ALLEVIATION OF FROST DAMAGE TO PEAR FLOWERS BY GIBBERELLIN

Late spring frosts are on the increase. They occur earlier, more frequently and more severely from year to year, thereby damaging fruit crops throughout Europe. In April 2003, early spring frosts damaged ca. 88% of the flowers of the early flowering 'Alexander Lucas' pear. Gibberellins may be used to increase fruit set after late spring frosts in many crops, including pear. However, adverse effects of gibberellin applications on pear trees include small fruit size, abnormal shape and poor return bloom, These effects may be due to the sole use of the homologue, GA₃, at excessively high doses. It is also unclear whether GA is more effective when applied directly after the frost, i.e. before flower opening, at full bloom or both. Hence, the objective of the present work was to investigate the optimum timing of the combined application of GA₃ and GA₄₊₇ at low doses in order to improve parthenocarpic fruit set of pear exposed to spring frost before full bloom. Additionally, gibberellin effects on fruit quality, size and return bloom were also investigated. Frost-affected cv. 'Alexander Lucas' pear trees were treated with gibberellin GA₃ + GA₄₊₇, either immediately after the frost at the white bud stage or thereafter or both. Untreated pear trees served as the control. Both initial and final fruit set were significantly increased by a combined application of GA₃ + GA₄₊₇ at full bloom, without adversely affecting return bloom, but June drop was also enhanced. There was no decrease in fruit quality or fruit size after any of the GA applications and no increase in abnormally-shaped, elongated pear fruit.