The most common symptom of
rust on irises is yellowing and browning leaves;
closer observation will
reveal rust-colored spots formed by pustules.
(Photos courtesy of Missouri
Botanical Garden PlantFinder)
Iris rust is a specialized parasitic fungus that
can attack any type of iris. While it depends on the living host to complete its
life cycle, the spores can survive on dead tissue in the garden to start the
cycle again in spring. Each type of rust is specific to its host, so
different types of plants cannot infect each other. However, the spores are
easily carried long distances by wind, so closely related plants growing nearby
may transmit the infection.
Rusts get their name from the color of the reddish
brown, powdery pustules full of spores that form on leaves. Symptoms
typically include leaf yellowing, withering and early leaf drop. The infected
areas are frequently surrounded by yellow tissue. The condition is favored by
atmospheric moisture, including rain, dew and overhead irrigation, therefore
it is best to irrigate so that the water does not remain on leaves longer
than a few hours.
The following strategies may assist in the control
of rust in your garden:
Follow good sanitation practices. Since leaves, stems and flower stalks can
harbor the fungus, remove all debris from the garden.
Remove infected areas. As soon as rust pustules are noted, remove the
infested part of the plant. Spores are airborne, so caution should be used
during the removal process.
Apply a preventative fungicide. Rust is best managed by fungicide application
beginning when new growth appears in the spring and continuing for 5 or 6
applications every 7 to 10 days. Currently recommended fungicides to control
rust include chlorothalonil, myclobutanil and mancozeb.
Plant a resistant cultivar. Cultivars differ greatly in their susceptibility
to rust. If rust is a perpetual problem in your garden, check with other
gardeners in your area to find those cultivars that might be better