由Sclerotium rolfsii引起之馬拉巴栗白絹病

Southern blight of Malabar chestnut caused by Sclerotium rolfsii in Taiwan

調查彰化縣溪洲地區與水稻輪作田所栽種馬拉巴栗植株或苗株，觀察到部分田區植株地上部出現萎凋、葉片黃化、乾枯及死亡現象；檢查罹病部位附近土壤表面或栽植穴的殘根上會出現白色菌絲與菌核。一般田區發病率介於12-20%之間，嚴重時發病率可達75%以上。於外銷出貨的馬拉巴栗苗編苗，發病率可達80%以上。經分離純化所得F215020、F215022及F215028菌株，以形態特徵，輔以ITS序列，並經柯霍氏法則（Koch's postulates）完成病原性測定，病原菌鑑定為Sclerotium rolfsii Sacc.（有性世代為Athelia rolfsii（Curzi）C. C. Tu & Kimbr.），造成之病害稱為馬拉巴栗白絹病，英名為Southern blight of malabar chestnut。病原菌在馬鈴薯葡萄糖瓊脂（PDA）培養基上，菌絲白色、有如絹絲狀、生長快速，4-5天即可長滿培養皿。菌絲長滿培養皿之後2-3天，菌落表面開始有白色毛球狀產生，近球形，漸漸均勻增厚而轉為褐色，直徑約1-2 mm，此為菌核（sclerotia）。本病原菌菌絲在12-36℃下皆可生長，最適生長溫度為32℃，生長速率介於10.2-17.5 mm/day。本病害適合發病溫度介於16-36℃，以28℃時發病度最高。

Several Malabar chestnut seedlings showed the symptoms of leaf yellowing, plant withering, and even plant death in a few fields which planted Malabar chestnut plants and rotated with rice in the Xizhou Township of Changhua County. White hyphae and sclerotia usually observed on the soil surface beside the diseased plants or on the residual roots. The disease incidence were 12-20% in general in the field survey, but more than 75% in some severe fields and more than 80% in some cases of bare-rooted seedlings which were ready for export. Therefore, the fungal isolates F215020, F215022 and F215028 were isolated and chosen to conduct the pathogenicity test for completing Koch's postulates. According to the morphological characteristics and the molecular sequence of internal transcribed spacer (ITS) of ribosomal DNA, the pathogen was identified as Sclerotium rolfsii Sacc. (teleomorph Athelia rolfsii). The disease was named as southern blight of Malabar chestnut. The colony of the pathogen on PDA plate was white and silk-like. Mycelium grew fast and could fill up the petri dish in 4-5 days. Next 2-3 days, there were white hairy balls nearly spherical began to produce on the surface of hyphae. And then many spherical white-turning-brown sclerotia (diameter 1-2 mm) produced on the surface of agar media. The fungal pathogens grew at 12-36℃, optimally at 32℃ with the growth rates 10.2-17.5 mm/day on PDA plates. The suitable temperatures for disease development were at 16-36℃, with the highest disease severity at 28℃.