Calcium content in apple fruits after thinning treatments in relation to crop load, fruit size and leaf area
2002
Schroder, M. | Link, H.
Ammonium thiosulfate (ATS) and urea thin apple flowers but damage spur leaves. This may have negative effects on fruit Ca content and incidence of physiological disorders. To verify this hypothesis, single trees (9 replicates/treatment) of the apple cultivars Elstar‚ Jonagold and‚ Golden Delicious were treated with ATS (10 g/L at full bloom or at petal fall) or urea (40 g/L at the end of balloon stage or at petal fall) or were handthinned at full bloom. To simulate the leaf damage occurring frequently after ATS or urea treatments, the leaf area of untreated trees was reduced by removing 25 or 50% of each leaf at the end of balloon stage. Treatment effects on fruit set (fruits/100 flower clusters), crop load (kg/tree), fruit size (g), and on leaf area (size and % damage by lesions) were determined and compared to the Ca content of fruits in a given size class. Damaged leaf area measured 7 days after full bloom ranged from 4.4 to 18% in the ATS treatment depending on year and cultivar. Urea application damaged 8.5 to 21.8% of the leaf area. All treatments decreased Ca concentration in fruits only when crop load was reduced. Reducing leaf area by 25% as well as ATS application at petal fall had no significant or only a weak thinning effect and no or just a weak effect on Ca level in fruits, compared to the untreated control. Fruits of urea treated trees, compared to fruits of hand-thinned trees, did not differ in their Ca concentration when fruit set was similar. These results indicate that thinning effects of ATS and urea on fruit set are more important than leaf area for the regulation of Ca content in apple fruits. Decreased Ca concentrations in fruits in relation to fruit size and crop load will be further discussed.
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