Leafminers. This is the worst time of year for leafminer infestations. This one is specific to beet, chard and spinach. Cut off all the affected leaves, seal them up in a plastic bag and throw it away. Cover the plants with floating row cover or tulle to prevent the moths from laying more eggs.

Yup, leafminers. I covered my beets with row covers this year and they still look like yours.

In order for row covers to work, one must plant in a different site than in previous years --
In other words, a place where you haven't had leafminers previously.

hi all, thanks for the replies and information. now i know what it is and ways to prevent it.

@hamiltongardener - Thanks for the tip. I shall do this often to stop the spread!

Thanks.

I came here looking for answers of this as well. Mine wilted and some type of inchworms were chewing leaves. They were much smaller than hookworms and almost a vibrant green color. Even after I removed them all on several plants in different pots in different properties, they all declined a slow and steady death. I assumed it was more due to it being 90+ here in Florida and a bunch of rain. Will Swiss Chard last the summer in a hot and wet environment here in Florida?