I have planted many perennials this year. I planted 6 hybrid phlox only about 8 inches tall. The I planted Dianthsis and these are short varieties. ALL were blooming profusely. The I notices the Phlox(in full bloom) foliage shriveling(flowers still perfect) The plant parted at soil level and died. The Dianthsis did the same ..all perfect blooms laying in the dirt and the whole plant fell over with area at ground level and leaves in that area yellow.. Grubs? over water? I pulled Phlox out root ball was very small They had been planted about 4 weeks...I lost 5 but now the next seems to be doing the same.. Thank you Elena
This sounds like overwatering and possible root rot or crown rot from overhead watering.
This article will help you.