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.
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
We planted a mandarin orange tree spring of 2017 and got about a dozen fruits the following December. However, the tree didn't bloom this year. I fertilized in the spring and summer with citric fertilizer but no good. Several of the new leaves are curled (not brown). What am I doing wrong?
This article should help:
it's aso firmer than usual to bite into and less juicy, do You know why this may be?
This is, typically, due to bad watering practice. Not enough water is the culprit.
Here are some articles for more information:
We planted a tiny mandarin orange tree. Some of the leaves are starting to turn yellow. Could I be overwatering, or underwatering?
Overwatering or underwatering is certainly a possibility. Below are care tips that should help.
Also examine the plant's leaves closely; they may have organisms such as scale that aren't obvious at first. This can be easily treated with Neem oil. Your soil may also be lacking in nutrients, such as iron.
It is about 5 feet tall when I replanted it. I used miracle grow soil and added bone meal. This tree is over 50 years old and was started by my late father.HELP! Thanks in advance
It's probably in transplant shock and should recover. Keep it watered and continue with its normal care. Here are more causes of leaf drop:
Here's an article that will help:
I have a section my Golden Nugget Mandarin with long thorns and round fruit on some branches but not all. Is this rootstock and should I prune It Off?
This is a sucker that was let to get very mature. They steal valuable nutrients from your grafted tree, and need to be removed when young. You will find, if you follow the branch down, that it originates from below the graft point.
Removing it now will be the best option, but keep in mind that once they get this mature they can suffer from infection when pruned.
Make sure that you shears are sterile, and you wipe the cut with peroxide or a 10 percent bleach/water solution after pruning.
These articles will help:
I had about 10 limes this year but now all I have are spikes no leaves
No, this is a sign of distress. Disease, over/underwatering, insect pests, and many other environmental factors can cause this, so diagnosis without clear photos of the tree and area around will be difficult.
In the meantime, I can include one of our articles on the care of the tree. This will allow you to know what care the tree needs in order to be healthy: