Macadamia trees in New Zealand

I have two Macadamia trees in my New Zealand garden near Wellington. One is a Beaumont and the other is a Nutty Glen, both planted about five years ago. The site is well sheltered from strong winds, and both are growing well. The ground is over an ancient river bed, and is well drained. The Beaumont has this year produced a few racemes of flowers. New leaves are healthy looking, and deep green, but older leaves are yellow between the veins, and develop brown/purple marks, which dry up, and fall off. Is this a sign of a virus/bacteria attack, or a pH or soil deficiency of some kind? Any advice would be appreciated. John
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