Effects of water stress control in apple trees by misting

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Abstract
Summary Comparisons of intermittent misting with ‘pure’ (deionized or rain) and lake (high in Ca(HCO3)2) water on both irrigated and unirrigated Cox’s Orange Pippin trees on M.26 rootstock were made from June to September in 1973–6 to investigate the effects of alleviating water stress on fruit production. Mist treatments were applied only on days of relatively high evaporative demand, and irrigation frequency was determined by soil moisture tensiometers. In seasons of relatively low soil moisture stress the mist effects masked those of irrigation where the treatments were combined, but in seasons of high soil moisture stress and high evaporative demand the mist and irrigation effects combined to give the best yields of prime grade fruit. Both irrigation and misting reduced water stress and their effects were additive in this respect as well as on cropping; this supported the concept that environmental and physiological (cropping) stresses are additive, the reduction of environmental stress permitting the tree to withstand a higher cropping stress, and vice versa. Misting increased fruit bud production, decreased fruit set per 100 flower clusters, and increased fruit number per tree but did not affect fruit size or relative trunk growth rate. Irrigation increased fruit size and relative trunk growth rate but not fruit number per tree. Both mist and irrigation improved fruit quality by reducing rough russet and fine cracking. Misting with lake water practically eliminated bitter pit but this advantage was offset by a deposit of CaCO3 on the fruit that could, however, be removed by dilute citric acid solution. Misting with pure water also reduced bitter pit significantly in two years out of three without apparently improving the Ca level in the fruit or affecting fruit size. Irrigation consistently reduced soft rots. The relative effectiveness of misting and irrigation could not be compared in terms of the amount of water used because individual trees were misted and each application of mist was dispersed very rapidly away from the treated trees.

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