https://www.nature.com/articles/d41586-019-00875-7

Luak, your trees always look healthy.  You do a great job caring with them.

What is the makeup of a "gray sand soil "?  Containerized citrus trees absolutely require to be grown in a well draining aerated medium.  There are several commonly used mediums, but the number one used medium for container plants is a 5-1-1 which is 5 parts bark, one part peat and one part builders sand (some use perlite in place of sand).  Newer growers of citrus should not feel badly about losing a tree or two, as the average life of a container citrus tree is less than two years.  However, as one gains knowledge, this changes rapidly.  There are  container citrus trees that are over 100 year old.

behlgarden, are you sure about leafminer on the tree's first flush of the year?  Seanny is correct in that the leaf miner does not attack until the second flush. 

Pepsi, through its partnership with local distributor Suntory, has also offered special flavors just for Japan. Now, this spring, Pepsi Japan is set to roll out its newest creation, featuring one of Japan’s most delicious and refreshing traditional seasonings. Pepsi was impressed at how many Japanese desserts manage to provide a burst of sweetness that seamlessly transitions into a clean, crisp finish, and one way they do this is with yuzu, a Japanese citrus fruit.

Yuzu’s flavor falls somewhere between an orange and a lemon, sharper than former but not as intrusive as the later. While the fruit is rarely eaten directly, it’s popular flavoring for cakes, dumplings, and sweet bean jams, and now Pepsi is promising a crisp, satisfying yuzu aftertaste for Pepsi Japan Cola.

Rust mites causes damage to the peel, however the inside of the fruit is not affected by the mite.  You should be able to rid the mites with a good horticultural oil spray.  Spay early in the morning.

https://news.ucr.edu/articles/2019/03/14/35-million-givaudan-gift-will-protect-citrus-collection

Of the trees that I have, I rate them in order:  Grapefruit, Valentine pummelo, Cara Cara navel, Dekopon,, New Zealand Lemonade, Ponkan all of which are in ground trees.  The rest (lemon varieties, mandarins, kumquats, )  fall into the OK class.

Trees on trifoliate have good to excellent internal fruit quality with all common scions.  Fruit typically have high solids, acids and juice content. I have various citrus cultivars on Flying Dragon, and there fruits are all of normal size.  .

Just an old leaf.  A citrus leaf has a life span of approximately 18 - 20 months. When a leaf is no longer able to carry out all that is demanded by the tree it is quickly discarded.  There is no welfare program for old leaves.  For citrus trees the time of greatest leaf fall is during the tree's blooming period.

In the 2018/19 season, the world's orange production will increase by 4.2 million tons compared to the previous year and reach 51.8 million tons.

One further note:  From the very first sign of flowering, never let the tree's root system go dry.  Doing so will quickly cause the drop of most or even all flowers greatly reducing the crop.