A common disease specially on tomatoes is the Septoria leaf spot, this particular disease can affect other plants as well. Although Septoria leaf spot is not necessarily fatal for your tomato plants, it spreads rapidly and can quickly defoliate and weaken the plants, rendering them unable to bear fruit to maturity. The disease start off somewhat circular and first appear on the undersides of older leaves, at the bottom of the plant. As the disease develops, the spots will get larger and may merge together. If left untreated, Septoria leaf spot will cause the leaves to turn yellow and eventually to dry out and fall off, thus weakening the plant, sending it into decline and causes sun scalding of the unprotected, exposed tomatoes.

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Source: The Spruce