Answer: Iris sometimes develop a fungal disease when the weather has been wet, the plants are overcrowded, or the soil is on the acidic side. You can try to stop the spread of the disease by removing the infected leaves. If that doesn't help matters, an application of a good garden fungicide should stop the disease from spreading. If your plants are overcrowded you should lift and divide them in late summer. If they have plenty of elbow room and good air circulation, they won't need to be divided, but you should pull the old leaves off the rhizomes in the fall. Removing this debris will keep the disease in the old leaves from overwintering and infecting the new leaves next spring.
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