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had little impact on slender thistle infestations. The CSIRO Division of Plant Industry has recently selected two new strains in Europe. These strains are virulent on Australian types of slender thistles. Strain IT2 from Italy affects C.pycnocephalus and strain FR3 from France affects C. tenuiflorus. After thorough host specificity testing we are now able to release these two strains in Australia.
Description and Life Cycle
 The slender thistle rust fungus has a complicated yearly life cycle involving five different spore stages. The most commonly seen are the honey-brown autumn-spring spores (urediniospores) which are first seen on the leaves of seedlings and young plants in autumn. The urediniospore bodies are reddish brown on the upper sides of leaves and purplish on the under side. Urediniospores have a generation time of 8 to 10 days and several generations are produced per season. They are dispersed by the wind, allowing rust epidemics to spread throughout the thistle population.
When temperature and humidity are suitable the spores germinate, the growing tips penetrate the stomata (leaf pores) and begin to ramify within the leaf tissue and destroy it.
Towards the end of spring, summer spores (teliospores) are produced on flowering plants and remain attached to the dead stems, leaves and thistle litter until autumn, when they germinate to infect the new generation of slender thistles. Rust infection starts on new seedlings in autumn and builds up to epidemic proportions by spring, covering most of the leaves and flowering stems.
 Impact
Infection of leaves and flowering stems by the rust fungus weakens the plant and reduces its ability to produce flowers and seed. This makes the thistles more susceptible to competition from desirable pasture species. Experimental field inoculations have shown that rust infection reduces plant height, plant mass and the production of viable seed. Slender thistle rust can kill plants and it has had a devastating impact in some areas.
Releases
The slender thistle is inoculated with rust urediniospores at "nursery sites" where populations of the rust are allowed to build up for later redistribution to other infestations. Nursery sites are selected by the Meander Valley Weed Strategy.
A release site should have a dense and persistent thistle infestation and be connected to other neighbouring infestations so that the rust can spread rapidly. The site should have a low priority for control by other techniques.
Integrated Control
Biological control cannot totally eradicate a weed but can only reduce the spread and density of infestations. In some cases control is achieved to the level where the weed is no longer of concern and no other control is necessary. More commonly, other methods are still required to achieve the desired level of control; however these need not be applied so frequently. Biological control should not be considered the complete answer to a slender thistle problem but can be used in conjunction with other control measures in an integrated management plan.
 
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