Answer: You are either seeing whiteflies or leafhoppers. It's difficult to control both the disease and the insect pests all at the same time. It might be easier for all involved to simply dig the irises up, cut off the diseased leaves, divide the rhizomes, amend the soil with compost and replant the rhizomes. Make sure the iris are getting full sunshine and they won't be so prone to fungal diseases. By digging, dividing and removing the diseased foliage you'll be controlling both the insects and the powdery mildew. Good luck with your irises.
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