Fruiting bodies of the rust fungus, Endophylum osteospermi on boneseed.

Management and control of boneseed: boneseed rust fungus, a highly promising candidate for biocontrol

CSIRO is finalising the risk assessment phase for the rust Endophyllum osteospermi, a highly promising biocontrol agent for boneseed, one of the 20 Weeds of National Significance.

• 4 October 2007 | Updated 14 October 2011

• Boneseed distribution in Australia
• Boneseed rust - impact
• Boneseed rust - host specificity

CSIRO in partnership with the Plant Protection Research Institute in Stellenbosch, South Africa, has been investigating for many years the systemic rust, Endophyllum osteospermi as a potential biological control agent for boneseed (Chrysanthemoides monilifera subsp monilifera), one of the 20 Weeds of National Significance (WoNS).

Financial support has been obtained from the Australian Government (Land & Water Australia, Defeating the Weed Menace Research and Development initiative) to finalise the risk assessment phase for this potential agent and apply for its release in Australia.

Boneseed distribution in Australia

Boneseed is a woody evergreen erect shrub with a rounded, often dense canopy growing one to three metres high. It is confined to South-Eastern Australia with major infestations on the:

• Mornington Peninsula, Victoria
• You Yangs Ranges, Victoria
• Mount Lofty Range, South Australia.

Small or scattered infestations occur elsewhere throughout Victoria, Tasmania and South Australia.

Isolated occurrences also occur in New South Wales and Western Australia.

Boneseed rust - impact

Improving effectiveness of the biological control program against boneseed has been identified as a high research priority in the National Strategy for this WoNS, because none of the six insect agents released so far have established in the field.

Boneseed rust is a highly promising biocontrol agent because it reduces growth and reproduction of plants by causing extensive deformation of infected branches (witches’ brooms).

In South Africa the rust appears to be the primary cause for the decline and death observed in some local boneseed populations.

The systemic nature of the rust is a desirable characteristic for biological control purposes as once the fungus is established within the host the infection is retained until the death of the witches’ brooms.

Witches’ broom caused by the rust fungus on boneseed in South Africa.

Boneseed flowers mostly from late winter to spring.

Boneseed rust - host specificity

The boneseed rust is only recorded in South Africa on a small group of related plants of the genera Chrysanthemoides and Osteospermum (Calenduleae: Asteraceae).

As there are no indigenous representatives of the Calenduleae in Australia, the non-target plants most at risk from this rust fungus are the introduced, ornamental species belonging to this tribe.

Because of the nature of boneseed rust, which develops visible symptoms only one to two years after infection of its host, an initial series of host-specificity tests were performed on detached leaves of plant species of the approved test list.

Using microscopy techniques, these tests were to determine whether the rust was capable of penetrating epidermal cells of non-target plant species (as it successfully does on its hosts).

Additional tests carried out on leaves still attached to plants of some non-target species, as well as the target weed species, confirmed accuracy of results obtained with detached leaves.

Since penetration of epidermal cells does not necessarily imply that the infection process will continue and be successful, more tests on whole plants of the species where penetration occurred in initial tests are underway to determine if the fungus is capable of colonising tissue of these species.

Results from these additional tests will provide the necessary information to fully assess the risk of significant impact on these non-target species should the rust be released in Australia.

Read about Dr Louise Morin: using fungi to fight Australia’s weeds.