Rust of Prickly Ash

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Puccinia andropogonis has a complex life cycle involving five distinct life stages and two host species. The life stages are pycniospores, aeciospores, urediniospores, teliospores, and basidiospores. The fungus overwinters in the telial stage on big bluestem and little bluestem. There are nine currently recognized varieties of Puccinia andropogonis, and each variety occurs in the aecial stage on just one of nine plant species in the summer. Puccinia andropogonis var. xanthoxyli is found exclusively on northern prickly ash, hence the common name Rust of Prickly Ash.

On infected plants it appears as orange, pustule-like structures (uredinia) on the upper and lower leaf surfaces. It will occasionally also appear on stems.

The uredinia are the fruiting structures of the fungus. They are round to oval and up to 3 ⁄16″ long. They contain a dusty mass of thousands of microscopic, orangish-yellow summer spores (urediospores).

On northern prickly ash it first appears as swollen, yellowish-green spots on the upper leaf surfaces and twigs. On the undersurface of the spot there are numerous, tiny, rust pustules. The leaves and twigs are disfigured but the infection is not fatal to the plant.

Big bluestem and northern prickly ash

Summer on northern prickly ash, spring in big bluestem

Sources

4, 7, 26, 29, 30, 77.

Infestation

Stem

Upper leaf surface

Lower leaf surface