| 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 are 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 |
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