Early Blight - see link below and the many discussions here on Early Blight and how to treat it.

Remove all infected foliage from the plants ASAP. Fungus disease requires regular use of fungicides.

Dave

Here is a link that might be useful: TAMU - Early Blight

I opened the page that Dave referenced. Judging from that link and its description, probably yours is not a case of early blight. But you can take any protective measure about that.

As I look at the picture, I see a general pillowing along with, maybe, some environmental damages. To me, the plant appears to be mal nutritioned. Maybe some application of fertilizer can help.

So I inspected the underside of an affected leaf and there is a fine white spider web like substance with dark specks?

Then you most likely have spider mites in addition to the Early Blight. The webbing indicates the mites although the adults are usually reddish orange, the nymphs are a dark brown-black. See pics linked below.

There are several sprays available for controlling spider mites but effectiveness is debatable. If only one plant and only a few affected leaves then hand squishing and affected leaf removal is as good a control as anything. With a bad infestation then buying ladybugs helps or you can use insecticidal soap or insecticidal oil or neem,

Dave

Here is a link that might be useful: Spider mite pics