Fruit trees grown in orchards and home gardens are as susceptible to disorders as other plants in the landscape. Yellow foliage on a fruit tree is commonly caused by nutritional deficiencies.
Deficiency of iron, manganese and magnesium in fruit trees causes foliage to turn yellow. Sometimes the fertilisers used in trees also contain herbicides that cause foliage to yellow and thicken. Nutrient deficiencies are common in trees growing in overly alkaline or poorly drained soil.
Iron deficiency causes foliage to lose colour, turning yellow with only a network of green veins. Leaf edges often start to die. Manganese deficiency commonly affects only the older foliage; young foliage is affected in more severe cases. A deficiency of magnesium is frequently seen on apple trees during late summer. Foliage yellows and drops prematurely.
Use fertilisers that contain chelated iron to address iron deficiency in trees. You can also apply sulphur to the ground outside the tree drip line. Often the tree is unable to access the nutrients in the soil; the addition of sulphur raises the acidity of the soil and makes nutrients easily accessible to the tree.