Effect of Thinning the Crop of 'Kawano Natsudaidai' Trees after Partial Defoliation by Typhoon September, 1991.

Over 80% of the leaves from 28-yr-old 'Kawano Natsudaidai' (Citrus natsudaidai, Hayata) trees were removed by typhoons on September 14 and 27, 1991. To allay the question of whether fruit thinning would hasten the recovery of the damage, four thinning treatments were applied : 1) the entire crop was removed in Octover; 2) half of the crop was eliminated in October; 3) all fruits were removed in November; 4) no thinning was done. In addition, relatively undamaged trees were observed as a control.1. Two months after the typhoons, the amount of dead wood was not significantly different among treatments, but nine months later, trees in the unthinned plot had more dead limbs than did trees in the completely defruited plot.2. Trees completely thinned in October had more new fall shoots than did trees in the three other treatments.3. The new shoots which developed in the following spring produced about the same number of leaves, but more shoots per lateral shoot emerged on trees thinned in October than they did on unthinned trees. Completely thinned trees had more dead roots than did unthinned trees. All roots over 1 cm in diameter on unthinned trees died and their rootlets were very thin.4. The acidity of fruits on defoliated trees did not decrease, and the fruits failed to turn yellow. The sugar content of fruits from defoliated trees was lower than that of fruits harvested from healthy trees.Our results lead us to recommend that after a severe typhoon during which 'Kawanonatsudaidai' trees are defoliated, the crop should be completely thinned to aid in the speedy recovery of trees.