Leaf-yellow-spot, a physiological abnormality occurring in leaves of several chrysanthemum (Chrysanthemum x morifolium) cultivars is a very serious production problem in Japan. High temperature or high irradiation are possible physiological factors, which may lead to leaf-yellow-spot. In the present study, effects of air or medium temperature on the occurrence of leaf-yellow-spot in 'Seikou-nomakoto' were investigated. The nodal position with spotted leaves and rate of leaf-yellow-spot increased
with increasing day/night temperature. The nodal position with spotted leaves and rate of leaf-yellow-spot of plants grown on 30°C night air temperature were smaller than those grown on 25°C or ambient night air temperature. The days to first occurrence of leaf-yellow-spot showed no differences among medium temperatures. As occurrence of leaf-yellow-spot was not affected by root zone temperature, we recognized that occurrence of leaf-yellow-spot was promoted by high temperature or solar radiation on shoot, especially leaf. Occurrence of leaf-yellow-spot was reduced by long period high temperature and/or high solar radiation as plant growth reduced. Therefore, we thought that occurrence of leaf-yellow-spot was promoted by environmental condition as plant growth would promote.