調查葉片發病等級，再換算被害度(2)先接種胞子懸浮液後，約五天後開始發病時再施藥，經10天後調查發病葉片之等級，再換算被害度。結果以先噴藥再接種者，其被害度較先接種再噴藥者為低，藥效以58%鋅錳滅達樂可濕性粉劑及75%四氯異苯腈可濕性粉劑500倍效果最佳。 Cucumber downy mildew caused by Pseudoperonospora cubensis was quite Wide-spread and serious on cucumber, melon, muskmelon and loaf in Taiwan. The lesion formed at 4-5 days after inoculation on detached leaves of cucumber and melon under 15, 20 and 25℃. Sporulation at 5-6 days under 15 and 20℃, 6-8 days under 25℃. No lesion formation or sporulation under 10, 30 and 35℃. The optimal temperatures for sporangial germination were observed at 10℃ and 15℃ after 6 hours of incubation, the percentage of germination were 95.18% and 97.52%, respectively. Germination decreased at higher temperature. Most sporangia produced seven zoospores with an average of 6.7, a few sporangia produced 3 or 11 zoospores. The sporangia of Pseudoperonospora cubensis were incubated at 30℃ for one day, and then transferred to 15℃ , additional four hours were necessary to germinate, the germination percentage was 20.87%. As long as at 15℃ for 24 hrs the germination percentage increased to 63.11%. If the sporangia were incubated at 35℃ for one day first arid then transferred to 15℃, another 24 hrs were needed for germination, and the germination percentage was only 7.01%. Even at 15℃ for 48 hrs, the germination percentage was also increased to 22.42% only. If the infected leaves with sporangia were incubated at 30℃ for one day then moved to 15℃, the sporangia also took 4 hrs for germination. While the infected leaves with sporangia were incubated at 35℃, it required 24 hrs for germination. On the other hand, the germinability increased with increasing relative humidity, at the RH of 55,81, 95 and 100% the germination percentage were 29.85, 66.30, 89.02 and 97.53%, respectively. The percentage of germination of sporangium was influenced by agar concentration of water agar medium. When the agar contents were at 2.0% and 3.0% the germination percentage were 99.30 and 96.9%, respectively, after 24 hrs of incubation at 15℃. As agarcontent reached 8.0%, no germination was observed. Sporangia did not germinate on the leaf juice of Cucurnis melo. When the sporangia were incubated on V-8 juice, it showed that the higher the concentration of V-8 juice, the lower the germinability was observed, arid 35% of V-8 juice inhibited, germination. In soil with 30% water content the sporangia did not germinate even after 24 hrs of incubation. After 4 days of incubation, the germination percentage was only 2.66%. Therefore, the germinability increased with increasing water content. The sporangia could survive at least 14 days under 30% of water content in the soil. The downy mildew fungus of cucurbits maintained its infectivity in spore suspension after 6 of 24 hrs preparation, however, after 48 hrs, no infectivity was observed. The relationship of disease severity increment was meteorological factors was analyzed. Which indicated that downy mildew disease was positively correlated with rainfall at 7 and 8-14 days before disease occurrence; but negatively correlated with average relative humidity. Fungicide trials showed that the disease rating was lower on the plot which the seedlings were treated with fungicides first and then inoculation was made. Among the fungicides tested, 58% Metalafyl + Mancozeb wp and 75% Chiorothalonil wp showed the best effectiveness.