Evaluating the efficacy of a preharvest combination of calcium and boron as foliar application to reduce sunburn on 'Cripps Pink' apples
Sunburn severely reduces external apple fruit quality destined for the fresh market. Under local conditions, a 20 to 40% sunburn incidence has been reported on sensitive cultivars like Granny Smith with the resulting reduction in pack out and income due to unacceptable fruit finish. Thus, there is an urgent need for affordable technology to reduce sunburn incidence consistently in the apple industry. This need is exacerbated by the higher temperatures predicted in future for the Western Cape region. Previous results showed a significant reduction in sunburn incidence in Golden Delicious apples after a preharvest foliar application of a combination of Manni-Plex (MP) Ca (calcium) and MP B (boron). This paper investigated the possibility of applying a similar treatment to a bi-coloured cultivar, ripening much later during the season, which is also susceptible to sunburn damage. Combinations of Ca and B with various formulations (MP Ca, MP B, Solubor and calcium nitrate) were applied on bearing Cripps Pink trees at the Welgevallen Research Farm, Stellenbosch, during three consecutive seasons. Treatments were applied weekly for six consecutive weeks from 40 (2013/14) or 28 (2014/15 and 2015/16) days after full bloom (dafb), for three consecutive seasons. Sunburn incidence was evaluated visually on individual fruit. Fruit were categorized into five sunburn categories according to an adaption of Felicetti and Schrader (2008). In 2013/14, the control, showed a significantly higher sunburn incidence compared to the treatment with a combination of MP Ca and MP B. During 2014/15, fruit were harvested later and showed improved colour at harvest, but also masking of sunburn. No significant differences in sunburn incidence were found between treatments. This emphasises potential underestimation of sunburn incidence in bi-coloured fruit due to masking of sunburn damage to fruit, resulting in poor internal quality. However, results for 2015/16 showed a significant reduction in sunburn incidence of class 1 fruit with all formulations of the Ca and B combination compared to the control when applications commenced 28 dafb.
Lötze, E., Daiber, S. and Midgley, S.J.E. (2018). Evaluating the efficacy of a preharvest combination of calcium and boron as foliar application to reduce sunburn on 'Cripps Pink' apples. Acta Hortic. 1217, 61-68
Malus domestica, Solubor, calcium nitrate, early season, masking