I live in Gran Canaria and am having problems with a young Mandarin tree.
The tree is potted and in full sunlight. Since I bought it in March it has flowered twice, but both times within 2 weeks of appearing, the fruits have turned black and fallen off. Any advice much appreciated.
Certified GKH Gardening Expert
Without photos it will be hard to say. My guess is overwatering. What you describe is a fungal infection, and it is not easy to cure them in citrus.
There will be a few things that you are going to need to care for your citrus in your area.
Iron sulfate, Dolomitic lime, and citrus fertilizer will be necessary every year, with fertilization happening every few months.
Curing the fungus will require a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Now, without at least of 200 watts of Horticultural lighting (Don't use cheap LED's for citrus. I recommdend a cheaper CMH lamp if you are on a budget) you will not expect fruit on the tree. Even with bright window lighting, it will not be enough to let them fruit. Your growing season isn't long enough.
Here is an article that will help you with proper care of Mandarins: https://www.gardeningknowhow.com/edible/fruits/tangerine/mandarin-orange-tree-care.htm