Tomatillo leaf issue

[GoGayleGo]

Can someone help me out on what's happening here? It's gotten progressively worse through the season, and the hot pepper plant in the box with one of the tomatillos is also exhibiting this symptom. Obviously it's too late to address this year, but I'd like to understand it and perhaps prevent/control it in the future.

There are no bugs visible to the eye, as far as I can see.

Thanks!