Isolation and Evaluation of Potential Microbial Biocontrol Agents for Management of Root Lesion Nematodes

Pests and diseases are a significant factor affecting the production volume and quality of wheat in Australia, and throughout the world. Of the nematodes affecting wheat, Pratylenchus are of most concern due to their prevalence throughout, and their economic impact on the Australian grains industry. Chemical nematicides are not available to this industry, and so control of this pest relies on host plant resistance and crop rotations with non-host plants to prevent damage. However because of their wide host plant range and incomplete levels of resistance in current wheat varieties, sufficient control levels are not always achieved. Biological control, involving the deliberate release or increase of a biocontrol agent into a target area is an alternative method of pest and disease management, and was investigated in this work.
Twenty one farms throughout the Riverina district of New South Wales were sampled for microbial biocontrol agents known to be effective against plant pathogenic nematodes. Three hundred and thirty three isolates from the genera Purpureocillium, Bacillus, Pochonia or Lecanicillium, and Trichoderma were isolated. This collection is the most significant single collection of microorganisms with nematode biocontrol potential from the Riverina district, and from New South Wales. Over eighty of these isolates were ranked for efficacy against Pratylenchus thornei, with results showing a range of activity levels against this plant pest. Thus efficacy can be considered isolate specific. Six microorganisms were chosen for investigation as combined treatments, wherein complementary modes of action and the ability to affect multiple nematode life stages are proposed to contribute to greater levels of control and reliability in the field.

Laboratory trials confirmed that the six biocontrol microbes were able to colonise the rhizosphere of wheat, and some isolates were able to penetrate into the root system and colonise endophytically. Thus their establishment in the location where nematodes initiate plant damage was confirmed. Extracellular products were found to contribute to the control of P. thornei for the six microorganisms, but most significant was the finding that parasitism was involved in nematode biocontrol for Trichoderma harzianum. Until as recently as 2015, the mode of action of Trichoderma species against plant pathogenic nematodes was considered to be unclear (Li et al., 2015). Laboratory competition assays, wherein the biocontrol agents were co-inoculated to determine compatibility, showed that some microorganisms were incompatible in combinations. Thus combinations of biocontrol microbes need to be chosen with care.
One isolate each of Purpureocillium lilacinum, Bacillus pumilus, and Lecanicillium lecanii were tested in combinations in pot and field trials for biocontrol of P. thornei. The results demonstrated that for the isolates tested, there was no difference in using combinations as compared to using the microorganisms singly. Extrapolating laboratory results to predict performance of biocontrol microbes in pot and field trials proved unreliable. Therefore a combination of research methods will be required in future investigations of microbial biocontrol agents for the control of root lesion nematodes. Interactions within the nematode-plant-biocontrol system are complex, and detailed studies of these and further components of the system will be necessary in the future. However, the benefits warrant continued investment into research and development of this environmentally sustainable control method.