In my not-so-humble opinion chlorothalonil is worthless. Years ago I used a
bunch of it with essentially no beneficial results.
You would be much better served by going back to your "copper"
fungicide and by adding a spreader-sticker. But be careful.
I have posted a reply in the thread "
Can you store Copper-Soap solutions?"
with detailed information on the use of copper fungicides but be sure you
are starting with a 10% concentrate of copper octanoate.
Addition of Revitalize could help although it may be late in the season
for that.