Influence of Time of Thinning on Yield, Fruit Quality and Return Flowering of 'Fuji' Apple

Abstract

A study conducted over two growing seasons examined the effect of time of hand thinning of 6-year-old 'Fuji'/M.9Nagano apple trees on fruit quality and yield at harvest in 1999 and return flowering, yield and fruit quality in 2000. Primary thinning (PT) at 7, 17, or 28 days after full bloom (AFB), in combination with secondary thinning (ST) at 60 days AFB (when crop load was adjusted to a union form level based on one fruit for every 55 leaves) proved effective for obtaining 300 g fruit at harvest. Trees receiving PT at 7, 17, 28 and 37 days AFB followed by ST at 60 days AFB showed atleast 50% of the fruits larger than 300 g at harvest in 1999. No thinning, primary thinning applied later than 37 days AFB, or ST only applied at 60, 74, or 94 days AFB failed to result in the 300 g average fruit weight standard. However, only the no thinning treatment resulted in reduced soluble solids levels at harvest. Flower buds as a percent of total terminal buds in 2000 ranged from 2 to 75% and was clearly related to time of primary thinning in 1999. Primary thinning as late as 28 days AFB resulted in ad equate return flowering. All trees involved in the 1999 trial were thinned in 2000 with PT at 30 days AFB followed by ST at 60 days AFB. This resulted in 2000 yields ranging from 5 to 166% of those recorded in 1999, depending on the 1999 thinning regime. Only the trees receiving primary thinning by 28 days AFB exceeded yields obtained in 1999. In conclusion, this trial shows that PT within 28 days AFB in combination with ST for deciding crop load within 60 days AFB is the desirable practice for obtaining large size (around 300 g) quality fruits and enough return flowering for full cropping of 'Fuji'/M.9 apple trees at Nagano, Japan.