被忽略的稻熱病初次感染源－－『帶病秧苗』

The neglected first inoculum of rice blast pathogen Pyricularia oryze on the mats of rice seedlings

"在台灣水稻栽培制度下，嘉南地區稻熱病（rice blast disease）主要好發在第一期作的清明節與梅雨季前後的高濕度環境。雖然許多研究嘗試透過田間孢子量及環境氣候監測進行病害發生預測，但都未能明確掌握病因與病害流行間之關聯性。近年田間調查結果發現，近一成左右剩餘秧砧常在插秧後2-3週就出現嚴重稻熱病危害，但同時間本田植株卻未有相同發病情勢。從2019年3月份嘉南地區稻熱病爆發後取得的發病地區空拍圖顯示，當地稻熱病大爆發的田區病害發生分佈與水稻插秧方向具一致性，顯示稻熱病的發生與秧苗有關。針對發病田進行稻種抽樣檢測，台南11號與台農71號種子的稻熱病菌（Pyricularia oryzae）帶菌率分別為1-3％與6-11％，病原在稻殼的護穎（sterile lemma）及米粒胚（embryo）對側的種皮（testa）部位可同時被檢出，此結果與前人文獻相符（3, 8, 9）。綜合以上結果，顯示以未經處理的稻殼作為苗土介質或消毒未完全的帶菌稻種作為種原，都可能增加秧苗帶病、成為初次感染源的風險，進而造成稻熱病在田間以種傳病害形式傳播爆發。"

"The epidemic of rice blast disease in central and southern Taiwan usually occurs in early April and May, before the rainy season when the weather is with high humidity. Weather information and the amount of pathogen spores have been used to establish the forecasting system, but the best timing to control the disease is yet to be found. Recent field monitoring results indicated that around 10% of the unplanted seedlings left in the field in a bunch were severely infected by rice blast fungi 2-3 wks after transplanting, while most of the plants in the same field remained healthy. The aerial shots of blast fungi infected rice fields in Tainan and Chiayi areas showed that the disease pattern of the fields is concurrent with the seedling transplanting direction, indicating a positive correlation between disease incidences and the transplanting seedlings. Random sampling of the seeds planted in those diseased fields showed that cultivars Tainan 11 and Tainung 71 seeds had 1-3% and 6-11% infected rates, respectively. Pathogen Pyricularia oryzae could be observed colonizing on the sterile lemma and testa, and this result is consistent with previous reports(3, 8, 9). In conclusion, unsterilized seeds or infected hulls mixed in the rice seedling planting materials would increase the disease incidence of seedlings and seedlings as the first inoculum introduced to the field would cause rice blast epidemics when the weather condition is suitable for P. oryzae infection."