Thanks everyone. I have been using FP at 1 tsp per gallon once a week on this tree and also have Osmocote Plus in the pot (both contain minor and micronutrients). I will give it a few treatments with the Epsom salts and see what happens. I'm still not sure about the timing of the insect soap - the label says 3 treatments a year which I am already over so maybe I will just stick with neem for the mites. There is, however, a more ridiculous problem. The periodic cicadas are out and whenever I see any on any of my citrus I always shoo them away but I noticed this today: One got by me. She has cut slits in the branch and laid eggs inside. This is very upsetting. That branch will die within a few months and the eggs will hatch and nymphs will travel down the trunk and into the roots where they will feed on the roots. I'm not at all worried about this on any in-ground tree as they have roots to spare. But this is a large tree in a very small pot with no roots to spare. I'm considering using Bifenthrin on this tree now to hopefully kill the eggs and nymphs. It is not labeled for foliar application in the US (it is in Brazil with a PHI of 7 according to this document). I don't know what to do. Ideas? The thought of 500 nymphs sitting in that pot for 17 years eating roots (have you seen the size of full-sized cicadas?) is pissing me off. They will almost surely kill this tree that I've worked so hard to keep alive. Even if I have to sacrifice this crop of lemons (ie throw them away because of a chemical pesticide) I would rather that than lose the tree.... anyone have any experience with container fruit trees and cicada damage?

That water droplets on foliage + sunlight can burn foliage due to the water droplet serving as a mechanism by which light can be focused on a particular point efficiently enough to cause temps high enough to 'burn' foliage is a horticultural myth - ain't gonna happen. It appears to be a mechanical injury of some sort - or possibly scars related to a physiological malady called oedema, which occurs when plants take up more water than they can rid themselves of via transpiration. Al

Looks like a soil issue... salt? Too high pH? Are you by any chance watering with water from a water softener? You would be surprised how many plants are killed that way, as water softeners replace minerals with salt. Can you tell us what type soil you have? Looks like it may be planted next to a wall; if so, is your house new? Sometimes around new buildings concrete or lime is left in the soil and that will kill a lemon tree.

Thank you all for those answers. I keep my citrus indoor but I've put them one or two week on my patio in the middle of summer. I don't know if it's sunburn because white spot continue to appears months after I've put my plants indoor and away from the sun. I think it's a chemical burn most likely. One time, and one time only (hopefully) I used a "water spray" for human face on my citrus to hydrate them. But the water contain sodium, sulfate, bicarbonate, chlorine, nitrate, silicon dioxide... :(