Effect of prohexadione-Ca (Regalis®) on the effectiveness of NAA and BA used for fruitlet thinning in apple trees

Twelve-year-old apple trees cv. Lobo/M26 were sprayed soon after bloom with Regalis (R) (10% prohexadione-Ca) at 200 g/100 L to control shoot growth. The next season, half of the trees were again treated with Regalis (R). NAA and BA were also applied to some of the trees treated with Regalis (R) as a means of fruitlet thinning. The trees treated with one preparation only, as well as those not treated at all, were used for comparison. The effect of the above treatments was evaluated over a two-year period. The total shoot length was reduced by about 33% in the first year of treatment with Regalis (R). The next season, these trees showed a tendency to more vigorous growth than the untreated control. The repeated use of Regalis (R) caused an inhibition of this unfavourable effect. Thinning by means of NAA or BA of the trees treated earlier (4-7 days) with Regalis (R) did not modify shoot growth significantly. Generally, these trees were growing similarly to those treated with Regalis (R) only, with the exception of the trees on which Regalis (R) and BA were used during both seasons. In this case, a significant reduction in the number of shoots per tree and, consequently, the weakest growth of trees was observed. Application of Regalis (R) in the first season had no significant effect on fruit set, the number of fruits per tree and yield in the year when the preparation was used and also the next season. NAA and BA reduced fruit set and the number of fruits per tree in both years. In the first year, the combined use of Regalis (R) and the thinning preparations produced the strongest thinning effect, especially when BA was used for thinning. In the second season, both thinning preparations caused a reduction in fruit set in the trees treated with Regalis (R) to a level similar to that in the trees thinned but not treated with the retardant.