Answer: What to do depends entirely on the cause. Take a closer look at your plants and see what your investigation yields.
Ants and spots like mildew suggest the presence of aphids (there are at least three that attack iris). Ants feed on aphid honeydew and will actually protect them from predators so they can "herd and milk" the aphids for the sweet stuff. Mold spores also grow happily on honeydew. If they're there, wipe or wash as many of them from the leaves as you can, and apply a neem-based repellent (such as Burpee's Japanese Bettle repellent, which keeps lots of other bugs at bay) to prevent reinfestation.
The stress of transplanting can make plants more succeptible to disease as well. Iris are prone to a number of fungal leaf spot diseases and viruses. In general you can keep infections from spreading by using a copper-based fungicide (contact Gardens Alive, 5100 Schenley Pl., Lawrencville IN 47025; ph# 812/537-8650). If you want a professional opinion before applying any treatments, contact your agricultural extension office (ph# 404/371-2821), and see if someone can take a look at your iris. Hope this helps!
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