Hello! My parents have a young Yuzu tree that they planted in March 2016.

The tree has been growing steadily and increasing foliage, but lately(the last two months) a lot of the leaves started to turn yellow and leathery brown. The tree gets watered regularly and was last fertilized a month ago. I have attached some photos. Any idea on the cause of the problem? Could it be simple sunburn, or something more serious like a fungus or mineral deficiency/ toxicity?

I have attached some photos: