Calcium hydroxide sprays for the control of black spot on apples - Treatment effects on fruit quality

To reduce the number of fungicide sprays for the control of black spot Venturia inaequalis, a novel approach has been developed by the Tasmanian Department of Primary Industry and Fisheries. This involves the modification of the leaf surface pH, by the use of calcium hydroxide, to a level which inhibits the germination of fungal spores. Unfortunately, the foliar residue from the calcium hydroxide may reduce photosynthesis and hence affect potential yield and fruit quality. For this reason four field trials were established. The three treatments tested were an unsprayed control, four sprays of calcium hydroxide, and calcium hydroxide mixed with copper hydroxide at the final 2 application dates. At commercial maturity, fruit were harvested and assessed for quality. Fruit parameters measured varied between cultivars ('Golden Delicious','Fuji' and Red 'Delicious') and included yield, colour, skin strength, incidence of russet, weight, shape, firmness, ethylene production, sugars, starch, water core and alcohol insoluble solids. Calcium hydroxide alone did not affect any of the fruit quality parameters measured whilst the calcium plus copper spray increased fruit firmness and fruit alcohol insoluble solids in some trials. For 'Golden Delicious' the combined spray mixture increased the incidence of fruit russet to an unacceptable level. The commercial significance of these findings is that apple trees treated with calcium hydroxide for the control of black spot have similar yields and fruit quality to unsprayed trees. These findings demonstrate the crop safety of this treatment for the control of black spot on apples. The addition of copper hydroxide had some positive effects on fruit quality, however, the increase of russet limits the commercial application of this treatment.