Oscar,
Just food for thought. I've noticed a small bug on my mango trees, the likes of which I have never noticed before. They can fit through the eye of a regular sewing needle and are jet black and skinny, with a pointed tail. They are tiny, but in sufficient numbers, I have seen them destroy bunches of leaves. I doubt it's something relatively new to Guam because the symptoms they cause here on island has been long documented. They prefer surfaces of young leaves (probably fruit as well) and they cause ugly russeting on my mango leaves. No fruits yet to test my theory.
They seem to favor my NDM and Gold Nugget over all others. Do you think that some sort of insect like what I described actually prefers certain cultivars? And maybe...just maybe some years, natural predators keep their populations down enough to cause less-obvious damage?