NAA is used extensively for fruit thinning of apples to increase fruit size and to promote return bloom. In some cultivars, even if thinning is achieved, fruit size at harvest may be less than expected based on crop load. CPPU, N-(2-chloro-4-pyridinyl)-N.-phenylurea, has been shown to increase fruit growth in apples, grapes, and kiwi. We evaluated combinations of NAA and CPPU on thinning, fruit growth and return bloom in Redchief `Delicious', `Elstar', and `Gloster'. CPPU was applied at 5 mgμL–1 (based on 0 to 10 mgμL–1 response curve) in combination with 15 mgμL–1 NAA as high-volume sprays at 7 to 10 mm KFD. Yield and fruit size distribution (on total yield) were used as index of response. In `Delicious', CPPU (3-year study) increased % large (70 mm+) fruit, but in the presence of NAA % large fruit was reduced 2 of the 3 years. CPPU did not induce significant thinning. There were no significant effects on color or soluble solids; firmness was increased slightly and seed number reduced. The L/D ratio was increased and uneven lobe and carpel development was common. CPPU had no significant effect on return bloom in presence or absence of NAA, but NAA increased bloom in both the presence and absence of CPPU. With `Elstar' (2-year study) there was no significant thinning with either chemical, but CPPU increased mean fruit size and % large (70–80 mm) fruit over nonthinned, but not significantly greater than NAA alone. There were no significant differences in firmness, color, soluble solids or seed number. NAA + CPPU did not inhibit fruit growth or cause excessive uneven carpel development. Frost damage reduced crop load in `Gloster' where results were similar to `Elstar' except seed number was reduced by the NAA + CPPU combination.
Evaluation of Fruit-thinning Criterion as Branch Diameter in Citrus Cultivar ‘Harumi’
