J.D. Mantecon. 2009. Importance of potato late blight in Argentina, and the effect of fungicide treatments on yield increments over twenty years. Cien. Inv. Agr. 36(1):115-122. Late blight (Phylophthora infestans) is the most destructive fungal disease of potato (Solanum tuberosum) in Argentina, reducing yields considerably. During the 1986-2005 growing seasons, fungicide treatments to control potato late blight were performed using mancozeb (Dithane M80 WP, Dow AgroScience Argentina, 1986-2005) every 7 d, metalaxil+mancozeb (Ridomil MZ 72 WP, Syngenta Agro Argentina, 1986-1995) every 14 d, and mefenoxam+mancozeb (Ridomil Gold 68 WP, 1996-2005) every 7 d. Blight severity was rated live times each season on the basis of the percentage of diseased foliage caused by late blight. Five visual assessments of infection were used to calculate the area under the disease-progress curve (AUDPC, foliar blight x day). Marketable (>60 g) potatoes and total tuber yields were recorded each growing season. Late blight was prevalent and severe each year, reducing yields significantly particularly for marketable tubers. The average yields increased on fungicide treated plots was 41.8% for marketable tubers and 35.6% for total potato yields. Under the climatic conditions of southwest Argentina, total crop loss on non fungicide treated plots was not obtained in 20 years; therefore late blight was not a completely limiting factor for potato production. However, fungicide treatment appears to be essential to obtain high quality yields.