Did you get fertilizer on the plant leaves and not wash it off that can cause dead spots. I know copper sulfate spray will make dead spots on leaves.
I read something about leaf spots 2 days all I can remember is it has something to do with certain minerals that are not available in the soil. I don't remember where I read it. Soil needs, calcium, potassium, phosphorus, magnesium, iron, sodium, zinc, nitrogen. You can not get all of these in any factory made fertilizer that I know of.