Rust of Alfalfa
Rust on alfalfa is caused by the fungus Uromyces striatus. In general, rust is most problematic from mid season onwards when cutting is delayed. Severe rust can occur on fall regrowth and fall-seeded stands may be weakened or killed. Rust appears as numerous small, powdery, brick-red pustules on leaves and petioles. Both sides of the leaf may have pustules, which break through the outer layer of leaf tissue and spores are released. Spores can be brushed from pustules easily. Premature defoliation can occur on susceptible alfalfa cultivars. Stems can also develop pustules.
U. striatus survives on living host tissue in the southern United States. Rust spores are slowly spread to northern states via the wind. Water is required for spores to germinate and warm weather favors rust development. U. striatus can also survive as mycelium on alternate hosts within the Euphorbia genus.
Rust on alfalfa is best managed by planting resistant cultivars, although only a limited number are available at this time. Ensure timely harvest to avoid leaf loss and reduce spore production.
Prepared and reviewed by the Alfalfa Pest Management Working Group.
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