Petunia's

I plant them every summer and have never had a pest problem with petunia's before but this year it seems that soon after a flower blooms it has tiny holes all over it's petals. The leaves seem okay, what could this be ? Also it has been very cool at night for the past 2 weeks and they seem to be blooming less than usual for this time of year. Should I prune them back a little, would that increase their bloom?
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