Leaf Spot (fungus – Cladosporium iridis): Irregular, eye-spots which vary in size and run together. They have a grayish center with dark brown borders. The disease usually is more or less confined to the upper surface of the leaf. Carefully remove and burn old iris leaves in the fall or early spring.
Leaf Spot (fungus – Heterosporium gracile): Common leafspot of iris in Central Texas. Disease shows up on both upper and lower leaf surfaces as 1/8 to 3/8 inch diameter, irregular spots with dark brown centers and outer edges. Remainder of spot is light tan. Good sanitation is best control. Removal and burning of old leaves each fall is desirable.
Basal Rot (fungus – Fusarium oxysporum): This soil fungus enters the roots of bulbous iris and then penetrates the base of the plant and the scales of the bulb. Bulbs should be dried rapidly to avoid possible injury. Cull and discard damaged bulbs.
Soft Rot (bacterium – Erwinia carotovora): A foul-smelling soft rot of the rhizome. At first, the leaves begin to turn brown and dieback progresses until the entire plant is dead. The disease is favored by wet, heavy soils or shady locations that tend to keep the ground continuously moist. Plant iris in a well-drained soil and avoid crowded conditions in the bed. In the fall, destroy all dead leaves and plant residue in the iris bed. In season when soft rot is severe, take up the rhizomes in August or September. Cut out the destroy all rotted portions of the rhizomes. Plant in a new location or sterilize the soil. Avoid excessive watering during the growing season. The disease is easily spread to new iris plantings during lifting and dividing operations by spades and other tools. Control iris insects such as the iris borer.
Rust (fungus – Puccinia spp.): Rusty-red, powdery pustules break out on both sides of the leaves, causing considerable damage to many varieties. Destroy all fallen leaves before growth starts in the spring.
Mosaic (virus): Yellow striping or mottling of the leaves and flowers. The potato and peach aphids transmit this virus. Discard all infected plants. Control aphids that transmit the virus.
Other Disease: Iris are attacked by several species of nematodes and Sclerotium rolpii. (See discussion in other sections for more information.)