Macadamia trees in New Zealand

I have two Macadamia trees in my 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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Hi John, It sounds like a soil deficiency. Magnesium deficient soil makes older leaves yellow with wide green veins. If you've been using high potassium fertiliser on the trees that could cause the magnesium deficiency. You could try watering the trees with some epsom salts added to the water, I don't know the ratio off hand but a google search should turn some info up for you (it's in the order of 200gms per 10 lt, I *think*).
Do you have a good nursery nearby that you could take some of the affected leaves to for a diagnosis?
Good luck with it :) A
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