Hi- check the top and bottom leaf surface for signs of sucking insect pests like whitefly or leafhoppers. These insect pests have piercing/sucking mouthparts. Their feeding results in many small white dots (stippling). Brushing the plants will cause them to fly up if they are present.

A floating row cover can prevent this type of feeding injury next time you plant these crops:

https://extension.umd.edu/resource/floating-row-cover

If there are no signs of insect pests the symptoms may have been caused by high ozone levels. Swiss chard and spinach are very sensitive:

https://ssl.acesag.auburn.edu/pubs/docs/A/ANR-0913/ANR-0913-archive.pdf (pages 3-4)

http://www.ozoneinjury.org/crops/index.php?option=com_content&view=article&id=14&Itemid=9

Jon