My tomato plants have developed black spots on their leaves; see the photo. It started with the older bottom leaves, but now it seems to have spread to most of the leaves. The plants are growing in containers. I water them regularly, but have not used any fertilizer. The water is calcium rich. The tomatoes themselves seem fine, the first few are starting to develop a bit of color.

Should I be concerned? Is there anything that I can do? Using pesticides is not an option here.