Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Iris (Rhizomatous)

Disease (causal agent) Symptoms Survival of pathogen and effect of environment Comments on control
Bacterial soft rot*
(Pseudomonas spp.)
Wilting and dying of fans accompanied by a soft, wet, foul-smelling rot of rhizome. Fans separate easily from rotted rhizome or topple over. In infected tissue. Favored by warm, wet weather, but rot may continue when conditions are dry. More serious when rhizomes are buried when planted. Bacteria enter through wounds, so prevent injuries to plant. Remove infected plants, cut away rotted tissues, and allow cut surfaces to thoroughly dry before replanting. Plant high or on ridges so that the top of the rhizome is not covered.
Leaf spot*
(Mycosphaerella macrospora, Didymellina macrospora, conidial state; Heterosporium gracile)
Circular to elongated spots 0.125 to 0.25 inch in diameter and up to 1 inch in length. At first, yellowish flecks appear. Spots later turn light brown and have a distinct red border. If severely infected, leaves of some cultivars die back. Dark green spores may be found in spots. On living and dead leaves. Favored by wet weather. Fungus also infects bulbous iris and other iris species. Collect and burn or bury dead leaves. Where practical, cut off infected parts of leaves. Protect foliage during wet weather with chlorothalonil, myclobutanil, mancozeb, or copper fungicides. Addition of a wetting agent may be necessary.
Rust *
(Puccinia iridis)
Reddish brown, powdery pustules on leaves. Infected areas frequently surrounded by yellow tissues. Cultivars differ greatly in susceptibility to rust. On living iris leaves. Spores are airborne. Favored by atmospheric moisture (rain, dew, overhead irrigation). Irrigate so that the water does not remain on leaves longer than a few hours. Chlorothalonil, myclobutanil, and mancozeb used to control leaf spot will also help control rust.
Southern blight
(Sclerotium rolfsii)
Rhizomes and leaf bases become rotted. The fungus is visible as a white, cottony growth on the surface and in the soil, and as small, brown resting structures (sclerotia). In soil as sclerotia. Favored by high temperatures and wet soil. Attacks many other plants. Avoid planting in infested soils. Fumigate soil before planting. To prevent spread, drench infested areas with PCNB or mix granular form with planting medium before planting.
 
Virus or viruslike disease Symptoms Host range and natural spread Comments on control
Mosaic
(Iris virus 1)
Light and dark green mottling and yellow stippling of foliage. Mottling and stippling are especially prominent on young leaves. Mosaic is most severe on bulbous iris and some rhizomatous species. Iris germanica and its hybrids are only slightly stunted and sometimes exhibit no symptoms. Iris family (Iridaceae). Transmitted by aphids. Oncocyclus iris and their hybrids can be severely damaged. Serious on Tigridia. Rogue infected plants. Control aphids.
Rhizomatous irises are also susceptible to root knot nematode** (Meloidogyne hapla). Rhizome rot caused by Sclerotinia convoluta and bacterial leaf blight caused by Xanthomonas tardicrescens, do not occur or are rare in California.
* For additional information, see section on Key Diseases.
** For additional information, see section on Nematodes.
Text Updated: 11/20