Leaf miners are a group of insects (sawflies, beetles, flies and moths) whose larvae live in the tissues of leaves and 'mine' their way through the leaf of the plant. Although visually unattactive, they do no real damage to the plant.
The damage they do is often mistaken for some other problem or disease such as a fungal disease, to be sure it is a leaf miner, tear the affected leaf in half and hunt the larvae down. If you spot a leaf miner on your plants, you can tear the leaf off and discard it or pinch the larvae and kill it.
Pesticides don't work well in regards to leaf miners because they cannot penetrate the leaf barrier. Floating row covers can prevent the adults from laying eggs.Check your leafy crops for frequently for egg and hand crush them.
See Below for leaf miner damage and larvea. The top two are serpentine leaf miners and the third is a blotch leaf miner.