looks to me like minor insect damage. i have similar spots on mine and have a few white flies buzzing about but nothing serious enough to bother with fixing...it's also similar to what spider mites leave behind but i don't think you can blame them for what you've got...looks like a plant/leaf sucking insect tho.

i wouldn't worry about it.

fert burn starts on the edges and works inward leaving brown, dead-dry crispy leaves...