Leaf Spot Diseases of Iris

There are several potential causes of leaf spots on Iris including fungal infection, bacterial infection, various viruses, insect damage and various environmental factors. Of these factors fungal leaf spot is the most common and widely spread. Leaf spot diseases greatly mare the appearance of Iris plants and can kill whole leaves and eventually plants if not brought under control.

Fungal Leaf Spot of Iris

Iris Fungal Leaf Spot Disease (also known as leaf blotch of iris) is caused from the fungus Mycosphaerella macrospora  (syn. Davidiella macrospora), the sexual stage of the fungus, and Cladosporium iridis (syn., Heterosporium iridis, H. gracile), the non-sexual stage of the fungus, (referred to as, imperfect state). This is a common and widely spread disease of iris, especially rhizomatous species, most notably bearded iris (although both bulbous and rhizomatous are affected). “Other hosts of the leaf spot fungus include daylily (Hemerocallis), freesia, gladiolus, and narcissus” (University of Illinois, (1982)).

Fungal leaf spot on Iris leaves.

Identification and Life Cycle of the Fungus

When the milder temperatures of spring arrive the overwintering fungi Mycosphaerella macrospora (the sexual stage of the fungi that were produced in the autumn to survive the winter in plant debris) begin producing masses of non-sexual spores (conidia) and to a lesser extent sexual spores (ascospores) during periods of high moisture and or humidity (University of Illinois, (1982)). The fungal spores are spread mostly by splashing water but also by insects, wind, human activity etc. to new growth, where it enters the plant either through stomata or by penetrating the epidermis within hours of contact. Following this primary infection, small, either circular or oval yellow spots with water soaked margins appear. As the spots enlarge the centers turn brown and dry and may be surrounded by a reddish border. The fungi continue to reproduce non-sexually throughout the summer during periods of high moisture and humidity. As the disease progresses the spots coalesce and entire leaves can die. Stems and flowers can also be affected.

Fungal leaf spot on Iris leaves.

How to Manage the Disease

Keep a close eye on iris plants for signs of the disease, especially following rainy periods and high humidity.

Fungal leaf spot on Iris leaves.
  1. Remove infected plant material and black bag it, being sure to disinfect hands and tools after contact with infected iris plants.
  2. Avoid overhead watering.
  3. Avoid going in the garden while leaves are wet (including morning dew).
  4. Insure good air circulation around your iris plants, so that leaves dry quickly.
  5. Divide iris plants as needed to keep them from becoming crowded.
  6. Remove plant debris in the fall to prevent the overwintering stage of the disease.
  7. Plant varieties that are most resistant to the disease such as Siberian iris (I. sibirica), crested iris (I. cristata) and I. graminea species (Creswell T., (2021)).
  8. Avoid bearded iris where the disease is a problem, as they are most susceptible.
  9. If the disease is advanced and persistent a fungicide can be applied in the spring before infection and again in the fall, where permitted. Be sure to check what is registered for use against fungal leaf spot of iris in your area first.
Fungal leaf spot on Iris leaves.

Other diseases and factors that can cause spotting on iris leaves:

  • Rust: is also a fungal disease caused by the fungal species Puccinia iridis. Damage is characterized by small rust coloured flecks that are often surrounded by a yellow boarder. These flecks can appear on either side of the leaf surfaces. The flecks enlarge and are filled with rust coloured fungal spores that have a powdery texture that can stain hands and clothing when touched. As the disease progresses whole leaves can die back.
  • Ink Spot: Is a fungal disease of Iris reticulata bulbs. Iris caused by the pathogen Drechslera iridis. These iris bloom in early spring about the time of snow drops. The disease is characterized by the appearance of reddish brown elongated spots with chlorotic margins and gray centers. Dark fruiting bodies form in these diseased areas during high moisture. The disease can quickly cause plants to yellow and die. On the bulbs ink-like stains occur.
  • Botrytis blight (gray mould): Is caused by the fugus Botrytis cinerea. The fungus attacks a large range of plants from herbaceous annuals and perennials to trees and shrubs, even vegetables. The disease can infect flowers, leaves and tubers. The disease is initially characterized brown spots that begin to develop a fuzzy, grayish mold. As the disease progresses whole leaves and flowers can die back. High humidity favours its development.
  • Bacterial leaf spot (Bacterial leaf blight): is caused by the bacterial pathogen Xanthomonas campestris pv. tardicrescens). The disease is characterized by large, faint, irregularly shaped, water soaked spots. The spots expand rapidly during moist, mild weather and quickly turn yellowish to light brown then develop grayish centers. As the blotches coalesce entire leaves die. Flower stems can also be infected. The disease is more prevalent after a mild winter.
  • Viruses: There are several viruses that can infect Iris and symptoms can very from mild to bold mosaic patterns, green stippling and stunting. They can be difficult to diagnose without a laboratory. Many are the result of insect vectors such as aphids which vector mild mosaic, iris severe mosaic and cucumber mosaic.
  • Insect pests (spider mites, aphids): Spider mites feed on leaves by sucking the juices out of the plant. Their damage appears as white stippling on the leaves. Aphids also feed by sucking plant juices out. Their damage appears as spotty yellow discolourations, puckering, leaf distortion and leaf yellowing. Plants may be sticky, due to the honeydew excreted by the aphids and this honeydew may attracts ants and wasps and develop a black sooty mold.

Photo Credits: All photos taken by the author.

References:

American Meadows, (2021). Bearded Iris Care: How To Avoid 5 Common Growing Problems. Retrieved from: https://www.americanmeadows.com/blog/2016/07/19/bearded-iris-care

Creswell T., (2021). Purdue University. Iris Leaf Spot. Landscape report. Retrieved from: https://www.purduelandscapereport.org/article/iris-leaf-spot/

EPPO Global Data Base, (2002). Cladosporium iridis (MYCOMA). Retrieved from: https://gd.eppo.int/taxon/MYCOMA

Flynn P., (1994). Department of Plant Pathology, Iris Leaf Spot. ISU Extension and Outreach. Iris Leaf Spot. Retrieved from: https://hortnews.extension.iastate.edu/1994/8-12-1994/irs.html#:~:text=Leaf%20spot%20is%20a%20common%20disease%20of%20iris%2C,leaves%20that%20are%20surrounded%20by%20a%20water-soaked%20border.

Hansen M.A. , Virginia Tech., (n.d.), (last mod. Dec 2014 ). Cladosporium iridis (heterosporium leaf spot on iris). Retrieved from: https://wiki.bugwood.org/Cladosporium_iridis_(heterosporium_leaf_spot_on_iris)

Mooreman G.W., (2016). Pennsylvania State University. Iris Diseases. Retrieved from: https://extension.psu.edu/iris-diseases

Oklahoma State University Extension, (n.d.). https://extension.okstate.edu/programs/digital-diagnostics/plant-diseases/iris-leaf-spot.html

Oregon State University, (n.d.) Iris, Bulbous and Rhizomatous (Iris spp.)-Viruses. A Pacific Northwest Extension Publication. Retrieved from: https://pnwhandbooks.org/plantdisease/host-disease/iris-bulbous-rhizomatous-iris-spp-viruses

Pacific Northwest extension, (n.d.). Iris, Bulbous and Rhizomatous (Iris spp.)-Leaf Spot. Pacific Northwest Pest Management Handbook. Retrieved from: https://pnwhandbooks.org/plantdisease/host-disease/iris-bulbous-rhizomatous-iris-spp-leaf-spot#:~:text=Cause%20Mycosphaerella%20macrospora%20%28asexual%3A%20Heterosporium%20gracile%29%2C%20a%20fungus,overwinters%20on%20dead%20leaves%20and%20other%20plant%20remains.

University of Illinois, (1982). Iris Leaf Spot. Report on Plant Disease, RPD No. 628, Department of Crop Sciences. Retrieved from: https://ipm.illinois.edu/diseases/rpds/628.pdf

University of Georgia. Center for Invasive Species and Ecosystem Health. (rev. Oct. 2018 Cladosporium iridis (Fautrey & Roum.) G.A. de Vries. Retrieved from: https://www.prevalentfungi.org/subject.cfm?id=19282

Photo Credits: all photos by the author.

Updated on Mar. 8, 2022

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