Answer: Iris blight, or leaf spot, overwinters on dead leaves and other plant debris. Spores are produced in spring and infect leaves during warm wet weather. Acidic soils accentuate the disease. To control, remove and destroy dead and infected leaves in the fall. Maintain proper plant spacing to provide good air circulation, and add lime to maintain soil pH above 6.
You can apply a fungicide such a Daconil or Bayleton before the disease appears (usually when plants are 6"-8" high) and repeat every 10-14 days while conditions favor disease development.
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