Comprehensive diagnostic analysis of pastures affected by dieback

Summary

Pasture dieback is a complex condition involving a range of interacting biotic and abiotic factors. The role and impact of these factors are poorly understood. This project aimed to clarify the range of pathogenic organisms associated with pasture dieback across multiple regions in Queensland, as well as northern New South Wales (NSW), to assess for pathogenicity. The knowledge of multiple causal agents or co-factors of pasture dieback has improved and the impact they can have on pasture productivity and business profitability.

Objectives

The objectives of this project are:
1. Improved knowledge of the potential causal agents or co-factors of pasture dieback using pathogenicity testing.
2. Gain knowledge of pathogenic organisms present in the pasture affected by dieback from geographical areas where plant and soil samples have not been previously collected.

Key findings

The knowledge of multiple potential causal agents or co-factors of pasture dieback has improved through the analysis of plant and soil samples from affected and unaffected areas, and the pathogenicity testing of multiple fungi and the pasture mealybug.

Multiple field surveys across pasture dieback affected regions in Queensland (southern, central and northern) and subsequent sample analysis indicate it is unlikely that different casual agents of pasture dieback occur in different regions of Queensland.

No evidence was found for a link between fungal pathogens and pasture dieback. However, some known fungal pathogens do occur in conjunction with pasture dieback e.g. buffel grass blight (Pyricularia grisea).

Microbiome research demonstrates very similar microbiomes across single point sampled symptomatic and non-symptomatic plants and that there was not a clear, bacterial or fungal population strongly associated with symptomatic plants to indicate a possible single causal agent for dieback.

Virology research conducted during the project indicate a range of viruses present in both symptomatic and non-symptomatic plants. Some of these are novel i.e. have not been recorded previously. Others represent new host or geographic records for known viruses. Grass velariviruses warrant further investigation as it remains possible that these viruses play a role in pasture dieback.

Analysis of soil chemistry and nutrient levels, and investigation of ground pearl and nematodes, do not provide clear evidence of a causal link with pasture dieback.

Replicated field research demonstrated that uncontrolled, high-density infestations of the pasture mealybug can cause the rapid onset of dieback symptoms, decline in plant growth and plant death. Control of pasture mealybug infestations at early onset of symptoms resulted in recovery of pasture productivity.

Benefits to industry

The outcomes of this project provide empirical evidence of the role of pasture mealybug on one variety of pasture and indicate no evidence of a single fungal, bacterial or viral pathogen associated with pasture dieback. Current project outcomes will guide future research into effective management solutions to restore pasture productivity and business profitability.

Future research

Despite the significant research effort and improvement in knowledge of pathogenic organisms associated with pasture dieback, further research is required to overcome the limitations encountered and to answer new questions that emerged during the project.

These include:
1. New field-based sampling experiments to track impacts of pasture dieback over time.
2. Investigation of the mechanism/s of pasture dieback, with focus on pasture mealybug as the primary causal agent.
3. Undertake wider screening of sown/improved and natives grass species to the pasture mealybug.
4. Undertake additional analysis of soil sample data.