LEAF SPOT OF THE APPLE 'GOLDEN DELICIOUS': A HORMONAL DISORDER
Leaf spots and leaf drop of Golden Delicious have long been known in Europe and the USA and a literature summary has been published by Jonkers (1973). A complex of many factors is involved, but the disorder can be reproduced in growth rooms. It is optimal at 21°C and is promoted by conditions of low light and vigorous growth. Spots are mostly observed in well-kept orchards where, independent of weather, they appear around mid-August, often with further outbreaks at the end of August and early September. It is found on both light and heavy soils. No fungus has ever been detected and virus is not involved. Spotted leaves show Ca and Mg deficiency and an excess of K. Fankhauser et al. (1976) in Switzerland found a foliar spray of MnSO4 + MgSO4 effective against the spots, but we have been unable to confirm this.
Jonkers, H. (1981). LEAF SPOT OF THE APPLE 'GOLDEN DELICIOUS': A HORMONAL DISORDER. Acta Hortic. 120, 253-253