Leaf Miner

This damage appears to be caused by a leaf miner. A small fly or moth lays its eggs beneath the surface of the leaf and when the eggs hatch, the larvae tunnel thru the leaf tissue, consuming the nutrients in the foliage. Once the damage has been done, nothing will reverse it. Chemical pesticides are not effective against this pest because most products won’t penetrate thru the leaf surface although organic products containing Spinosad have often
worked to prevent further infestation. If you do use Spinosad, spray in the early evening after the bees have returned to their hives. Once the spray is dry it is safe for beneficials. However, once the damage has been done to the leaf, it is not
reversible. Usually the damage done by these insects is not going to kill the plant unless the damage is extensive.

infestation is extensive. Here is a link that you might find helpful:
http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/PESTS/vegleafminers.html