Foliar application of urea during bloom increases fruit size in 'Williams' pears

Abstract

In preliminary screening of materials for potential bloom-thinning of ‘Williams’ pears in Oregon, USA and Rio Negro, Argentina, treatment with urea in 5% (w/w) or 7.5% solutions was identified as effective in enhancing fruit size. Other materials tested (ammonium thiosulfate, lime sulfur, fish oil or calcium chloride) caused excessive crop reduction, did not significantly increase fruit size, or caused injury to fruit. Urea treatments at 80% bloom were more effective than treatments at 20% bloom. In Oregon, a 5% urea spray at 80% bloom reduced fruit set in one of three years of trial and increased the proportion of large fruit in the crop (211 grams or larger) in one year. A 7.5% urea spray at 80% bloom increased the proportion of large fruit in the crop in all three years. In Argentina, 5% urea at 80% bloom increased the proportion of large fruit (7 cm or larger) in each of two years as measured by the yield of the first of three successive size-based harvests. In general, a higher concentration of urea (7.5%) resulted in greater yield reduction and a greater increase in fruit size. The results suggest that both crop load reduction and fruit nutrition may be playing roles in the fruit size enhancement observed in this study. Studies are currently underway to identify the distribution of nitrogen from urea applied as a 5% foliar spray at 80% bloom within the fruitlet.