As Susanne wrote it is probably a manganese deficiency. A manganese deficiency shows on the newest leaves having green veins WITH green border areas to the side of the veins, on an otherwise yellow leaf. Manganese deficient leaves remain NORMAL SIZE. Manganese deficiency commonly shows up when the tree is flushing new growth, but not always. I can't see your picture all that well, so I will also mention it might be a zinc deficiency. A zinc deficiency looks very much like a manganese deficiency --- green veins with a green border area to the side of the veins, and also showing on the newest leaves, but in a zinc deficient leaf the leaves are substantially dwarfed in size. You can foliage spray with a dilute solution, or apply the fertilizers to the root zone, using manganese sulfate or manganese nitrate to correct the manganese deficiency. The same procedures using zinc sulfate should take care of a zinc deficiency.