That's not back spot. That is sooty mold, a fungal organism that grows on honeydew, the sugary excretions of sucking insects. In itself, it is harmless to the plant, unless it covers leaf surfaces to the extent it interferes photosynthesis. It is more of an after-the-fact warning sign of an insect infestation.

The causal insects may or may not be present now but you will need to keep an eye out for them and treat when or if you locate. You could be looking for things like aphids, whitefly, thrips, scale, etc.