紅龍果低溫貯運褐斑病特性分析及採後溫湯處理防治技術開發

Abstract

紅肉種紅龍果(Hylocereus polyrhizus Britt. & Rose)是我國新興外銷果品， 5℃海運3週以上遠程市場常發生褐色病斑，嚴重的病害果腐是長程貯運品質劣變主要原因之一。經本試驗分離鑑定，Alternaria是主要造成低溫貯運期的病原菌，具有潛伏感染特性，採收後或是貯藏期間才會顯現病徵，故無法在採後選別剔除遭感染的紅龍果果實。Alternaria菌絲在5℃生長仍能緩慢生長，5℃培養1週時孢子發芽率即可達到83.2±1.2%，至4週孢子發芽率更高達99.8±0.2%，顯示5℃貯運無法完全抑制Alternaria，因而成為紅龍果長程貯運的重要病害。‘大紅’與‘蜜寶’紅肉種紅龍果採後以60℃溫湯處理2分鐘，可使5℃冷藏4週、20℃回溫3天之‘大紅’ 病害面積顯著減少至22.5±2.8%，較對照組67.7±6.5%低，冷藏3週‘蜜寶’病斑面積也降至13.0±2.4%，也較對照組21.1±3.5%低，果皮無水浸熱傷害且對果實品質、色澤無負面影響。以商業化溫湯處理機操作流程處理‘大紅’、‘蜜寶’、‘富貴紅’海運外銷至加拿大溫哥華(海運25天)，‘大紅’和‘富貴紅’溫湯組果實病害面積各為23.2±1.9%、19.3±0.4%，比對照組(38.2±4.3%、45.9±1.0%)低，‘蜜寶’溫湯組果實病害面積27.5±3.9%，與對照組果實27.0±5.1%無顯著差異，但所有果實外觀仍因病斑明顯喪失商品價值。‘大紅’與‘蜜寶’貯前浸泡藥劑溫湯，經4週低溫貯藏回溫後，‘大紅’、‘蜜寶’果實病害嚴重程度分別為7.09±1.06%和3.29±0.56%，較對照組病斑面積53.6±3.9%及51.1±3.8%低，果實外觀具有商品價值，無果皮藥害、風味品質不受影響，且農藥殘留量也符合我國與目標市場安全容許量。因此，該處理技術有潛力應用於紅肉種紅龍果長程外銷採後流程，以改善紅龍果長程低溫貯運病害果腐問題，提高到貨可售率與延長櫥架壽命。

Red-fleshed pitaya (Hylocereus polyrhizus Britt. & Rose), also known as dragon fruit, is a newly-emerging exported fruit in Taiwan. However, fruit rot of red-fleshed pitaya is one of the factors limiting long-term cold storage or transportation of the commodity. After three-week storage at 5℃, symptom of white-sunken lesions appears on the peel surface of a fruit which strongly affected its appearance quality and shelf-life. Through isolating from symptoms, the pathogen was identified as Alternaria spp. based on the morphological characteristics of the fungi, which caused sever fruit rot of pitaya during cold storage and transportation period. The Alternaria spp. exhibit latent infection feature, namely the symptoms will not appear until postharvest stage. Low temperature of 5℃ could not completely suppress the mycelium growth of the Alternaria. Moreover, the spore germination percentages were 83.2±1.2% and 99.8±0.2%, respectively, after 1- and 4-week incubations at 5℃. These properties make Alternaria spp. a major pathogen being responsible for pitaya decay after long-term cold storage. The disease lesion percentages of ‘Da-Hong’ and ‘Mi-Bao’ pitaya treated with 60℃ hot water immersion for 2 minutes followed by 5℃ storage were 22.5±2.8%, which was lower than the untreated control 67.7±6.5%, in ‘Da-Hong’ after 4-week storage plus 3 days at 20℃ and 13.0±2.4%, which also lower than the untreated control 21.1±3.5%, in ‘Mi-Bao’ after 3-week storage plus 3 days at 20℃. The treated fruits showed no heat damages on peel and had no negative effect to flavor quality. Commercial operating process of hot water immersion with ‘Da-Hong’, ‘Mi-Bao’ and ‘Fu-Gui-Hong’ shipping to Vancouver, Canada (took 25 days) was performed. The disease area of ‘Da-Hong’ and ‘Fu-Gui-Hong’ treated fruits were 23.2% and 19.3%, lower than untreated fruits 38.2% and 45.9%. The disease area of the ‘Mi-Bao’ treated fruits were 27.5±3.9%, which was not significantly different from untreated group (27.0±5.1%). But the appearance of all the fruits was still unmarketable due to the lesions. In contrast, the disease lesion percentages of ‘Da-Hong’ and ‘Mi-Bao’ immersed in fungicide formulated with hot water were 7.09±1.06 % and 3.29 ± 0.56%, which were lower than 53.6±3.9% and 51.1±3.8% of the untreated controls, respectively. This prestorage treatment did not significantly caused peel phytotoxicity and flavor quality decrease as well as the fungicide residues of the fruit were also below the maximum limitations of Taiwan and target markets. Therefore, this prestorage treatment is expected to be promising to incorporate into the process of postharvest handling in pitaya to reduce Alternaria rot, improve the salability after arrival at international markets, and extend the shelf-life.