Japan annually produces 2.4 million tons of potato on 79,700 ha. There are four
cropping seasons stretching from 24° to 45° N: a winter crop at Okinawa, spring and autumn crops at Kyushu and southern Honshu, and a single summer crop at Hokkaido. Using the crop growth model LINTUL-POTATO-DSS, actual yields were compared with potential yields. The spring and summer crop yields of all prefectures varied from 10 to 43 t ha−1 and were 16t ha−1on average (not weighted by area) ; the actual/potential yield ratio varied between0.21
and 0.63 and was 0.40 on average (not weighted). Actual and potential yields of the autumn crops were half those of the spring crops. Actual yields of the Hokkaido summer crops in 21 years decreased from 35 to 32 tha−1 because of a change of varieties where as potential yield remained stable at 56 tha−1. Increased temperature would reduce the potential yield to 53 tha−1
but would move it up to 60 t ha−1 by lengthening the growing season and to 77 t ha−1 when the CO2 effect on growth is taken into account. The actual/potential yield ratio at Hokkaido was bench marked against that of other growing regions in the world, showing that this area compares well with that of other efficient production systems in northern Europe and America.