So tomatoes can be afflicted with many different viruses, bacteria, fungi and/or moulds. One that Brooklyn Grange is fighting is Septoria.
It can happen when it is hot and humid and there has been a lot on rain… and we have had all of those! The fungi will not go away but it can be controlled by removing the infected leaves. These leaves can not be composted, they have to be destroyed.
The fungi can also be controlled by spraying copper. Mixed with water and poured into a backpack like sprayer/pumper you look like a member of the ghost busters team. (That might be as close to a pic of me as you might get ladies and gentlemen).
First signs… lower leaves first start spotting. Then round, yellow or water-soaked spots appear on the undersides of leaves. They quickly emerge on leaf tops and turn to black or brown with tiny black dots in the center. The spotting may work its way up the plant and can infect stems.
Prevention… crop rotation, raised beds… better drainage, good air circulation… wide spacing, water the soil… not the plants, mulch with black plastic, or use hybrid varieties.