BIOLOGY: P. punctiformis is a naturalized fungus infecting Canada thistle.  The rust fungus undergoes four life cycles.  Aceiospores emerge in the spring and are visible as yellow fragrant spores on the undersides of leaves. Basidiospores become systemic and damage Canada thistle through summer.  Teliospores are brown and fuzzy on the undersides of leaves in the fall.  

DESTRUCTIVE STAGE: all

IMPACT TO HOST: Systemic fungal infections can completely kill individual Canada thistle plants.  Fungus do not spread well on their own.  Management like mowing or grazing will increase impact.

REDISTRIBUTION: Collect leaves infested with teliospores and release in new Canada thistle.  Spores must come in direct contact with rosettes for successful establishment.