Answer: Based on your description, I'd say the plants developed powdery mildew. This fungal disease occurs when plants are grown under conditions where humidity is high and days are warm but nights are cool. Some plants are more susceptible than others to powdery mildew. Try increasing the air circulation around the planter and providing plenty of room between plants. While not totally immune, these plants seem to resist powdery mildew: lobelia, dianthus, celosia, marigold, nasturtium, petunia, scabiosa and statice.
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