Answer: Sounds like a standard fungal disease and irises are prone to them if they are overcrowded or leaf debris is not cleared from the garden at the end of the growing season. The fungus that's attacking your plants overwinters on dead leaves and other plant remains, so be sure to clean the iris bed up and remove any plant debris. Spores are produced in spring and infect leaves, especially if the weather is warm and wet. To help prevent infection, and the spread of infection, give your plants enough space between so the leaves are not touching one another. You can apply a garden fungicide such as lime-sulfur to the leaves if you notice spots beginning this spring. Don't spray until the leaves are 6-8 inches high. You may have to spray every 10-14 days if the weather remains warm and wet. In August, after flowering has finished, dig and divide the clumps, then replant so that there's adequate air circulation around each plant. Best wishes with your irises!
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