Species in the Noogoora burr complex[1] are problematic weeds in primary production, the environment and the Australian community. Nationally significant impacts on productivity and profitability are experienced in summer cropping (e.g. cotton, sorghum, maize and pulse industries), in pastures and in rangelands (particularly wool production). Noogoora burrs are also hosts of insect pests and pathogens such as Verticillium wilt (Verticillium dahliae), an important crop disease. They are a serious threat to riverine ecosystems, habitats and native species, impacting 36 vegetation communities, including 11 ‘Endangered Ecological Communities’ in NSW alone. While a range of herbicides control Noogoora burrs in cropping systems, all cause off-target damage and are inappropriate for use in sensitive situations such as the riverine areas.

This project aimed to develop a bioherbicide to manage species in the Noogoora burr complex. Bioherbicides contain naturally occurring fungal pathogens that are specific to the target weed. Among their many benefits, they are safe to use in sensitive environmental and production areas and are highly cost effective when compared to traditional herbicide development. Having said this, bioherbicides require free water from dew or rain for the pathogens to develop and for plant death to occur. We overcome this global limitation through the use of a complex emulsion.

Our research builds on existing knowledge of the pathogen Alternaria zinniae and its ability to kill plants of the Noogoora burr complex. Initially, our research sought to better understand the taxonomy within the species complex since such uncertainty can constrain biological control efficacy. DNA barcoding and next-generation sequencing showed us that the Noogoora burr complex consists of two distinct genetic groups and a wide range of hybrids (not the 4 morphologically distinct species (morpho-types) originally described). The first group encompassed the species Xanthium cavanillesii and X. italicum, and the second encompassed X. occidentale and X. orientale. Fortunately, our bioherbicide is equally effective across these groups, supporting the validity and use of this bioherbicide against the Noogoora burr complex.

DNA barcoding also confirmed the presence of the Verticillium wilt pathogen in all groups of the Noogoora burr complex. Isolates identified belonged to a range of strains including the defoliating VCG1A and non-defoliating VCG2A Verticillium wilt.

The project was highly successful in achieving all its aims. It has progressed the potential to deliver an environmentally friendly bioherbicide alternative for the integrated management of Noogoora burr species. We anticipate partnership with land managers in testing this product as it moves towards a commercially viable product. Future research should extend the use of the complex emulsions to other pathogen/weed combinations to achieve control of other established weeds in primary production and environmental ecosystems.

[1]Four species comprising the Noogoora burr complex have been taxonomically described in Australia (X. cavanillesii, X. italicum, X. occidentale and X. orientale), while only a single species (X. strumarium) is defined in the Americas, the source of the burr. In practice, intra- and inter-specific variation exists, with some plants not readily conforming to any of the published taxonomic descriptions.