Abstract
The effect of urea and biuret on leaf burn was investigated. Urea and biuret were applied as foliar sprays in factorial combinations at the rates of 0,1, and 2% (w/v) and 0,0.05,0.1, and 0.15% (w/v), respectively, in late April to 2-year-old 'Braeburn'/'Malling-Merton 111' ('MM. 111') apple (Malus domestica Borkh.) trees in a greenhouse. The experiment was repeated in the field in late May and in August using 11-year-old 'Golden Delicious'/'Malling 7A' ('M.7A'). In August, additional urea [4% (w/v)] and biuret [0.4% (w/v)] treatments were applied. Biuret alone at all rates did not cause leaf damage. Within the range of concentrations used, urea was largely responsible for leaf burn and chlorosis following foliar sprays. However, presence of both chemicals in the spray solution increased leaf injury in April, but not at the later applications. Growers should use low-biuret urea within reasonable costs.