Have u had cool wet temps like I have here? Do the plants in the area of that plant have similar type of damage?
My guess is fungus. Looks like Anthracnose, it is a fungus floating around here and got inside and wiped me out of 50 clones and 20 moms. U will notice it everywhere, on all kinds of plants, some worse than others. It doesnt effect fruit, flowersm, or buds, just leaves, stripping the plant to sticks in a few days in the right conditions.