Between 1973 and 1979, studies were undertaken in North Thailand to quantify the impact ofCercosporaleaf spots,Mycosphaerella arachidisDeighton (Cercospora arachidicolaHori) andM. berkeleyiJenkins (Cercosporidium personatum(Berk. & Curt.) Deighton), and rust,Puccinia arachidisSpeg., on rainfed groundnut production and to examine control measures including comparisons of fungicides, spraying regimes (commencement time, interval and duration), varietal response to spraying and the addition of oils and wetting agents. Yield responses to fungicide treatments were between 50% and 113%. The most effective fungicide treatment was benomyl 50% w.p. in combination with maneb 80% w.p., used at application rates of 148 g and 1100 g a.i./ha/application. Chlorothalonil 80% w.p., zineb 75% w.p. and a commercial equivalent of benomyl + maneb were less effective. In years of average rainfall distribution, spraying could be left until 50 days after sowing for early wet season sowings. Spraying at 10 day intervals was superior to 15 or 20 day intervals. Yields continued to increase with spraying continued to 90 days but were only marginally economic for local varieties when continued beyond 80 days. Higher yielding groundnut varieties were proportionately more responsive. The addition of oils or wetting agents did not allow a reduction in fungicide application rates. Spraying for the control of the diseases under farming conditions was economical when assuming yield increments in response to disease control would be 50% of those achieved experimentally.