ISHS Acta Horticulturae 636: XXVI International Horticultural Congress: Key Processes in the Growth and Cropping of Deciduous Fruit and Nut Trees
BETWEEN YEAR AND WITHIN YEAR VARIATION IN CHEMICAL FRUIT THINNING EFFICACY OF APPLE DURING COOL SPRINGS |
| Authors: | T.L. Robinson, A.N. Lakso |
| Keywords: | apple,Malus x domestica, chemical thinning, cropload, temperature, sunlight, carbohydrate supply |
| Abstract:
Field chemical thinning studies were conducted in 2000, 2001 and 2002 with 6 to 8-year old ‘Gala’ and ‘McIntosh’ trees on M.9 rootstock and ‘Delicious’ apple trees on M.26 rootstock. Single application sprays of a tank mix of 75 mg l-1 of 6-benzyladenine (BA) (formulation VBC-30001) plus 600 mg l-1 of Carbaryl (formulation Sevin XLR Plus), or 7.5 mg l-1 of Naphthaleneacetic Acid (NAA) (formulation Fruitone N) plus 600 mg l-1 of Carbaryl were applied at 3 or 4 day intervals beginning at petal fall until 28 days after petal fall (DAPF). In all three years, a prolonged cool period following petal fall resulted in slow fruit growth and poor thinning during the first 10 DAPF. Thinning effectiveness was poor when fruits were smaller than 10 mm and improved when fruits were relatively large between 12 and 20 mm. In 2000, the best thinning efficacy coincided with a warm period, but in 2001 and 2002 the best thinning efficacy coincided with one of the coolest periods during the experiment. Weather data from each year was used to calculate daily carbon supply by the tree and demand by the shoots using a carbon fixation model. The difference between estimated carbon supply and estimated demand by the shoots was an estimate of the carbon available to support the growing fruits. Comparisons of the estimated carbon available for fruit growth with the curves of thinning efficacy showed that in the first 10 DAPF, when thinning response was poor, estimated carbon supply to the fruits appeared to be adequate (non-limiting) for fruit growth despite the cool temperatures. Although total estimated carbon supply was reduced due to the cool temperatures, estimated shoot demand was also reduced resulting in an adequate supply to the fruits. From 10-20 DAPF was a period when estimated carbon supply for the fruits was less than adequate due to higher estimated demand by the shoots. This coincided with the timing of the best thinning response. A possible explanation for the thinning efficacy responses is that a deficit in carbon supply supporting young fruitlets growth is a necessary pre-requisite for the fruit thinning chemicals to induce fruit drop in the developing fruitlets. In the 3 years of our study, the best thinning and the greatest deficits in carbon supply both occurred once fruit size was greater than 10 mm. These results may indicate that in cool years it is better to wait for relatively large fruit sizes before fruit thinners are applied. | |
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