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UC Pest Management Guidelines


Close-up side view of two white blossoms of Yemenese iris.

Floriculture and Ornamental Nurseries

Iris (Bulbous)

Disease Control Outlines

(Reviewed 3/09, updated 3/09)

In this Guideline:


Disease (causal agent) Symptoms Survival of pathogen and effect of environment Comments on control
Bacterial blight
(Xanthomonas campestris pv. tardicrescens)
Elongated, water-soaked spots or lesions that usually occur near the base of leaves. Under moist conditions, lesions rapidly enlarge causing leaf to turn yellow and collapse. Flower stems may be infected. Bacteria survive in infected bulb scales and tissues. Rhizomatous iris also is susceptible. Disease is favored by warm, moist conditions and by injuries and frost damage. Avoid excessive overhead irrigation. Space plants wide enough to promote air circulation and rapid drying of foliage. Discard infected plants. Use pathogen-free planting stock.
Basal rot
(Fusarium oxysporum f. sp. gladioli)
Stunted, yellowed plants. Basal plate and scales are affected by a firm brown rot. Blue mold may develop as a secondary rot. Fungus survives as chlamydospores in soil for several years. Spread by infected bulbs. Disease is favored by warm soils (above 57°F). Gladiolus, crocus, Ixia, Tigridia, Tritoma, and freesias are also attacked. Do not plant in infested soil for 3 to 4 years or fumigate soil with methyl bromide-chloropicrin mixture. Dip bulbs in thiabendazole. Fusarium oxysporum f. sp. gladioli has shown some resistance to thiabendazole and to benzimidazole fungicides.
Black slime
(Sclerotinia bulborum)
Plants yellow, wilt, and die or fail to emerge. Diseased plants tend to occur in clumps. Below ground shoots and bulbs are covered with a mass of gray fungus. Infected parts contain pockets of gray or white mycelium and black sclerotia. Favored by cool weather. Fungus survives as sclerotia in soil for several years. Rotate out of iris for 3 to 4 years. Include PCNB in bulb dip.
Blue mold
(Penicillium spp.)
Plants are stunted, off-color, lack flowers, and prematurely die. Blue-green mold on rotted bulbs. Also common as on stored bulbs. Wounds caused by insects, harvesting, sunburn, etc. are necessary for infection. Late or early digging favors disease. Frequently starts on corms stored incorrectly. Avoid very early or very late digging. Avoid injuries. Cure bulbs rapidly and provide good ventilation during storage. Heat cure bulbs within 5 days of digging. Dip bulbs in thiabendazole. Some Penicillium spp. have shown resistance to thiabendazole and other benzimidazole fungicides.
Fire (Leaf spot)
(Mycosphaerella macrospora = Didymellina macrospora, conidial state Heterosporium gracile)
Oval to elliptical leaf spots with pale yellow or reddish brown borders. As the spots become old, centers turn tan. Spots are often near tips of leaves. Flower buds, stems, and bulbs may be infected. Dark green spores may be seen in the spots. Disease is favored by mild temperatures (50° to 70°F) and wet conditions. Spores are airborne. Fungus also infects rhizomatous iris. Dig bulbs annually. Protect foliage with myclobutanil, chlorothalonil, or mancozeb. Destroy old leaf tissues.
Ink spot
(Drechslera iridis)
Dark reddish brown elongated spots with chlorotic margins. Older leaves develop gray centers. Dark spore masses may be visible on lesions. Usually older leaves are infected. Irregular inky-black stains occur on Iris reticulata bulbs. Disease may be severe on plants undug for 2 years. Disease is favored by mild (68° to 77°F), moist conditions. Fungus survives on infected bulbs and debris. Not common. Dig bulbs every year. Remove and destroy all debris; rotate on a 3-year basis. Protect foliage with mancozeb.
Nematode
(Ditylenchus destructor)
Plants are stunted. Black streaks occur along veins of the outer husks. Outer husks become shredded at the base and the basal plate becomes honeycombed and grayish. Nematodes survive in bulbs. Damage is worse in cool, moist climates. Other hosts include alfalfa, potato, sugarbeet, tulips, and some weeds and fungi. Harvest bulbs 7 to 10 days earlier than normal. Treat dormant bulbs in hot water (110°F) for 3 hours. Cool and dry promptly. Disinfect tools, trays, etc. by heat treatment such as steam or hot water at 185°F. Do not replant infested fields for 2 years.**
Rust
(Puccinia iridis)
Reddish brown powdery pustules on leaves. Cultivars differ in susceptibility. Fungus survives on living iris leaves. Spores are airborne. Favored by condensed moisture and overhead irrigation. Irrigate so that water does not remain on leaves longer than a few hours. Chlorothalonil, myclobutanil, and mancozeb used to control leaf spot will help control rust. Remove and destroy old infected leaves. more info *
Southern blight
(Sclerotium rolfsii)
This disease is also called crown rot or southern wilt. Outer leaves turn yellow. Eventually all leaves are affected. Leaf bases and bulb are affected by a soft rot. White mycelium is present on bulbs and in soil. Small, tan to reddish brown sclerotia are found in and on bulbs and soil. Sclerotia survive in soil. Disease is favored by warm (77° to 95°F), moist soil. May be spread by infected bulbs and anything that moves infested soil. PCNB mixed with soil before planting helps. more info *
Bulbous irises are also susceptible to gray mold * (Botrytis cinerea), gray bulb rot (Rhizoctonia tuliparum), root rot * (Pythium irregulare), neck rot (Rhizoctonia solani), iris mosaic virus, mosaics, and black storage molds (Rhizopus sp., Aspergillus sp.).
* For additional information, see section on Key Diseases.
** For additional information, see section on Nematodes.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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