general
types
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diseases
Rusts and Smuts In t r o d u c t i o n The rusts and smuts belong to the Basidiomycota, the group of fungi that includes the mushrooms. Rust fungi attack a large number of herbaceous perennials. The common name “rust” comes from the reddish to orange, rusty-colored pustules, or blister-like swellings, that rust fungi produce on leaves, scapes, or stems of host plants. Rust fungi are obligate parasites, which means that they need living plants to survive, and therefore seldom kill plants. Nevertheless, rusts harm ornamental plants’ appearance by reducing flower production, and detracting from overall plant health and vigor. Severe infections can result in premature leaf drop. Smut diseases are far less common than rusts on herbaceous perennials, and are seldom major concerns. The smut fungi can infect leaves, stems, and flower parts depending on the smut species, host plant species, and timing of infection. The smuts are named for their sooty black spore masses, which commonly replace infected plant parts.
Brown rust on chrysanthemum.
Rusts Rust pustules on Helianthus.
Symptoms Rust symptoms are different on the primary and alternate plant hosts. On leaves, scapes, or stems of most herbaceous perennials, rust fungi produce blister-like swellings, called uredinial pustules, which may be reddish, orange, yellow, white, or dark brown, depending on the species of rust. On alternate plant hosts, these same fungi can produce aecial pustules that are typically larger than uredinial pustules and are usually yellowish in color.
Ecology While most parasitic fungi have only one or two spore stages and a single plant host, rusts lead more complex lives. Some rusts have as many as five spore stages and two plant hosts. For some rusts, two types of host plants are needed to complete the disease cycle. These dual-host rusts produce specialized spores called urediniospores on one of the host plant species, and another type of specialized spores called aeciospores on the other plant host. Rust diseases with this lifestyle are said to have “alternate hosts”. For most rust diseases of herbaceous perennials, the perennial plant is the host on which the urediniospores form. Blister-like pustules called uredinia break open on the plant surface to expose the rusty-colored urediniospores. This stage is the most damaging to the plant: since urediniospores can cause new cycles of infection in
Rust pustules and uredinospores on daylily.
Teliospores forming in daylily rust pustules.
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