This looks more like a nutrient deficiency than a leaf disease, severe chlorosis, downward curling leaves, interveinal necrosis. The brown spots are too uniformly distributed and similar in shape for a fungal or bacterial leaf disease. I think the problem is lower, in the stem or roots. There may be a shortage of mineral nutrients or unavailable nutrients because they are bound to clay particles. With no expertise in mango growing or it's nutritional needs (do mangos have soil pH requirements? mycorrhizae?) , I can only hazard a guess. I would place the symptoms more in the range of something like iron, zinc, calcium or boron rather than one of the major nutrients (N,P, or K).