Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00000790
11/12/99
IRIS DISEASE PROBLEMS
Bacterial soft rot enters plants through wounds caused by
the iris borer. Infected leaves have a water-soaked
appearance. Healthy leaves fall over when the rhizomes
rot. Rotting rhizomes have a foul odor. Cut off leaves
showing the water-soaked areas. Destroy rotted rhizomes
and control the iris borer. Plant rhizomes shallow and
disinfect tools with 70% denatured alcohol.
Bacterial leaf spot causes translucent spots of irregular
shape (Vis. 1). These appear dark green in reflected light.
The spots may merge in rainy weather to involve the whole
leaf. Remove and destroy spotted leaves early in the
growing season. Remove and destroy the tops in fall.
Blossom blight causes blossom spots and blights in rainy
seasons. Remove blighted blossoms. The disease is
also called leaf blight and is caused by Botrytis cinerea.
Leaf spot causes irregular spots which vary in size and
may run together. The spots have grayish centers with
dark brown borders. The disease is usually in the upper
half of the leaves and is more serious after the plants
have bloomed. Premature death of the leaves weakens the
rhizomes. The causal organism is Didymellina macrospora.
Plants infected with rhizome rot fail to grow in the
spring. The fungus develops in cool wet weather. No
chemical control is listed for this disease caused by
Botryotinia convoluta.
Crown Rot is worse when plants are crowded. The leaf and
flower stalk bases are attacked. The infection spreads to
the rhizome causing a rapidly developing dry rot. Remove
infected plants. The disease is caused by Pellicularia
rolfsii and no chemical control is suggested.
Mosaic infected plants have yellow striping or mottling on
the leaves and flowers. The flowers are smaller then
normal. Discard infected plants and control aphids which
spread the disease.
Related information.
Pesticide Recommendations
Fungal and Bacterial Leaf Diseases
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This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
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