A series of unfortunate discoveries have led me to learn all about the spinach leaf miner, which is wreaking havoc on my spinach, chard and beets. Here is the innocuous appearance of one infected chard leaf.
Holding leaves up to the light shows the perpetrators at work. The little brown patch at right is a clutch of eggs.
A nasty case on some spinach.
Evil grubs (now deceased).
There appears to be no organic solution other than to remove damaged leaves (not into the compost!), check for eggs on the backs of leaves and get rid of them (but make sure you do not knock them into the soil, where they can still hatch).