Peach [Prunus persica (L.) Batsch, Peach Group] tree productivity is improved if trees are thinned early, either in full bloom or when the fruit is recently set. Chemical thinning reduces the high cost of manual thinning and distributes the fruit irregularly on the shoots. The effect is similar to a late spring frost that mostly affects early flower buds on the tip of the shoot. To simulate frost damage (or chemical thinning) and evaluate the effect of fruit distribution on production, fruit growth of several peach cultivars—'Catherine', `Baby Gold 6', `Baby Gold 7', `O'Henry', `Sudanell' and `Miraflores'—and the nectarine [Prunus persica (L.) Batsch, Nectarine Group] `Queen Giant' was studied in the central Ebro Valley (Spain) in 1999 and 2000. The factors investigated were the intensity of thinning and fruit distribution on the shoot (concentrated in the basal area or uniformly placed). The treatments were performed at 30 days after full bloom in 1999 and at bloom in 2000. For `Baby Gold 6' and `Miraflores' and when fruit load was high after thinning (over four fruit per shoot), a high concentration of fruit on the basal portion of the shoot had a negative influence on final yield and fruit size. The intensity of thinning (or simulated frost) greatly affected fruit diameter but was also strongly related to cultivar, tree size, and length of shoots. Thus, relationships between thinning intensity and fruit diameter varied, even among trees of the same cultivar.
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