Iris Diseases and Pests

Irises are remarkably free of diseases and pests. The few that bother these hardy plants can be corrected, usually with a minimum amount of fuss.

Fungal Leaf Spot:

Unsightly black or brown spots that appear on the bearded iris leaves. Generally, leaf spot does not harm the plant, but it is best to prevent this condition by starting with healthy rhizomes or by using dormant spray (copper sulfate) during the late fall and winter. Leaves that show Leaf Spot need to be continually removed and destroyed. Solutions of Funginex or Daconil are common commercial sprays. For a more organic solution of baking soda and water can be utilized. Dissolve 1 teaspoon baking soda in a quart of water, add a few drops of liquid soap to the mix to help it cling better to the foliage, spray infected plants thoroughly. Another unusual (and pungent) remedy for fighting fungal diseases is manure tea. This formulation fights blackspot, as well as mildew and rust, while providing foliar nutrition. Place one gallon of well-composted manure in a 5 gallon bucket and fill with water. Stir the mixture well and let sit in a warm place for three days. Strain the mixture through a cheesecloth or mesh and use the resulting tea to spray disease affected plants (the solids left behind can be applied around the base of the plants as added fertilizer).

  • Bearded Iris rot damage

Soft Rot:

A bacterial infection of the iris rhizome. It is characterized by a soft, mushy, ill smelling rhizome. To protect the rhizome, soft rot must be removed, else the entire rhizome may be lost. Immediately scrape away and completely remove the rot and allow the rhizome to dry in the sun. Carefully treating the rhizome with a 5% solution of ultra bleach and water can correct this problem. For spot treatments, a Clorox wipe is quite handy! Rhizomes with large infections need to be lifted and soaked in the 5% bleach solution and allowed to completely dry before replanting.


This is a gray-black mold that appears as a patch on the stalk that, if unchecked, makes the rhizome become pithy with badly deteriorated roots. Removal of the infected plant is recommended, although a fungicide drench may help. Treating plants in the Fall and Spring with a fungicide can be a preventative measure; the fungus grows and spreads in the Fall but is not evident until Spring. Remove and destroy (do not compost!) old or dead leaves in the Fall as a preventive measure as well..


These pesky little pests can suck the juices from forming flower buds and cause malformed blooms. They are also notorious for hiding along the flower stalk and the leaves, tucked in down at the base of the fan. For small numbers, the easiest method of control is the organically sound method of pick-and-squish. For larger infestations spraying a 10% solution of a mild liquid dish soap (such as Ivory) and water on the affected parts works well. For a solution with a bit more power, commercial sprays of insecticidal soap are also quite effective. Just be sure to do follow up control every three to five days to catch hatching eggs.


Another pest that likes to live down in the flower stalks and leaf fans; sometimes even in the flower. Again, the easiest control for simple problems is the pick-and squish method. Heavier duty controls of insecticidal soap or other garden-safe pesticides may be required for large infestations.


We all know what they are, and in the Pacific Northwest, these pests can be active all year! A hungry slug can climb to the top of a 4 foot flower stalk to get to those soft, tasty flower buds; he can also munch an Iris reticulata to the ground in one feeding. The best solution is constant vigilance in using whatever your favored method of control is. Control methods include bait, beer traps, crushed egg shells, copper wire and even sharp sand. Watch closely for signs of baby slugs and start control immediately.


Yes, weeds are pests! Prevention is a key factor here. Removing a weed before it has the opportunity to go to seed prevents having lots of baby weeds to deal with. Large infestations, or large weeds, can be handled by judicious use of Round Up or broad-leaf weed killers; carefully painting or sponging it onto the weed leaf surface will prevent damage to your more desirable plants. Commercial soil surface treatments like Preen have been proven to be effective, although use around newly planted rhizomes seems to significantly impact growth of immature increases.