What is wrong wiht my Spinach?
That is most likely is leaf miner, based on what i can decipher from the picture. The damage is caused by a tiny fly that lays eggs on leaves and then the larvae tunnel into the leaf. Adult flies also feed on the leaf. The problem seemed to arrive much earlier this year. You may be interested in going to link for more information and what you might be able to do about. Insecticides are effective but are best applied when you first begin to see the damage.