[early blight]
http://aggie-horticulture.tamu.edu/p...ly_blight.html
Early blight is not a systemic disease, it is simply a fungus
growing on the outside of the plant. If you cut off infected
parts and spray it with a fungicide, the plant will just keep
on trucking. It is worse in humid environments than dry
environments.
Daconil is a preventative treatment. It will not kill early blight
fungus on leaves that are already infected. It prevents the
fungus from taking hold on healthy leaves, until it is washed
off by rain, at which point you need to treat the plant again.
I don't know how copper fungicides work exactly. I usually
cut off infected foliage and dispose of it away from the
garden, without spraying. With early blight, this is often all
that I need to do if the weather stays mostly dry. (With late
blight, once it takes hold the plant is a goner unless it has
some genetic resistance, because the infection spreads
really fast.)