The effect of nitrogen nutrition on bitter pit in apples

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Abstract
In a three‐year container experiment with “Cox's Orange Pippin"/M 9 in sand culture, the influence of continuous and seasonally varied ammonium and nitrate nutrition on the mineral content of the fruits and on the incidence of bitter pit was investigated. During the entire growing season (15 Mar. ‐ 15 Nov.) all trees received an equal supply of nitrogen, namely 9 mM/liter. Part of the trees was given only nitrate or only ammonium (NNN,AAA); another part was given nitrate and ammonium simultaneously (6N3A, 3N6A). Still further trees received ammonium and nitrate in seasonal variance (NNA, NAN, ANN, NAA, ANA, AAN). The content of K, Ca and Mg in the fruits was raised by supplying nitrate in summer (1 June ‐ 15 Aug.) and reduced by ammonium. The K/Ca ratio in the fruits increased during fruit development with both forms of nitrogen, but the increase was cosiderably greater with ammonium nutrition than with nitrate. Whereas bitter pit could not be observed at harvest time, it became evident to varying degrees during storage. Fruits from trees given exclusively ammonium nutrition were infected by bitter pit to 70%, whereas the incidence of bitter pit was much lower (25%) in the case of nitrate nutrition. The best performance (only 8% infection) was found in trees given ammonium in spring and nitrate in summer and autumn.

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