One of the problems that is common with tomatoes in our area is Early Blight. Early Blight is a fungal disease and is rather difficult to control once it has infected a plant. Plants that have been stressed tend to be perfect targets for Early Blight.
Blight on tomatoes starts out with concentric brown spots on the leaves. If left unchecked, blight can quickly destroy entire leaves and eventually the entire plant.
If you notice yellowing leaves starting near the base of the plant that have a concentric brown spots, cut the leaves off and be very careful that the infected leaves do not touch healthy plants or leaves. Don’t be shy. Any blight left on the plant can quickly spread so try to cut off all diseased leaves.
Also, neem oil is an organic fungicide that can help control blight. Neem can help suppress the growth of blight until the plant can grow new leaves.
This process can leave some fruit exposed to sun scald but it is worth the effort. Maintaining vigorously growing plants is the best way to keep from getting blight. Make sure and water plants, especially during dry periods. And try to water the soil without allowing water to splash leaves.