ID help: dirty-looking leaves

[soozbz]

Hi all,

Can someone help me identify this disease? I've got four plants- one sweet 100, one sungold, one celebrity and one Cherokee purple, planted in two Earthboxes. Something is killing the leaves on all four of them, but I'm not sure what it is. The leaves start looking kind of dirty or dusty with very small spots - see attached photos. Then they turn yellow/brown starting around the edges, develop a papery texture, and die.

I've been pruning off the most affected leaves and accompanying stems in hopes of preventing further spread, but it hasn't worked - by now pretty much every single leaf is infected. Somehow the plants are still growing fruit, but not at the same rate as before, since I've had to cut off a lot of the stems. But I'm not sure how much longer the plants will last without leaves!

Any pointers as to what this is and what I can do about it? I wonder if it might be septoria, but I haven't seen any photos of that that match up precisely to what I'm seeing here.

Thanks!

Susan