Salt maybe in some situations. I have the same issue on some varieties in the greenhouse. They get fresh ground water and only very little fertilizer once a month. Seen this also after up potting as well. Fresh potting mix should no contain a lot of salt or any fertilizer. On the other hand, I do not see this on my trees planted in the ground. In my mind, the difference is less direct sun, lower air circulation, higher heat and humidity in the greenhouse possibly causing stress. Not sure but it is my best laymen guess before hearing about salt. Thoughts?