Effect of nitrogen foliar fertilizers for blossom thinning on apples
2002
Handschack, M. | Alexander, A.
Between 1994 and 2000 the effect of the blossom thinners urea, calcium nitrate, ammonium thiosulphate (ATS) and AZOLON fluid (methylene urea solution) on fruit set as well as on internal and external fruit quality was investigated on 7 apple cultivars. The mode of action of nitrogen foliar fertilizers with the exception of AZOLON fluid is based on desiccation and scorching of blossoms and leaves and on the resulting ethylene shock. The fertilizers exerted their thinning effect at rates of 1-6% depending on cultivar and year. In August the treatment with urea was still detectable through higher N leaf levels. The calcium content in fruits at harvest was not influenced after calcium nitrate sprays. Nevertheless the treatments with calcium nitrate improved colouring and fruit firmness at harvest. After treatments with urea and AZOLON fluid, an improved fruit size exceeding the effect through thinning was observed as well as a greener ground colour. When applying urea and calcium nitrate nearer to full bloom, russetting was increased. Therefore the use of urea and calcium nitrate is limited to pre bloom, while AZOLON fluid can be used during full bloom and ATS even later.
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