Rust

Rust pustules appear as powdery masses of yellow, orange, purple, black, or brown spores on leaves and sometimes on stems. Depending on the particular plant host, pustules can be found on either side of leaves. Severe rust can result in dried, dead foliage. The most significant impact of rust disease is the reduction of quality and marketability of the commodity.

The rust fungi are obligate parasites in the order Uredinales. Some have complicated life cycles that include up to five different spore stages and two distinctly different hosts. Other rusts produce less than five or sometimes only one type of spore and infect only one kind of plant. Stem rust (Puccinia graminis) of wheat alternates between barberry (Berberis spp.) and wheat (Triticum vulgare) and all five spore stages are produced. Rose rust produces four spore stages but only roses (Rosa spp.) are infected.

Urediniospores ("repeating" spores) are produced in pustules that appear yellow, orange, or brown as a result of masses of spores. These spores can re-infect the same host that produces them, making them the damaging phase of most rust diseases. Urediniospores are windborne and infect the plant through stomata. Water is required for short periods (6–8 hours or less) for germination and infection. Heavy dew is often sufficient. Once infection has occurred, water is no longer needed for continued development, and the infection and spore production will continue for the life of the leaf.

Some rusts, including rose (Phragmidium tuberculatum) may survive the winter on leaves that do not fall off the plant. Rose rust also survives as teliospores (dark, thick–walled, overwintering spores) that form in fall. These spores survive in a dormant stage on fallen leaves.

Rust diseases are favored by moderate temperatures that favor the growth of the host. Rust spores can be killed by high temperatures. Some rust infections, such as geranium rust (Puccinia pelargonii-zonalis), can be eradicated by hot water treatment of cuttings, although some damage to the host can occur.

Because water is necessary for infection, overhead irrigation should be avoided when rust is a problem. Eliminate alternate hosts.

Chrysanthemum white rust, caused by Puccinia horiana, and Gladiolus rust, caused by Uromyces transversalis, are under an eradication program in California. Gladiolus rust also causes disease on other members of the iris family such as Crocosmia, Tritonia, and Watsonia. See specific guidelines under the DISEASE CONTROL OUTLINE FOR CHRYSANTHEMUM and DISEASE CONTROL OUTLINE FOR GLADIOLUS.

There are several fungicides that can be used to protect plants from infection. Mancozeb as a protectant and myclobutanil as an eradicant are generally effective against all rusts while triadimefon is effective against only specific rusts.