Early blight (Alternaria solani)is the most important foliar disease of potatoes in the humid highlands of Papua New Guinea. Late blight(Phytophthora infestans)is not found. Two trials compared copper oxychloride, mancozeb and chlorothalonil, applied by knapsack sprayer at 7‐ and 14‐day intervals, with an unsprayed control. Copper oxychloride, the least expensive fungicide, gave the greatest yield in the relatively dry season (May‐September) at Taluma (altitude 2570 m) and was the only fungicide consistently to increase yield in the wet season (November‐February) at Rakamanda (1920 m). The extra benefit from spraying at 7‐ rather than 14‐day intervals was small. When sprayed every 14 days, starting at 31 days after planting, copper oxychloride (2–0 kg a.i./ha) increased the yield of cv. Sequoia from 21–8 to 26.2 t/ha (SEd 1.75; 28 d.f.) at Taluma, and from 26.4 to 300 t/ha (SEd 1.69; 28 d.f.) at Rakamanda. Copper oxychloride increased average tuber weight and specific gravity at Taluma but not at Rakamanda.