Can you scrape away some,of the ferriliser? Then water well. You prob got fertiliser on the leaves themselves so the rain would have helped a bit. Remove the brown leaves, leave,the floppy and avoid feeding. Hopefully....well, most likely....your dahlias will recover
I dont use Gromore types...they are fierce, caustic and aggressive if over applied. Organic fertilisers tend to be slower acting. Try to resist the temptation to apply fertilisers simply to hurry on growth. June is a time when plants will burst Imto growth amd flower very quickly so nature rather than fertiliser will encourage growth. Apart from possible scorching roo much fertiliser produces soft, slug attractive plants.