January 29, 2013
Sudden appearance of necrotic spots between leaf veins are generally a first indicator, followed by dropping of the affected leaves. This is the typical course of the disease, which due to its visible symptoms has been termed “leaf spot” and which eventually results in “premature leaf drop”.
The cause of the disease is still unknown; it has, however, become more frequent in recent years. Among the varieties most seriously affected are Golden Delicious, Jonagold and Gala. Early leaf drop most frequently occurs when hot and sunny days are followed by several cool, rainy days. Initially, the spots appear pale green, but turn a dark shade of brown within just a few hours. Four to five days later, leaf drop sets in. Older leaves from the middle part of the branches appear to be most susceptible. Once a tree has been affected, this scenario is usually repeated two to four times within the mid to late growth period. The final result is a reduction of the tree’s yield.
Experience has shown that a sufficient supply of the nutrients manganese and magnesium appears to help to mitigate leaf drop symptoms. In order to further clarify this connection, Dr. Duilio Porro from the Istituto Agrario di San Michele all’Adige (IASMA) conducted a four-year trial on behalf of K+S KALI GmbH, in the North of Italy (Salorno/ South Tyrol).
Within one vegetation period, Golden Delicious trees were treated with EPSO foliar fertiliser, while the control group was merely sprayed with water. All plots were equally treated with NPK-fertiliser (N:P2O5:K2O = 60:20:80).
The fertilisers used were
The quantity applied was 15 kg product ha-1, per application. The numbers of leaves dropped were counted for the months of July, August and September.
Comparison with the control group also showed a lower number of necrotic leaves for all EPSO fertilisation trials. Here too, the best results were achieved by EPSO Combitop®, followed by EPSO Microtop®.
Additionally, the trial series proved the excellent uptake rate of EPSO foliar fertiliser for apple trees. Analyses of leaves as well as of apples showed an increased content of micronutrients boron, manganese and zinc directly after foliar application.
Foliar fertilisation supplying manganese in combination with magnesium appears to help to mitigate the symptoms of leaf spot and therefore may reduce early leaf drop. We therefore recommend a preventive application of 10 – 15 kg EPSO Combitop® ha-1 after flowering, upon fruit set, followed by two more foliar applications three weeks apart (beginning of July and beginning of August).
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