Effect of urea spray accordance with growth phenology on yield and alternate ‎bearing of satsuma mandarin (Citrus unshiu)‎

To increase yield and reduce alternate bearing of satsuma mandarin trees, an experiment was performed in randomized complete block design during seven years. Treatments include: control; winter urea spraying (before flower bud differentiation); urea spraying at full bloom (at the opening of at least 50% of flowers) and summer urea spraying (after summer physiological abscission of fruitlets). Results showed that yield difference between “on” and “off” years in the first and second years was very high as from 94.19 kg for each tree in the first (on) year reached to 13.69 kg in the second (off) year which had about 80 kg difference. But yield difference in following “on” and “off” years (third and fourth years of experiment) reach to about 40 kg that was about half of previous alternate bearing cycle. Therefore, urea spraying adjusts alternate bearing during years of experiment, gradually. Treatments of urea spraying had significant difference on cumulative yield at 5% as the highest obtained from control and at flowering spray treatments. Also, effect of different urea treatments on alternate bearing index was significant. Control trees, had the most alternate bearing index and winter and spring spray treatments had the least alternate bearing index. All urea spray treatments increased fruit average weight compare to control. Therefore, based on the results of this project to modulate alternate bearing, winter and spring spraying of urea recommended.