WSU Extension

Hortsense

Caption: Iris leaf spot
Photo by: R.S. Byther
  
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Iris : Leaf spot
(revision date: 6/19/2015)


Biology
Leaf spot is caused by a fungus which overwinters on dead leaves and other plant material. Spores develop in spring and infect leaves. Several cycles can occur in warm, wet weather, and the disease thrives on plants growing in acidic soils. Tiny brown spots emerge with water-soaked edges, later turning yellow. Near season's end after bloom, spots may multiply and run together. Severely infected leaves may die. Older spots reveal grayish centers with red-brown borders.
Management Options

Non-Chemical Management
  • Remove and destroy dead and infected leaves in fall.
  • Maintain adequate plant spacing to improve air circulation.
  • Lime to keep the soil pH above 6.
  • Change iris planting locations after a few years.
Select non-chemical management options as your first choice!

Chemical Management


If the problem has been severe in previous years, fungicide applications may be useful. Make a fungicide application before the disease appears (when plants are 6-8inches tall) and repeat every 10-14 days while weather remains wet and warm.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Bonide Fung-onil Multi-Purpose Fungicide Conc
    Active ingredient: chlorothalonil  |  EPA reg no: 60063-9-4
  • GardenTech Daconil Fungicide Conc
    Active ingredient: chlorothalonil  |  EPA reg no: 67572-82-71004
  • Ortho Max Garden Disease Control Conc
    Active ingredient: chlorothalonil  |  EPA reg no: 239-2522
  • Spectracide IMMUNOX Multi-Purpose Fungicide Spray Conc
    Active ingredient: myclobutanil  |  EPA reg no: 9688-123-8845
  • This list may not include all products registered for this use.
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Caption: Iris leaf spot
Photo by: R.S. Byther