This is typical and almost inevitable. Tomatoes succumb easily to several leaf diseases (mostly fungus) which start on the lower leaves and work their way up. There are two approaches to dealing with it.

One way is just to grow your tomatoes as well as possible and hope your crop of ripe fruit wins the race against death from leaf diseases. This is what most gardeners do and it's fairly successful.

Or you can fight the diseases. Fungicides such as chorothanonil will help if applied consistently. Skim milk, diluted 1:4 with water has been shown to be preventative. 'Preventative' is the key word. Fungus diseases are tough to eliminate once they are established.

The best control I have seen is heavy mulch combined with pruning the lower branches early in the season. This seems to prevent splashing of soil borne fungus spores onto the lower foliage.

Jim

Not too late to start spraying with daconil. But next year make a note to start spraying early because it isn't a cure but a preventative. So, when spraying, it's important to cover the leaves that look healthy.

Egganddart, old leaves on any plant do not have the same capacity for photosynthesis as younger leaves. Further the canopy of leaves above intercept necessary sunlight.

So, those lower leaves may contribute nothing at all to the plant or the developing fruit.

digital Steve