- New research suggests disease suppressive soils can help mitigate against yield loss from RLNs in susceptible varieties.
- Yield loss in susceptible varieties can be up to 50% in wheat and 20% in chickpeas.
- Repeated studies in glasshouse and laboratory bioassays have consistently showed general suppressiveness to root-lesion nematodes exists in a variety of northern soils.
- Practices such as the use of tolerant or resistant varieties, crop rotation, no-till, stubble retention and good farm hygiene are generally effective in managing RLN populations but costly losses can still occur.
The battle against root lesion nematodes (RLNs) in the northern cropping belt has `gone to ground’ with new research suggesting that disease suppressive soils could play an important role in reducing yield loss.
RLNs (Pratylenchus thornei and Pratylenchus neglectus) cost Australian growers in excess of $250 million annually with yield losses in susceptible varieties of up to 50% in wheat and 20% in chickpeas.
Control currently relies on an integrated management program that includes the use of tolerant or resistant varieties, crop rotation and good farm hygiene and while these practices are generally effective in managing populations, costly losses can still occur.
Boosting the suppressiveness of soils to RLN is emerging as a new frontline defence in controlling and eventually reducing RLN populations when used in conjunction with a best practice integrated management program.
Disease suppression is defined as the ability of a soil to suppress disease incidence or severity even in the presence of the pathogen, host plant and favourable environmental conditions.
In essence, it relies on the vast array of organisms in the soil to provide a degree of biological buffering against pathogens.
The challenge for industry lies in gaining a better understanding of the suppressive nature of grain-growing soils within the northern region with the aim of providing growers with methods to enhance the suppressiveness of their soils to RLNs.
The level and variation of suppressiveness in northern soils has been the focus of a four-year research project jointly funded by the Grains Research and Development Corporation (GRDC) and Queensland Department of Agriculture and Fisheries (DAF) which involved 26 different paddocks/trials and more than 70 sampling sites.
The study included three long-term farm management trial sites where several fertiliser or tillage treatments were sampled and seven sites which compared nearby cropped, pasture or native/scrub remnant soils to gain an understanding of the impact cropping may have on suppressiveness to RLNs.
DAF Senior Soil Microbiologist at the Leslie Research Facility in Toowoomba, Dr Nikki Seymour said repeated studies of the different soils between 2010 and 2014 in glasshouse and laboratory bioassays consistently showed general suppressiveness to RLNs exists in a variety of northern soils.
“In the glasshouse assays, P. thornei increased only 2-5 times in unheated soil compared with 17 times when soil was heated prior to planting to eliminate the general soil biological community,” Dr Seymour said.
“Incorporating a small amount of unheated soil into heated soil (10% unheated) reduced multiplication rates by 60-89% showing that specific organisms in those soils were also contributing to this suppression and that it was not a physical or chemical property of the soil per se affecting multiplication.”
Researchers also tested the difference in suppressiveness between soil from the 0-15cm and the 30-45cm layers in the profile given that Pratylenchus nematode populations tend to be highest to depths of 30-60 cm.
Dr Seymour said results showed that soil from 0-15cm was generally more suppressive than soil from 30-45cm.
Information on the soil characteristics and crop management histories was also analysed to help researchers understand the mechanisms influencing these results but to date, analyses have not shown any specific correlation between particular soil chemical (macro and micro nutrient levels) and physical (pH, texture, clay content) attributes and suppressiveness.
One area of the work which raised as many questions as answers was the survey of over 130 northern soils for the presence of natural enemies Pasteuria bacteria, nematode trapping fungi (NTF) and predatory nematodes to RLNs.
Dr Seymour said glasshouse trials to increase populations of Pasteuria on hosts of P. thornei had been conducted but multiplication of the bacteria was slow to negligible using traditional methods.
She said further study was required to better understand the lifecycle and ecology of the Pasteuria bacterial parasite that interacts with P. thornei.
“Very little is known globally about this organism and no known research on the presence of Pasteuria on P. thornei had been conducted in Australia prior to this project,” she said.
“Also, traditional methods of isolating NTF are being compared with new molecular methods being developed in collaboration with Dr Helen Hayden from DEPI Victoria.
“This work successfully showed NTF could be detected in our soils using DNA extraction and TRFLP techniques and subjective quantification of levels in soil was also possible.
“Levels of these fungi were much higher in top soil layers which correlate nicely with the findings of our suppressiveness assays and indicate that these organisms may be having some impact on RLN multiplication.”
The impact of farm practices on the suppressiveness of soils to RLNs was studied as part of the project with results showing that factors such as pastures in rotation had a limited impact on soil suppressiveness.
While cropped soils were generally found to be just as suppressive as nearby native grassland or scrub, frequent tillage on the cropped soils should significantly increase nematode multiplication.
“Growers practicing no-till, stubble retention practices and cropping when soil moisture allows are probably doing the best they can to enhance their suppressiveness in the top soil,” Dr Seymour said.
“Without these practices, we estimate that RLN multiplication would be significantly greater especially in top soils and therefore lead to much greater losses in productivity of susceptible crops.
“As RLN multiply right down the soil profile, practical means of increasing soil biology through improving carbon deposits are needed to reduce multiplication further.
“Research in this project has identified specific antagonists of the RLN – including some for the first time - in our northern grain growing soils however more targeted research is required on how to measure and enhance of these organisms in our soil.”
Elise McKinna, DAF Media & Communication Officer
07 3087 8576
Sarah Jeffrey, Senior Consultant Cox Inall Communications
GRDC Project Code