Septoria leaf spots start off somewhat circular and first appear on the undersides of older leaves, at the bottom of the plant. They are small, 1/16 - 1/8 in. (1.6 - 3.2 mm) in diameter, with a dark brown margin and lighter gray or tan centers.
As the disease develops, the spots will get larger and may merge together. If you view them under a magnifying lens you may see the fruiting bodies of the fungus, which look like dark brown pimples, as shown above. This is one of the symptoms that distinguishes Septoria leaf spot from other leaf spotting diseases.
Although the symptoms usually occur on the older, lower leaves, the disease can develop at any stage in the tomato plant's life. They may also appear on the stems, as shown in the photo on the next page, as well as the blossoms and calyxes. They rarely affect the fruits.
If untreated, Septoria leaf spot will cause the leaves to turn yellow and eventually to dry out and fall off. This will weaken the plant, send it into decline and cause sun scalding of the unprotected, exposed tomatoes. Without leaves, the plant will not continue producing and maturing tomatoes. Septoria leaf spot spreads rapidly.
Don't lose hope, the next page will outline some steps to take to control Spetoria leaf spot, should it find it's way into your garden.