Efficacy Evaluation of Four Fungicides on a Water Agar Platform Smeared with Rust Fungal spores

The study is to develop a water agar platform method for quickly evaluating the efficacy of four fungicides applied to obligate rust fungi on crops and trees. This method uses a 9 cm sterile Petri dish filled with a thin layer of 3% water agar. A rectangular platform (6 × 1 cm) connected with five 3 cm dilution lines was designed for the water agar plate. The platform works as an in vitro field where test fungal spores are mixed with four fungicides. After approximately 10 minutes of mixing, five streaks are made with an aseptic loop to tow the treated spores outside the platform along the dilution lines to their ends. Four rust uredospores collected on-site from diseased leaves, including corn rust, grape rust, fig rust, and frangipani rust, were assayed against four common fungicides in Taiwan. Our results showed that mancozeb (80% WP) is the best agent to control corn rust, while chlorothalonil (75% WP) was the best for controlling grape, fig, and frangipani rusts. Bordeaux mixture (6:4) also had good efficacy for fig rust. The inhibition of germination and the morphological modification of spores inside and outside the platform can also be observed and compared with each other. If necessary the microscopic observation can be extended for two more days. Our results showed that chlorothalonil completely and permanently inhibited spore germination within and outside the platform, suggesting that chlorothalonil is a therapeutic fungicide. Conversely, the Bordeaux mixture experiments showed that uredospore germination rates outside the platform were higher than within the platform, indicating that it is only an inhibitive chemical for the rust uredospore. Our assessment of the morphological modification of spores showed that mancozeb at a 500X dilution causes germ tube and mycelia shapes of fig rust to be prominently different from normal blanks. The germ tubes are shortened, with stunted ends, while some mycelia show a slimmer shape. However, some of the germ tubes and mycelia outside the platform area recovered to near normal, indicating that mancozeb plays only an inhibition function. The practical information obtained from this method is quite useful for farmers, tree owners, plant doctors, or tree doctors when they want to select a fungicide to reduce the severity of a disease, and to avoid the inappropriate use of non-effective or resistance-inducing fungicides. Due to the low cost and simplicity of operation, this water agar platform method is especially suitable for plant medicine students or researchers who prescribe fungicides and conduct fungicidal screenings in the fields.

關鍵字

water agar platform ； fungicidal screening ； germination inhibition ； morphological modification ； corn rust ； grape rust ； fig rust ； frangipani rust

並列摘要

本研究設計一種可以快速評估4種殺菌劑對作物或樹木絕對寄生之銹病菌藥效的方法，本方法係使用9公分之無菌塑膠培養皿，先倒入3%的水瓊脂製備成薄層平板，再於底面之一側畫出一個6×1公分之長方形平台塗佈區，由此平台區依每1公分的間隔向另一側共拉出5條各長3公分的稀釋線。此平台區作為藥劑與真菌孢子的作用場域，當約等量的孢子懸浮液與藥劑在平台區均勻混合約10分鐘之後，即以無菌接種環將孢子群自平台區沿著稀釋線拉曳出平台，直到其末端。依此方法已自田間病株採集4種銹病菌，包括玉米、葡萄、無花果及緬梔之銹病菌夏孢子，並選取4種常用殺菌劑與之在平台上進行藥效的測試。結果發現4種藥劑中防治玉米銹病菌最有效的是鋅錳乃浦，而對抗葡萄、無花果及緬梔之銹病菌則以四氯異苯腈最具高效，但波爾多液對無花果銹病菌的防治效果亦極佳。於上述防治率測試之同時，亦觀察平台內與平台外稀釋區各銹病菌孢子發芽率及形態學之差異性，必要時可延長觀察2日，結果發現四氯異苯腈可讓無花果銹病菌夏孢子之發芽完全抑制，且平台外的孢子皆無法恢復發芽能力，說明該藥具有「殺菌」之藥效。與此相反的是波爾多液，經拖曳至平台外時，即見增高之發芽率，說明該藥僅屬於「抑菌」藥劑。在夏孢子發芽的發芽管及菌絲顯微形態學之研究上，發現稀釋500倍的鋅錳乃浦可使平台內的孢子發芽管變短及呈鈍尖狀，另使菌絲變細、變短。當被拖曳至平台外時，即見有恢復為較正常的發芽形態，也可供說明該藥僅屬於「抑菌」藥劑。利用此一水瓊脂平台法所得的是田間病菌及殺菌劑藥效的實況資料，故可供農民、樹木業主、植物醫師、樹木醫師在選擇已登記之防治用藥時，避免用錯藥劑及避開易生抗藥性藥劑的有力參考。由於本方法具有低成本、操作容易之優點，故特別適合植物醫學系所學生及研究人員，可供作為田間殺菌劑處方選擇及藥劑篩選的廣泛應用。

並列關鍵字

水瓊脂平台；殺菌藥效評估；發芽抑制；形態學變化；玉米銹病；葡萄銹病；無花果銹病；緬梔銹病

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