By Bazcoleman
United Kingdom 
My miniature orange tree which grows in a pot in my conservatory has developed creamy yellowish blotching on many of its older deep green leaves. The blotches seem to follow, roughly, the path of the veins or between the veins. Also, newer leaves are completely pale yellowish with barely visible green veins.
Otherwise the plant seems healthy with plenty of miniature, but, as expected, inedible, oranges.
There are no visible insects. I am trying a citrus feed and a systemic insecticide, but would appreciate any advice. This plant "belongs" officially to Mrs Coleman so re-potting in the compost bin is not a option.
Barry Coleman
27 Jun, 2011
Answers
Maybe some magnesium deficiency, too. A half teaspoon of Epsom salts dissolved in a gallon of water should help that, at least for the next 2-3 months.
28 Jun, 2011
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Hi Baz, it sounds as though your soil is suffering from iron deficiency. Orange trees need the same sort of soil as camelias and rhododendrons, so add some Sequestron to the water next time you give it a soaking. You can buy sachets of this from any Garden centre, and it adds much needed iron to the soil. It'll be easier to control with the plant being in a pot.
Hope this has helped. Good luck with your orange tree. Annie (Cumbria)
27 Jun, 2011