If it's only that small section of one leaf, likely not a problem.

Please post an image of the entire plant.

also include some info on indoors.. outdoors.. where you are.. potted.. nutrition program is potted.. etc ...

ken

It could possibly be Septoria Leaf Spot or Tomato Spotted Wilt Virus. Just like Jean said, could you post a picture of the entire plant and tell us which section of the plant you removed the leaf from?

Septoria Leaf Spot: caused by the fungus Septoria lycopersici. Infection usually occurs on the lower leaves near the ground, after plants begin to set fruit. Numerous small, circular spots with dark borders surrounding a beige-colored center appear on the older leaves. Tiny black specks, which are spore-producing bodies, can be seen in the center of the spots. Severely spotted leaves turn yellow, die and fall off the plant. The fungus is most active when temperatures range from 68 to 77° F, the humidity is high, and rainfall or over-head irrigation wets the plants. The fungus is not soil-borne, but can overwinter on crop residue from previous crops, decaying vegetation and some wild hosts related to tomato. Prevention/Cure: Clean out any plant debris from the bed and give tomatoes repeated applications of fungicide with chlorothalonil (very good) or copper fungicide, or mancozeb (good) to keep the disease in check.

Tomato Spotted Wilt Virus: spread by a tiny insect called thrips, which acquires the virus by feeding on one of many infected weed or ornamental hosts, and then spreads it to the developing tomato plants. Several weeks after transplanting the tomato plants into the garden, random plants may appear stunted, and younger leaves may be marked with bronze or dark spots, or have prominent purple veins. Often the upper foliage will become twisted and cupped as the bronze areas expand. Fruits may have yellow spots. Younger plants may wilt and die, but older plants may survive and bear discolored fruit that may not fully ripen. Prevention/Cure: Weeds in the garden area during the winter may harbor both the thrips and the virus. So, remove the old crop debris, till and mulch the garden for the winter to keep weeds and thrips down for the next year. There is no cure for a plant with TSWV. Removing infected plants immediately from the garden may help reduce the incidence of disease on additional plants. However, feeding by thrips can transmit the virus to plants within minutes. Because of this rapid infection time, insecticidal sprays may be of no use for the home gardener.