| Summary: | Alternate bearing is a phenomenon in some varieties that after high yielding year (On year) which fruits are very small and non marketable, flowering decrease significantly and yield will decrease so much that harvest costs is more than grove income occasionally. This bearing cycle that is usually biennial, has high severity in Unshiu mandarin as an important commercial citrus variety of Iran specially in Mazandaran province and can cause severe losses for citrus producers and citrus industry. In this study for decrease of fruitlet density in "on years" and regulating of crop production in continuous years, an factorial experiment was conducted on 18 years old Unshiu trees grafted on Poncirus rootstock in RCBD with three replications. In this four year's study, which was conducted in Kotra Citrus Experiment Station situated in Tonekabon city, effects of tree spraying with NAA on fruitlet thinning was evaluated. Surveid factors were NAA concentrations (200, 300 and 400 mg/L) and time of tree spraying (30 and 50 days after full bloom) and obtained results were compared with check (unsprayed)trees. Results showed that tree spraying with 400 mg/L at 30 days after full bloom can cause significant differences with check trees and can decrease fruitlet density at high yielding years significantly and improve crop quality on off years.
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