Slender Thistle Suppression with the Slender Thistle Rust Fungus (DSE Vic)

December 1998 Slender thistle suppression with the LC0146 slender thistle rust fungus ISSN 1329-833X Keith Turnbull Research Institute, Frankston

Common and Scientific Names dead stems, leaves and thistle litter until autumn, when they germinate to infect the new generation of slender Slender thistle rust fungus thistles. Puccinia cardui-pycnocephali Sydow Rust infection starts on new seedlings in autumn and builds Background up to epidemic proportions by spring, covering most of the leaves and flowering stems. The slender thistles Carduus tenuiflorus and C. pycnocephalus are Regionally Controlled Weeds in all Catchment and Land Protection Regions in Victoria except Mallee, Wimmera and North Central, where they are not declared noxious weeds. A strain of the slender thistle rust fungus has occurred in Australia since early this century. It infects both species of slender thistle and is common where the thistles are found. However this particular strain of rust has had little impact on slender thistle infestations. To improve control of slender thistles the CSIRO Division of Plant Industry selected two new strains of the rust in Europe. These have been tested to ensure that they present no danger to native plants or plants of economic importance. The strains are virulent on Australian types of slender thistles. Strain IT2 from Italy affects C. pycnocephalus and strain FR3 from France affects C. Figure 1. Autumn-spring spore bodies on upper side of leaf. tenuiflorus. Both strains have been widely released 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 stages 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 (Fig. 1) and purplish on the under side (Fig. 2). Urediniospores have a generation time of 8-10 days and several generations are produced per season. They are dispersed by the wind, allowing rust epidemics to spread throughout the thistle population. Figure 2. Autumn-spring spore bodies on leaf under side. When temperature and humidity are suitable the spores Sporing bodies of the rust tend to develop next to the veins germinate (Fig. 4), the growing tips penetrate the stomata of the leaf. A yellow ring develops around the active (leaf pores) and begin to ramify within the leaf tissue and sporing body where the leaf tissue has been damaged. destroy it. Towards the end of spring, summer spores (teliospores) are produced on flowering plants and remain attached to the

Impact Nursery sites are selected by NRE staff in consultation with land owners and Landcare groups. Infection of leaves and flowering stems by the rust fungus (Fig. 3) weakens the plant and reduces its ability to A release site should have a dense and persistent thistle produce flowers and seed. This makes the thistles more infestation and be connected to other neighbouring susceptible to competition from desirable pasture species. infestations so that the rust can spread rapidly. The site Experimental field inoculations have shown that rust should have a low priority for control by other techniques. infection reduces plant height, plant mass and the The vials of spore inoculum should be stored in a freezer production of viable seed. Slender thistle rust can kill and kept frozen until the day of release. The spores are plants and has had a devastating impact in some areas. mixed into a suspension in water and applied using a spray atomiser or paint brush. The selected virulent strains of the rust were first released in Victoria in 1995. Over 50 releases have been made. Moderate to high levels of rust have subsequently been observed at many release sites and the rust has spread long distances in some areas. Establishment success has been confirmed at 38 of the initial release sites. Integrated Control Biological control cannot totally eradicate a weed but can 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. It is a technique that should be used in conjunction with other control measures in an integrated management plan. Further Information For further information on the biological control of slender thistles contact: Keith Turnbull Research Institute, PO Box 48, Frankston, Victoria, 3199 Tel (03) 9785 0111 Fax (03) 9785 2007 Figure. 3. Slender thistle infected with the slender thistle rust. The spots on leaves and stems are the urediniospore bodies. Acknowledgments Compiled by Ian Faithfull, Peter Stevens, El Bruzzese and Sue Darby, September 1996. Revised December 1998.

Funding for the thistle biological control program has been provided by NRE’s Catchment Management and Sustainable Agriculture Division, The Woolmark Company and Meat and Livestock Australia. The biological control of thistles program is supported by the Cooperative Research Centre for Weed Management Systems.

The advice contained in this publication is intended as a source of information only. The State of Victoria and its officers do not Figure 4. Germinating spores of slender thistle rust. guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore Releases disclaims all liability for any error, loss or other consequence which The slender thistle is inoculated with rust urediniospores at may arise from you relying on any information in this publication. 'nursery sites' where populations of the rust are allowed to build up for later re-distribution to other infestations.