Looks like the beginning of Early Blight. It starts on the bottom and works up. It is a soil born fungus that usually starts because of water splashing soil upon the lower leaves. Once it starts it is very difficult to stop. Prevention is the best thing and preventing it is also difficult. The best thing you can do now is to remove the affected limbs and spray with Neem Oil.
Must I apply Neem oil all season? How frequently? I’ve removed all leaves which in any way resemble the picture I attached and will buy neem oil this weekend.
Since early blight is basically incurable, the object is to have a harvest of your tomatoes. If you can keep the early blight to a minimum this is entirely possible. So, here is what I'd do. The first thing is to place mulch around the base of the plant. Second is limb and sucker removal, not just leaf removal. There will be limbs and there will be suckers down close to the soil. Remove these so it will be difficult for the fungal spores to get on the leaves. Next is to spray both sides of the leaves with the neem. Spray about every 10 days or until harvest. If you planted determinate tomatoes you can probably have a full harvest because the tomatoes all ripen in a fairly short time. With indeterminate plants the fruit ripens all through the season and early blight will probably reduce the amount of harvest. It all depends on how quickly the fungus spreads up the plant. Neem slows it down but does not cure it. I don't think there is a complete cure. Do not overhead water and do not use a forceful stream of water. Prevention is the name of the game with early blight.
