Carduus pycnocephalus, Carduus tenuiflorus Slender thistles Slender thistle weeds invade southern Australian pastures, reducing pasture productivity and causing wool contamination. One biocontrol agent has been released to combat slender thistles. 1. Slender thistle rust *Puccinia cardui pycnocephali* The rust was first introduced accidentally into Australia at least 50 years ago. Subsequent release programs undertaken in 1995 resulted in the establishment of the slender thistle rust fungus in atleast 38 release sites where it has been shown to reduce the spread and density of slender thistle infestation.