The perennial iris produces a shot of early spring color. There are hundreds of types of iris, in almost every shade imaginable. They are hardy and vigorous and require little care if grown in the right conditions. Despite their easy growth habits, iris plants can be susceptible to some fungal diseases.
Didymellina Leaf Spot
The fungus Mycosphaerella macrospora (formerly known as Didymellina macrospora) causes didymellina leaf spot. While it can affect almost all irises, it's frequently a problem for bearded irises. Symptoms generally first appear during rainy or humid weather in the spring. Small brown spots with water-soaked margins and a yellow halo appear on the upper half the plant's leaves. As the temperatures rise, the spots grow, eventually developing a gray center with red-brown edge. Fungicides such as Systhane or Fore can control this fungus.
Bacterial Leaf Spot
The fungus Xanthomonas tardicrescens causes bacterial leaf spot. Symptoms include irregular spots with a water-soaked appearance near the margins of the upper leaf. The spots will lead to brown leaf tips. Bacterial leaf spot is more common during wet, humid springs. There is no known cure other than good sanitary conditions, like cleaning up dead leaves, watering the plants from above and trimming affected plants.
The fungus Puccinia iridis causes rust, which can lead to considerable damage to your irises. Symptoms include rust-red or brown powdery pustules on either side of the leaves and stems, which can lead to premature leaf drop-off. The spots will turn dark brown at the end of summer. The spots can contain millions of microscopic spores, and they're spread by wind and splashing water. To control rust, destroy infected plant parts and give your plants adequate air circulation.
Bacterial Soft Rot
Bacterial soft rot, caused by the fungus Erwinia carotovora, is common in newly planted as well as mature iris. Symptoms include soft, foul-smelling decay of rhizomes brought into the plant by the larvae of the iris borer. The plants may appear wilted or even partly dead by the middle of summer. Prevent the fungus by planting healthy rhizomes and eliminating the iris borers with hand-picking or an insecticide.