You have a form of blight probably this one. It is listed as one that attacks snapdragons in my plant disease book. These fungus live in the soil. Water hits the soil and it splashes on to the leaves and goes up the plant with water. The leaves die and fall to the soil carrying the fungus with it and the cycle begins again. If the area isn't large you could dig out the soil and replace it or try one of the fungicides mentioned here.

Botrytis blight (fungus)
Symptoms: Dead blotches appear on leaves, flowers, and
stems. Stems may rot and plants collapse. Flower buds may fail
to open. Diseased flowers decay and fall prematurely. Affected
areas may be covered with fuzzy fungal growth and spores.
Control: Destroy old plant debris in the fall. Avoid
wetting foliage. Prune out nearby shading vegetation. Provide
adequate plant spacing. Use fungicides such as
chlorothalonil, dicloran, ferbam, iprodione, mancozeb,
maneb, thiophanate-methyl, vinclozolin, or mancozeb +
thiophanate-methyl. Read the label for lists of specific
plants allowed for each fungicide.

might be a lot cheaper ... to just remove them.. and either go find some new Ss ... or some other plants ..

will never understand spending something like 20 bucks.. to save a two dollar annual ... not to mention the spray exposure.. etc ...

you can go back to snaps next year.. if its some passion ...

on reread.. i see you are basically doing such ... good ...

ken