It sounds like your tomatoes have one of the fungus diseases. They tend
to start on the lower leaves and work their way up from there.
Search the web for tomato diseases and I'm sure you will find pictures
of the problem your plants have.
This might be caused by over zealous watering. The leaves of tomato plants
should not be allowed to touch the soil. Water should be placed on the
ground gently rather than the water splashing on the leaves. A mulch helps
to alleviate this problem.
for specific problems. Read the instructions then click on "leaf". You
will find your problem there.
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