Grains Research and Development

Date: 31.10.2016

Soil test for nematodes critical for summer crop choices

Author: Toni Somes

Image of Dr Kirsty Owen

Dr Kirsty Owen from USQ’s Centre for Crop Health inspects a mungbean crop for signs of nematode damage at a 20-hectare trial site on a property at Jondaryan as part of a GRDC-funded research project.

PHOTO: David Martinelli, USQ

New research has highlighted the need for northern growers to soil test for root lesion nematode (RLN) species and population size ahead of summer planting to ensure the right crop choices are made to reduce the parasitic pests’ impact

GRDC-funded research by the University of Southern Queensland’s (USQ) Centre for Crop Health indicates some summer crops previously regarded as ‘resistant’ may now be ‘susceptible’ to specific RLN species.

The research was led by USQ nematologist Kirsty Owen, who also heads the Queensland team working on the national GRDC program researching the impact of nematodes on crop production.

RLNs are found in 77 per cent of paddocks in the northern region and samples tested using the South Australian Research and Development Institute’s PreDicta® B soil-testing service in 2015 showed Pratylenchus thornei are more widely distributed and found in greater, more damaging populations than Pratylenchus neglectus. PreDicta® B offers soil tests for both RLN species and the type of RLN present influences crop selection.

The potential for yield loss caused by RLN is up to 70 per cent in intolerant cultivars of wheat, with new data showing a 6.5 per cent yield loss in chickpeas with P. thornei.

Dr Owen says the latest research highlighted the need for growers to soil test for RLNs so they could plan crop rotations that targeted the correct nematode species.

“Effective management of the RLNs P. thornei and P. neglectus starts with growers identifying populations present in paddocks,” Dr Owen says.

“Very high nematode populations are reduced by increasing the number of resistant crops grown consecutively in rotations. It may take two or more resistant crops to reduce damaging populations, but with careful planning growers can minimise the impact of root lesion nematodes on farm profits.”

She says new research shows mungbeans were susceptible to P. thornei but were poor hosts of P. neglectus, while growing sorghum would cause populations of P. neglectus to increase and remain in high numbers to attack the next crop.

“Our research also showed that maize may not suffer yield loss due to P. thornei, but some hybrids of maize will cause populations of the nematode to build up and put the next crop at risk of attack.”

Dr Owen says the findings emphasise the importance of crop choice when it comes to reducing populations of specific RLN species. “We can’t eliminate nematodes, but with strategic crop choices and the right rotations growers can minimise their impact on farm profitability.”

Northern nematode pressure

In the northern region, paddocks with more than 15 P. thornei per gram of soil (or 15,000 per kilogram of soil) by the PreDicta® B test are classified as high risk for crops.

Even populations of P. thornei that are classified as medium risk by PreDicta® B, that is 2 to 15 per gram of soil (or 2,000 to 15,000 per kilogram of soil), can cause substantial yield loss of intolerant wheat varieties in warm, wet growing seasons conducive to nematode reproduction in the roots.

Therefore 2 nematodes per gram of soil (or 2,000 per kilogram of soil) should be regarded as the threshold for loss in intolerant crop varieties.

Steps for effective management

Dr Owen says once growers know which RLN species is present in paddocks and the population size, they could:

  • select tolerant varieties so yields are maximised; and
  • reduce the size of the populations by growing resistant crops.

Tolerance is the impact of root lesion nematodes on plant yield and resistance is the ability of the nematode to reproduce on the plant. Each species of RLN has a unique and broad host range including cereals and legumes. Within some crops there is variation in the susceptibility and resistance of varieties.

“Our previous field experiments with summer crops have demonstrated that crops such as sorghum and sunflower are moderately resistant to P. thornei and crops such as soybean and mungbean are susceptible,” Dr Owen says.

“However, one moderately resistant crop in a sequence is not enough to reduce damaging populations of P. thornei. Two or more resistant crops grown consecutively are needed to get on top of root lesion nematodes.

“For example, populations of P. thornei increased after growing most cultivars of mungbean and soybean compared to fallow and sorghum hybrids.”

Crop selection assisted by new research

Mungbeans and P. thornei

Dr Owen says new research shows mungbeans are susceptible to P. thornei but were poor hosts of P. neglectus.

The results showed P. thornei increased between 2 and 2.7 times after growing mungbean varieties and up to 8.5 times after growing black gram cv. Regur. In comparison, previous field trials have not detected any yield loss in mungbeans due to P. thornei, indicating that they were tolerant. 

“The take-home message for mungbeans is to be aware that they will cause P. thornei to increase and should not be included in rotations where you are trying to reduce populations of P. thornei,” Dr Owen says.Sorghum and P. neglectus

She says sorghum is an excellent crop to include in rotations where P. thornei is present because it is moderately resistant. However, sorghum is susceptible to P. neglectus.

Recently, hybrids of sorghum were screened in glasshouse experiments for resistance to. P. neglectus. In these trials there was a range of responses for sorghum from ‘susceptible’ to ‘moderately susceptible’.

“Our previous results have shown that forage sorghum hybrids are also susceptible to P. neglectus, with the sweet sorghum hybrids being most susceptible and the grain sorghum/Sudan grass hybrids being moderately susceptible,” Dr Owen says.

“Growing sorghum will cause populations of P. neglectus to increase and nematodes will remain in high numbers to attack the next crop. This research highlights the importance of testing soil for root lesion nematodes to identify which species is present in the paddock.”

Maize and P. thornei

Dr Owen’s team also investigated hybrids of maize in glasshouse experiments for resistance to P. thornei. These results indicate a range of responses to P. thornei for maize from ‘susceptible’ to ‘moderately resistant’.

“Our previous field trials have not detected any yield loss in maize, indicating that they are tolerant,” she says. “Although maize may not suffer yield loss due to P. thornei, some hybrids of maize will cause populations of the nematode to build up and nematodes will remain in high numbers to attack the next crop.”

Dr Owen advises growers to monitor RLN population change and soil test again in three to five years.

More information:

Kirsty Owen,
07 4631 1239,

kirsty.owen@usq.edu.au;

'Summer cropping options to manage root lesion nematodes' GRDC Research Update paper;

Predicta® B;

'Root-lesion nematodes' Tips and Tactics

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