What is it when there are black spots on the underside of a tomato leaf?
Answers:
I suspect your plant may have Septoria Leaf Spot (Septoria lycopersici).
This condition is not common in the South or on the Pacific coast, but Septoria Leaf Spot occurs in the mid-Atlantic and central states, and as far south as Arkansas and Tennessee. It florishes when temperatures are moderate and rainfall abundant. The disease destroys so much foliage that plants fail to make enough food to support an abundant crop. Absence of leaves exposes the fruit to sunscald. Fungus is most evident on plants that are just beginning to set fruit.
The first symptom of Septoria Leaf Spot is the appearance of water-soaked spots on the older leaves. Spots are rough and circular, with gray centers and dark margins. Later, dark dots are evident in the centers where spores are produced. Eventually all the leaves are affected and drop off, leaving only a few at the stem top. Fruits are rarely attacked.
To control this in tomatoes, plow under all crop and weed the refuse. Replant next season.
Other answers:
Probably a type of fungus. Spray with Daconil.
Probably a type of fungus. Spray with Daconil.