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.