Rust pustules on gladiolus leaf (Central Science Laboratory, Harpenden Archive, Gritish Crown, Bugwood.org)
Erupting rust spores (John W. Dooley, USDA APHIS PPQ, Bugwood.org)
Healthy gladiolus (Netherlands Bulb info Theflowerexpert.com)
Uromyces transversalis is the fungus that causes Gladiolus rust. Two different spore stages are found on the leaves. Urediniospores are produced all summer and are responsible for the orange lesions known as sori that erupt on both sides of the leaves. On occasion, teliospores will form later in the season. These appear black and are found surrounding the urediniospores.
This fungus is a disease of quarantine interest and of state regulatory concern. The fungus causes rust on leaves of Gladiolus spp. as well as other members of the Iridaceae family. This serious disease of gladiolus can destroy commercial gladiolus crops unless fungicides are used. Because of this, gladiolus flowers coming from areas in Florida and California known to have gladiolus rust must be shipped without leaves attached.
Gladiolus rust is native to southern Africa. It has since spread to many parts of Europe, Portugal, England, South America and Mexico. Florida reported finds in Manatee County in 2006 and 2007 and Hendry County in 2006, 2007 and 2008. Southern California reported a find in 2006. Eradication efforts continue in these U.S. locations. Gladiolus rust spores are windborne but movement of plants and cut flowers are primary pathways for introduction. While we do not have Gladiolus rust in Minnesota gladiolus fields, it has recently shown up on leaves of cut gladiolus flowers delivered from FL to MN. Those leaves have been collected and properly disposed of.
Initially, small yellow spots show up on leaves followed by orange rust pustules formed when rust spores emerge through the surface of the leaf. Black pustules that contain teliospores can eventually surround these initial pustules. The lesions formed by the eruption of spores progress transversally across the width of the leaves. The species was named for this transverse growth.
Contact the Minnesota Department of Agriculture via Arrest the Pest if you suspect you have found gladiolus rust in Minnesota.