"Adam Schwartz" wrote in message news:s%WOa.18897$H17.6046@sccrnsc02...
The blotches are of irregular shape. They cross leaf veins. The weather
has been somewhat stormy, but the blotches appeared before the storms. I
water the sunflower thoroughly once a day (it's in a pot). I have not
applied fertilizer but there is a slow release fertilizer in the potting
mix.

To be more specific, the blotches seem to stay between the veins for a
while, but eventually they overtake most of each leaf.

There are seven kinds of fungal leaf spot disease common to sunflower
and one bacterial and little treatment. Generally bacterial diseases
stay between the veins and fungal ones cross them but once it gets out
of hand even the bacterial ones cross leaf veins.
You can clip off affected leaves if you see any spread.