That looks very odd! Is it progressing in any way? Is there still growth where they're bleached out? Is new growth normal now in the affected areas? Did the affected area grow like that, or did normal growth turn like that?
If it's not some sort of poison or mold.... Which it very well could be. We can see that it's starting at the top of the plant. So that should narrow it down to a micro-nutrient deficiency, if it is indeed a deficiency of any sort. How could it possibly be sulphur on an island covered in volcanic sulphur? I don't think that could be it. I honestly have no idea. However I can remember seeing pictures of plants that were deficient in either Iron, or Zinc, that looked like that in a grow book. At any rate, I hope you figure it out. Because I'd really like to know what caused that.