It looks like scale damage. If you noticed some fuzzy or cottony white "stuff," those are often "crawlers" or the larvae stage of scale. Scale can leave behind yellow, blotchy areas where the insect sucked the leaf. Spider mites can also affect the undersides of the leaves (mites usually occur in dry conditions).
Fungal infection will cause dark necrosis. Alcohol can remove scale. Or you can spray the plant with a horticultural oil. If all else fails, then an insecticide.