SOUTH AFRICAN RUSTS WITH POTENTIAL TO CONTROL TWO MAJOR ENVIRONMENTAL WEEDS IN AUSTRALIA
L MORIN1, PB EDWARDS2,3, CA KLEINJAN2 and A WOOD2,4
1CRC for Weed Management Systems, CSIRO Entomology, GPO Box 1700, Canberra, ACT, Australia, 2601; 2CSIRO Biological Control Unit, University of Cape Town, South Africa; 3CSIRO Entomology, Winnellie, NT, Australia; 4Plant Protection Research Institute, Stellenbosch, South Africa.
Background and objectives
Exotic weeds such as Asparagus asparagoides (bridal creeper) and Chrysanthemoides monilifera (bitou bush/boneseed) pose a major threat to biodiversity and conservation in Australia's temperate natural ecosystems. Control by intensive methods such as the use of herbicides or mechanical methods has proved uneconomical or inappropriate for most areas. A. ;asparagoides is a climber, the seeds of which are dispersed by birds, which establishes itself in relatively undisturbed vegetation producing dense mats of rhizomes and tubers. The foliage senesces at the onset of the dry season (e.g. summer in winter-rainfall regions), and commences regeneration from its underground rhizomes before the start of the next rainy season. Chrysanthemoides monilifera is a woody evergreen shrub growing up to 2-6 ;m, the seeds of which are dispersed by birds and mammals. Infestations can develop into thick, impenetrable stands. Classical biological control using pathogens may offer a sustainable and environmentally-friendly solution for these two troublesome environmental weeds. This biological control approach involves the introduction and establishment of pathogens that infect the target weeds in their native range South Africa, into Australia. This paper reports on two rust fungi that are currently under investigation as potential biological control agents for these weeds.
Results and conclusions
During extensive field surveys in South Africa in the early 1990s, two rust fungi were identified as potential biological control agents for A. ;asparagoides and C. ;monilifera. The macrocyclic and autoecious rust fungus Puccinia myrsiphylli infects the cladodes (leaves) and stems of A. ;asparagoides . All spore stages (spermatia, aeciospores, urediniospores, teliospores and basidiospores) have been observed in the field. Severely diseased plants shed infected cladodes prematurely and produce few or no fruits. The fungus is found mainly on A. ;asparagoides. A morphologically distinct form of P. ;myrsiphylli has also been observed on rare occasions on the closely-related plant species A. ;volubile and A. ;alopecurus. Puccinia myrsiphylli has never been reported on cultivated asparagus in South Africa. The rust is commonly found in the winter-rainfall, even-rainfall and summer-rainfall regions of South Africa, wherever A. ;asparagoides occurs.
The microcyclic, systemic rust fungus Endophyllum osteospermi (formerly referred to as Aecidium osteospermi) infects the foliage and stems of C. ;monilifera . It produces spermatia, teliospores and basidiospores. The teliospores have the appearance of aecidiospores and are produced in aecium-like fruiting bodies. One to two years after infection, plants develop witches' broom symptoms with multiple, swollen stems and short internodes, and small and slightly chlorotic leaves. Infected branches produce few or no fruit and usually die within 1-4 ;years. The systemic nature of E. ;osteospermi 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 plant. E. ;osteospermi completes its entire life cycle on its only known hosts in South Africa: C. ;monilifera subsp. monilifera (boneseed), C. ;monilifera subsp. rotundata (bitou bush), C. ;monilifera subsp. pisifera and C. ;incana. The rust occurs widely in the winter-rainfall and even-rainfall regions of South Africa.
Rust fungi are considered excellent biological control agents because they can cause severe disease epidemics that reduce the competitiveness of weeds, disperse efficiently, and are generally host-specific. The host-range of P. ;myrsiphylli and E. ;osteospermi is currently being determined experimentally. Should high specificity towards the target weeds be confirmed permission to release the rust fungi in Australia will be sought.
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