Iris leaves have rust-coloured pustules. Could be Iris Rust. Find out more, including how it spreads and what fungicide to use to control it.
What is it?
A plant disease caused by one of several closely related fungi of the puccinia species, which forms rust-colored pustules, which consist of millions of microscopic spores.
What does it look like?
The leaves have rust-colored pustules which are powdery and on both sides. Pustules turn dark brown late in the season, and leaves may die off prematurely.
How does it manifest?
Spores spend the winter on iris leaves that haven’t entirely died back and infection usually starts or spreads in the spring when the plant begins to grow. Spores are spread through wind and splashing water. Occasionally the plant may be killed off prematurely, but others are more resistant and may not be so severely affected.
What can you do about it?
Any plants, which show signs of infection should be sprayed with fungicide containing chlorothalonil or ferbam, and resprayed every 7 days for two successive treatments. Remove and destroy all old, dying leaves in the fall. Wet weather favors rust, so water in the morning to allow the foliage a chance to dry before nightfall. An alternative is to plant rust-resistant varieties.