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New Zealand Plant Protection 69 (2016): 230-237

Postharvest foliar nitrogen applications increase Neonectria ditissima leaf scar infection in apple trees

G.H. Dryden, M.A. Nelson, J.T. Smith and M. Walter

ABSTRACT

Postharvest foliar nitrogen (urea) is often applied to apple leaves immediately after picking for bud fertilising, and/or during leaf fall for Venturia inaequalis control. During 2013-2016, ethylene diamine tetra-acetic acid copper (EDTA-Cu, to enhance leaf abscission), urea, calcium nitrate and Bud-Wiserô foliar treatments were applied alone or in combination to determine their effects on leaf scar infection by Neonectria ditissima in 'Braeburn' (2013-14), 'Scifresh' and 'Royal Gala' (2015-16) orchards. In 2013-14, leaf scar infection increased six-fold when 5% urea was added to EDTA-Cu and sprayed at the onset of leaf fall. In 2015-16, up to a nine-fold increase in leaf scar infections was observed. The timing of application was more important than the amount or form of nitrogen used. As a result of this research, the use of urea-based foliar nitrogen fertilisers for V. inaequalis before leaf fall are not recommended, and growers should consider all factors affecting Neonectria ditissima infections before applying nitrogen immediately after harvest.

Keywords: European canker, Nectria galligena, fertiliser, picking wound.

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