I have two orange trees which are losing leaves for the first time. One of the trees got bright green leaves. Why is that? I water once a month with citrus fertiliser and otherwise with insipid water.
It is complex to overwinter orange trees because they are tropical plants - and there are no different seasons in the tropics. Moreover, they have the same light conditions all over the year - and they are used to a lot of light.
When the plant is not able to perform photosynthesis adequately it is important to slow down the metabolism. Otherwise the plant will produce less sugar than it needs and will start to metabolise its own plant mass. So the damaged leaves will fall off.
That is why you have to winterize your orange trees as bright as possible at 5 to 10°C. Fertilisation and watering is not the big issue here. Stop fertilisation from May till September and water from dry to slightly wet.
As you say that this has happened for “the first time” it must have been okay during the last years. Orange trees are highly vigorous and the older they are the more consumptive they get. So maybe you have to fertilise more during the growth season. General fertilising recommendations are: Every 10 to 14 days, 2 g/litre – equivalent to about 20 ml. And take care that your trees get enough calcium carbonate when you use insipid water, but just as much that you do not block iron absorption which is very important.