Maybe you are watering too much, or aren't getting enough air circulation.
Without knowing exactly what they are it is hard to say, but assuming it is
caused buy a fungus, I would try to dry it out some. Other than that, I
would take some to an extension agent and have them give me an analysis and
Google on "black spot" and look at the pictures. Make sure that's what
the problem is.
What plants are being affected, edible or ornamental? Some treatments
for ornamentals aren't suitable for edibles. Since you asked about
tomatoes below, here's a good site.
Small black/dark spots, mostly on tender young growth? Could be the
four-lined plant bug.
Just noticed some damage out in my garden the other day.
"The spot damage inflicted by four-lined plant bugs may be misidentified
as fungal disease spots because of their comparable appearances. The
agility of this bug contributes to this misidentification because when
disturbed, the four-lined plant bug will drop to the ground or will hide
and is therefore rarely seen. In addition, when the damaged portion of
the leaf fall out, a shot hole will remain that looks similar to fungal
Pat in Plymouth MI ('someplace.net' is comcast)
Any technology distinguishable from magic is insufficiently advanced.
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