Don't deplete my goosefeet

Something is causing the leaves of my spinach and chard plants to deteriorate (both are members of the goosefoot family, which also includes beets, amaranth, quinoa, lamb's quarters, and purslane, if you're interested). It starts with a beige-yellow streak in the leaf, and then grows until, in some cases, the whole leaf is hanging limply and sometimes puffy. Here are some pictures:

I looked it up, and the best guess I could come up with is downy mildew. Perhaps due to the unusually hot, humid weather we've been having. Anyone care to make a better guess?

It's rather disappointing, but I'm not going to be too bothered by it. If all my spinach and chard die off, then all the more room for tomatoes, squash, and oats!