This is just a guess, but is your star fruit in full sun? I've read that they prefer part-shade. Maybe try shading it from afternoon sun while it is still getting established. I don't think that too much water would be the issue, unless it is actually standing in water.

The fruit is ripe when it turns yellow.

Too much water would be my first guess. They like it particularly well drained. It might just be going thru a leaf drop and might flush back without a problem. How well drained is the area where it is at?

Gary

Thanks for the responses, guys. The soil is mostly rocks and sand and doesn't retain water particularly well. The tree is in full sun, and it has been a brutally hot summer so maybe that is the problem. It did have a leaf drop before, but not this bad, and did flush back. I am just concerned because over 50% of the leaves are yellow. I recently planted some citrus and added some of the citrus fertilizer (miracle gro citrus & avocado fertilizer) to the soil around the drip line. I am wondering if that has anything to do with anything?