In commercial orchards of satsuma mandarin in Japan, mulching cultivation has been developed to produce high quality fruit. It is, however, difficult to manage irrigation to produce consistent characteristics such as high fruit quality, fruit productivity, and tree vigor. The principal reason for this is that no simple, easy, and reliable indicator has been developed to evaluate tree water deficits. In this study, we demonstrated that the morning maximum leaf water potential could be estimated using the fruit hardness in the evening of the previous day. To assist farmers in estimating fruit hardness, we developed a set of three silicone rubber balls with different degrees of hardness as a reference tool, and verified their effectiveness. The experiment was performed in 2008 and 2009, and revealed that irrigation management during July and August using the silicone rubber reference balls gave good results, with high fruit quality and high fruit productivity. Fruit hardness estimated using the silicone rubber balls therefore appears to be a good indicator of irrigation management during July and August for mulched citrus orchards.
