Manganese foliar application to prevent leaf drop

The effects of magnesium (Mg) and manganese (Mn) on apple leaf drop are not well documented in literature. Mn is involved as cofactor in many reactions of plant metabolism. One of its important functions is linked to the chlorophyll efficiency and in redox processes. Compartmentation of Mn++ in cells could compete with Mg nutrition: Mg uptake, therefore, can be strongly depressed by Mn cations and vice versa. A decrease in Mn content of leaves has a small effect on chlorophyll content but a great effect on photosynthetic oxygen evolution drop. Mn deficiency (on apple 15 mg Kg-1 dry weight in fully expanded leaves) is often linked to interveinal or blotched chlorosis both in young and in old mature leaves. Mn availability is reduced in basic soils, with a large organic matter content. These conditions are often registered in Non Valley (Trentino -Northeastern Italy) where early leaf drop is widespread. Normally leaf drop is linked to Mn deficiency. It tends to be more common when a cool, rainy period of 4 to 5 days is followed by hot, summer weather. The disorder is characterized by the development of necrotic blotches or irregular areas of dead tissue (brown blotches) in mature leaves. The present work aims at verifying the effect of foliar application of Mn in order to correct Mn deficiency and to prevent leaf drop. After a preliminary experiment carried out in 1996, the trial was performed on Golden Delicious at Cles (Non Valley), using a factorial experimental layout from 1997 to 1999. Sixty trees were tested for three different foliar applications of Mn (test, autumn and spring applications). The effect of rootstock on Mn nutrition was also tested in five different rootstocks (M9, M26, MM106, M11, Mark) each replicated four times. During the trial period leaf drop incidence, percentage of blotched leaves, greenness and nutritional status of the leaves were yearly measured throughout the growing season in different periods. Spring treated trees had higher leaf Mn concentration than autumn treated trees and control ones. Leaf drop incidence, leaf greenness and percentage of blotched leaves were statistically affected by foliar application. The spring combination was more effective in reducing leaf drop than autumn treatment. Rootstock effect was registered too on Mn and Mg nutrition. MM106 and M11 had the lowest leaf Mg concentrations, whilst Mark the highest values. M26 showed the highest values of leaf Mn concentrations, whilst M11 the lowest, in test trees and in autumn Mn foliar sprayed trees. The greatest disorders were associated with low values of both Mn and Mg. The role of Mn seems to be more important in determining of leaf drop. The rootstock choice (M9, M26 and Mark are preferred) could be therefore successfully used in prevent leaf drop.