It’s not the hot, dry weather that’s responsible for the brown blotches on chard. The spots are caused by beet leaf miners and about all you can do about it now is remove the infected parts of the plants.
In the Garden
Q: There are large brown blotches on the leaves of my chard. Was this caused by our unusually hot spring?
A: Those translucent blotches that look like they were scorched by the sun are caused by beet leaf miners, tiny black and yellow flies that lay their eggs on the leaves of spinach, beets and chard.