荔枝露疫病菌之特性與生態研究

Studies on Characters and Ecology of Litchi downy blight in Taiwan

荔枝露疫病為荔枝最重要的病害之一，為明瞭病害之發病生態與降雨的關係，自1998年起數年間於南投荔枝主產區調查病害初次發生的情形。結果顯示，連續降雨7-10日以上地區之荔枝果實最早出現露疫病病徵。另外，調查15個病害發源點，每個發源點均源自地面落果或靠近地面之果實，推測荔枝露疫病初次感染源應來自荔枝園之根系與/或土壤中的病菌。因此於病害發原點採集荔枝的根系與根圈土壤，以葉片誘釣法均可誘釣到露疫病菌。而將露疫病菌孢子懸浮液接種於3-6個月大之荔枝（黑葉）幼苗根部，6個月後可使90%以上之接種幼苗死亡，但不會引起3年生以上幼苗死亡。露疫病菌可自接種發病之荔枝病根組織分離得到，但無觀察到卵孢子，推測病原菌可能以菌絲形態長期存活於荔枝根部或根圈土壤，為主要存活藏所。荔枝果實採收後，罹病果園之新萌芽葉片亦會罹患露疫病，發病時間約在7月至10月，但成熟葉片不會染病。此外，果園中野生的荔枝與龍眼幼苗亦會染病，顯示除荔枝外，龍眼幼苗亦為露疫菌寄主之一。接種露疫病菌的荔枝幼葉可以檢測出卵孢子，但老熟葉片則不會被感染，而罹病果實亦無法檢出卵孢子。將人工接種露疫菌之果實埋於盆栽荔枝苗土壤中或置於土表，一個月內可自病果上分離到病原菌，但經過二個月後即無法測得。菌絲、孢囊、游走子在土壤中之存活時間，以孢囊最長，在一般田土中可存活一個月，游走子與菌絲約為15-30天。

"Litchi downy blight caused by Phytophthora litchii (=Peronophthora litchii) is one of the most severe diseases of litchi. To evaluate the correlation between disease ecology and rainfall, we surveyed the timing of the first disease occurrence in Nantou, the major litchi producing county, for several years since 1998. The results indicated that, in the areas with continuous rainfalls for more than 7-10 days, litchi showed the earliest downy blight symptoms. In addition, of the 15 investigated pre-infection sites in 5 orchards, all of the primary infected litchi fruits were found to be those dropped on the ground or those grew near the ground, suggesting the primary inoculum could come from roots and/or rhizosphere soil. The roots and rhizosphere soil adjacent to the primary infection sites were collected and assayed using the bait test with litchi leaves. All of them showed the existence of P. litchii. In the pathogenicity test by using P. litchii zoospores suspension as inoculum to infect the roots, 90% of the infected 3 to 6-month-old litchi seedlings died within 6 months post infection while 3-year-old ones were still alive. In the diseased root tissues, the pathogen could be isolated while no oospores were observed, suggesting that the pathogen might use mycelium as long-term survival structure which hide mainly in the litchi roots or rhizosphere soil. After the litchi fruits were harvested, new shoots but not mature leaves could be found infected during July to October in the orchards with downy blight history. Besides, diseased litchi and longan seedlings were found in the fields, indicating that, in addition to litchi, longan was one of the hosts of P. litchii in the wild. Oospores appeared in the young but not matured leaves inoculated with P. litchii. No oospore was found in fruits infected with P. litchii. While the pathogen infecting fruits were buried or placed on the soil of potted litchi seedlings, the pathogen could still be isolated from the diseased fruits after 1 month but not after more than 2 months. In the survival test with the field soil, sporangia showed the longest survival period for about 1 month. In contrast, hyphae and zoospores survived for 15-30 days."