Beneficial micro-organisms might be the key to overcoming soil-borne diseases. By Elaine Fisher.

Enhancing soil health and boosting its existing communities of beneficial organisms is one way pastoral farmers can attempt to counter the adverse impacts of soil-borne plant pathogens, AgResearch scientist Bryony Dignam says.

Bryony is one of nine scientists who late last year released the findings of their research in the AgResearch report; “Impacts of soil-borne disease on plant yield and farm profit in dairying soils”.

“Our results suggest that root pathogens on New Zealand dairy farms are most prevalent and damaging in the Waikato region where economic modelling estimated clover and ryegrass root disease to cost 211kg milksolids (MS) per hectare per year in milk production, and $909 per hectare per year in farm profitability. Given the scale of these costs, targeted management of soil-borne disease could present an economically viable approach to improving the resilience of these multi-plant multi-pathogen ecosystems,” the report says.

Bryony and fellow researcher Nigel Bell, Science team leader · AgResearch, say further research is needed into the role of beneficials and how they are managed.

There are few chemical options for controlling soil pathogens. Pasture management and working with the soil’s communities of beneficials might be the best solution.

In fact, the management of soil microbial communities and the role they play in disease suppression and potentially helping reduce constraints on clover and ryegrass pastures is the subject of Bryony’s postdoctoral research, commissioned by Agricultural and Marketing Research and Development Trust, (AGMARDT).

“Introducing beneficial micro-organisms into the soil is not easy. They would need to compete with existing communities to be able to establish and do the job we want them to do. Finding ways to boost existing beneficial communities and promote their positive activity may be part of the answer,” Bryony says.

The report’s authors believe future research should explore the quantification of key forage root pathogens using molecular based techniques. Nigel says this methodology has been successfully developed for predicting soil-borne disease risk in arable cropping systems in Australia.

“Next-generation DNA sequencing is allowing us to sequence, in theory, just about everything in the soil. That means we can see entire soil communities and so see the bigger picture of the interactions between pathogens and beneficials.”

Although the technology is expensive and developing it for use on mixed sward pastures is complex, Nigel says in future it may be possible to create a test for organic biomarkers which would enable farmers to not only test for NPK nutrient levels but also for the biological health of their soils.

The latest research was partly prompted by a recognition that while the economic impacts of soil-borne diseases on potential yield have been studied extensively in arable cropping systems, that information was lacking for agricultural grasslands in New Zealand and elsewhere. Bryony says changes in land use, intensification, the doubling of the national dairy herd and the increased value of pasture production in the past 25 years in New Zealand meant further scientific investigation into the resilience of highly productive pasture plant species was required.

The research report says soil-borne plant pathogens are known to form diverse and dynamic pathogen complexes under pasture, hindering both pasture establishment and persistence. In NZ, surveys have identified a range of ‘generalist’ and ‘specialist’ nematode, fungal and oomycete pathogens as being present and damaging in pasture systems.

Identifying exactly what pasture issues are caused by soil-borne plant pathogens is not easy, Bryony says. That’s partly because the damage is going on, out of sight, underground. Complex interactions with other factors, such as climatic conditions and pest damage, often disguise the impact of soil pathogens on pasture performance.

“Although direct production losses attributable to soil-borne diseases are difficult to quantify, the consensus of early studies from New Zealand estimate production losses to be between 40% and 50%,” the report says.

“As the pastoral sector has undergone increasing intensification, associated changes in soil fertility, botanical composition and livestock grazing from this process has led to new abiotic and biotic environments within which additional soil-borne disease pressure may develop over the life of a modern pasture. As such, increasing concern regarding the impact of soil-borne diseases on pasture growth and persistence is driving industry-led calls for increased investment in this area of research.”

An earlier AgResearch-led study highlighted the importance of defining an economic ‘baseline of soil biological constraints’ to pasture productivity to provide a foundation from which appropriate decisions regarding soil-borne disease control can be made.

The aim of the study was to determine the extent of soil biological constraints to the growth of three common pasture species (ryegrass, white clover and plantain) from three regions in NZ (Waikato, Canterbury and Southland).

The report estimated the economic costs of soil disease to pasture production on dairy farms for regions in which disease pressure (i.e., % increase in plant growth in pasteurised soils compared to original soils) was significant. To identify putative root pathogens, nematode groups were extracted from soil and microbial plant pathogens isolated from diseased plant roots. Relationships between region, environmental, soil physicochemical and biological properties and disease pressure were then investigated.

The study surveyed the soil-borne plant disease pressure of 30 farm sites across the three main dairy regions. Of these regions, Waikato had the most consistent and prevalent soil-borne disease pressure, where regional average disease pressure is estimated to limit white clover and ryegrass drymatter production in dairy soils by 35% and 19%, respectively.

At individual farm sites, disease pressure identified in the study showed that limits on white clover and ryegrass production could be as much as 74% and 38% respectively.

The differences found between the impacts of soil-borne plant pathogens between the Canterbury and Waikato farms in the study may be due to geographic location and climate with colder winters in Canterbury possibly reducing pathogen numbers. However, Bryony says some individual properties in Canterbury recorded populations not dissimilar to those on Waikato farms.

“The fact that the Waikato has been a dairying region for longer than Canterbury, giving soil plant pathogen communities a chance to become well established, may also be a factor.”

Nigel and Bryony hope the research may help farmers gain greater understanding of the impacts soil borne plant pathogens may be having on their pasture and so production. “We welcome contact from farmers who want more information,” Bryony says.

The full report can be found at: https://onlinelibrary.wiley.com/doi/10.1002/sae2.12009#.YbpkE8QPS2g