Apple trees typically have glossy, medium green leaves. When leaves turn yellow, either the weather has turned cold or the tree is suffering from some sort of disease and nutritional deficiencies. Over time, these can result in the yellowing of large numbers of leaves, known as chlorosis. In some cases, quick action is needed to return the tree to a healthy state.
When cold weather sets in, apple trees, which are deciduous, will drop their leaves in preparation for winter. The tree stops the flow of water and nutrients to the leaves at the end of their stems. This reduces the photosynthesis within the leaf and consequently the amount of chlorophyll that gives the leaf its green colour. Without chlorophyll to colour the leaf, other naturally occurring chemicals in the leaves become visible as the colour yellow, turning all the leaves on the tree to this colour.
Certain diseases can also cause apple tree leaves to yellow. According to Ohio State University, one example is called necrotic leaf blotch, which typically attacks Golden Delicious apples. In this disorder, the leaves typically develop brown spots, then turn yellow. Eventually, the tree may lose all its leaves. Scab is another disease that can yellow apple tree leaves, states North Dakota State University. Insect infestation can also weaken the tree, causing leaves to yellow.
Occasionally, trees will suffer a yellowing from a deficiency of proper nutrients in the soil. Low levels of minerals, including iron, manganese, copper, magnesium, sulphur or zinc, can cause leaves on apple trees to yellow. For example, a lack of manganese typically presents as yellowed leaves that retain green veins. Leaf size and shape may also be affected. Once soil testing is completed, quick and proper application of supplements can help return the tree to a healthy condition.