If you asking about the yellow colored leaves, they are just old leaves, The life span of a citrus leaf is approximately 18 months. Before the tree discards them, it removes all the usable nutrients from the leaf to reuse in other portions of the tree (causing them to turn yellow) then discards them, Nothing to worry about.

Thanks. I’m asking about the tiny fruit that has turned brown/black in the photo

I spy some webbing - it may be an insect, possibly a caterpillar burrowed in there and hiding.

Did you keep the tree well watered from the beginning of the bloom, until after fruit set? Irrigation during the above period is extremely important.

My hunch was that I overwatered. I really soaked the ground 2 times a week for a while. I also had some yuzu buds fall off another tree next to this one right after the flowering stage when the tiny fruit would appear. There are some webs, yes. Could that be the culprit?

FWIW, insects and diseases tend to affect weakened/stressed plants.

What do you mean over watered it?

It's not the amount of water that suffocates the roots but the lack of oxygen. What kind of mix do you use?

A porous one or a fine packed one? One the drains and dries out fast or slow?

Mike

Wherabouts in the world are you? If you are in Europe, this looks very much like the damage caused by the Citrus Flower Moth and its caterpillar, Prays Citri. But I suspect you are in US and I don't think this pest has arrived yet!

Carefully delve into the top of the shoot with the damaged buds and see if you can find anything suspicious living there. I don't think this is a watering issue.

FWIW, I get these tiny caterpillars that do similar type of damage - mostly to plants in the rose family - and they wriggle violently to escape and drop to the ground when uncovered.