THE EFFECT OF HIGH TEMPERATURE ON FRUIT SET OF 'TRIUMPH' PERSIMMON

In recent years, there have been declines in fruit yield and quality of the ‘Triumph’ persimmon (Diospyros kaki L.), and it was supposed that these tree responses were due to the climatic warming trend in the recent decade. The present objective was to examine the effect of high temperature on fruit drop under controlled conditions, and to evaluate tree-protection methods in the field. The fruit-drop rates of eight different persimmon cultivars, growing in the same plot, were recorded in spring for several years, and ‘Triumph’ was found to be the most sensitive. Under controlled-environment conditions the trees showed significantly lower fruit set under day/night temperatures of 35/18°C than at 22/18°C. The decline of fruit set was more evident when the trees were exposed to the high temperatures during development of flowers and fruitlets (March 15-April 16). Attempts were made to protect the fruits under field conditions by covering the trees with white net screens (30% shading) or by sprinkling water above or under the tree canopy at temperatures above a threshold of 27°C. Trees under these protective treatments showed higher fruit-set rates. It seems that high temperatures in spring may reduce fruit set, but that this response might be mitigated by shading or evaporative cooling.