Hello! I'm kind of new to gardening and I have trouble figuring out what's wrong with plants when they appear sick.

I have 4 cucumber plants living in an Earthbox, using the standard included earthbox soil and fertilizer and I've been noticing brown spots on the leaves - one of the plants is pretty badly affected. The others not so much and maybe not at all. It seems to be starting at the bottom leaves and working its way up the plant, with the top leaves mostly unaffected (yet).

I scoured the web think it might be Anthracnose. Anyone have any other thoughts? Can it be saved?

I'm pretty close to the beach in southern california, so it doesn't get very hot or cold and not much rain. I top off the earthbox water reservoir every 2 days so it never dries out.