[[abstract]]水稻臺農70號於民國74年第一期作命名，而自同年第二期作正式推廣。命名當時具有高產、良質、抗葉稻熱病，對穗稻熱病屬於中抗的優良品種，但於民國76年第一期作，即自正式推廣後僅四期作，即失去抗性，發生嚴重的穗稻熱病。本研究主要為探討本省稻熱病菌生理小種對該品種致病性之變化情形。供試稻熱病菌計78 個菌株（包括73年度採集的27個菌株及76年度採得51個菌株），並行人工接種於16判別品種及臺農70號，測定其生理型及對臺農70號之致病性，結果可類別10種生理小種，其中僅有race P─12及P─63兩種生理小種對臺農70號具有致病（S）反應。自73年度所採集的27個菌株中有僅3個菌株（1菌株為P─12，2菌株為P─63，佔11％），76年所得51個菌株中，有18個菌株（12菌株為P─12，6個菌株為P─63，佔35％）對臺農70號顯示感病（S）反應。此兩種生理小種，原來在本省已有存在，但其密度極低，自臺農70號急速大面積種植後，才增加其密度比例，致使該品種的罹病化。本省的栽培稻品種，有稉稻（japonica）及秈稻（indica）混合種值於田間，因此稻熱病菌生理小種之變化頗複雜。 Rice cultivar Tainung 70 was named in the first crop season and released in the second crop season of 1985, due to its high yielding, high quality, resistance to leaf blast, and moderate resistance to panicle blast. After 2 years (4 crop seasons), the resistance broke down and servere epidemics of panicle blast occurred in the first crop of 1987. Experiments were conducted to explore the pathogenic variation of races of Pyricularia oryzae in the rice cultivars to understand the reason of break down. Seventy-eight isolates (27 collected in 1984, 51 collected in 1987) of P. oryzae were artifically inoculated to the cv. Tainung 70 and 16 differential varieties for race studies. The reaction of the 78 isolates were classified into 10 physiologic races. Among the 10 races, there were 2 races, P-12 and P-63, pathogenic to Tainung 70. In terms of isolate and percentage, 3 isolates (1 belonged to race P-12, 2 belonged to race P-63) out of the 27 isolates or 11% collected in 1984 and 18 isolates (12 belonged to race P-12, 6 belonged to race P-63) out of the 51 isolates or 35% collected in 1987 were pathogenic to Tainung 70. These 2 races, P-12 and P-63, have already existed in nature but their populations were very low before the release of Tainung 70. When the acreage of Tainung 70 increased the population of the 2 races also increased with the time and then, the cultivar Tainung 70 became susceptible.