Variety choice and crop rotation key to managing root lesion nematodes

Image of Richard Daniel

Richard Daniel, CEO of the Northern Grower Alliance, says there there are large and consistent differences in the levels of susceptibility between wheat varieties.

PHOTO: Anthony Mitchell

Root lesion nematodes (RLN) can cause up to 70 per cent yield loss in susceptible wheat crops, and are particularly prevalent in the northern region. Growers should look for tolerant wheat varieties and use multiple rotations of resistant crops as crucial steps in the long-term management of RLN populations.

Conditions in the northern region are ideal for the survival of RLN, particularly the predominant species Pratylenchus thornei. These microscopic, worm-like parasites can reproduce rapidly inside the roots of plants, causing extensive tissue damage in intolerant varieties of wheat and chickpeas. Infected plants lose their ability to take up water and soil nutrients such as nitrogen, zinc and phosphorus, leading to stunted growth and a decrease in crop yields.

Know your nematodes

All growers in the northern region should presume they have nematodes in their paddocks, says Alex Gwynne, a grain grower on the Darling Downs, Queensland. For more than 30 years Mr Gwynne has dedicated 20 hectares of his land to RLN research.

“A lot of growers don’t know about them,” he says. “Until they’ve had crops affected by nematodes they don’t realise they’ve got them. Yet RLN are present in about 70 per cent of land in the northern region.”

The commonly accepted threshold for P. thornei numbers in the north has been 2000 per kilogram of soil (or two per gram of soil), yet Dr Kirsty Owen, a nematologist at the Queensland Department of Agriculture, Fisheries and Forestry (DAFF), says anything above 1000 P. thornei per kilogram (or 1/g) potentially has a high risk of causing substantial yield loss in intolerant crop varieties.

“It’s different down south where they have quite discrete risk ratings,” she says. “We might see up to 25 per cent yield loss with a population that starts at 1/g of soil, but in the south they might not see any effects until they’re up to 20/g.”

Researchers recommend growers ‘know before they sow’, and make use of the PreDicta B testing service before planting if their property’s RLN status is unknown.

Breeding for resistance

Root lesion nematodes 

  • Female Pratylenchus thornei nematodes are self-fertile and lay multiple eggs every day. 
  • Nematodes complete their life cycle in as little as six weeks, and populations can increase rapidly under the right conditions.
  • Root lesion nematodes (RLN) migrate freely between the roots and the soil, and can move long distances in soils carried by floodwaters.
  • RLN can also survive in fallow for long periods of time, lying dormant until there is a new, susceptible crop to feed on.

Key terms

  • ‘Tolerance’ ‘describes the impact of nematodes on crop yield. A tolerant crop will grow and yield well where nematodes are present.
  • ‘Resistance’ is a measure of a variety’s impact on RLN multiplication or build-up. A resistant variety will decrease nematode populations in the soil.

In paddocks with Pratylenchus thornei infestations, it is critical that growers avoid planting intolerant wheat varieties, but they must also focus their variety choice on options with improved levels of resistance to reduce the build-up of P. thornei populations.

While wheat breeding for resistance is expected to provide the next improvement in nematode management, there are no commercial bread wheat varieties totally resistant to RLN.

The 2014 Queensland Wheat Varieties Guide rates the varieties EGA EaglehawkPBR logo, LongReach LancerPBR logo, EGA GregoryPBR logo, LongReach GauntletPBR logo, EGA BurkePBR logo, EGA WyliePBR logo, SunguardPBR logo and SuntopPBR logo as having at least moderate tolerance to P. thornei in the northern region. This means that they will all yield well in the presence of P. thornei. However, there are differences in their resistance to P. thornei, LongReach GauntletPBR logo and SuntopPBR logo have a resistance rating of MR-MS and would likely reduce high populations, maintain moderate populations but may increase low populations. The other varieties listed have varying levels of susceptibility and would likely increase low to moderate populations of P. thornei and maintain high populations through the winter cropping season.

Although there are no wheat varieties totally resistant to P. thornei, says Richard Daniel, CEO of the Northern Grower Alliance (NGA), it is clear that there are large and consistent differences in the levels of susceptibility between wheat varieties.

“Growers with P. thornei present in their soils need to ensure they don’t grow highly susceptible varieties,” Mr Daniel says. “There are several wheat varieties that consistently produce larger P. thornei populations, such as StrzeleckiPBR logo, SunvexPBR logo and Sunco.

“They are varieties that have a good fit in a range of situations but should be avoided in paddocks where P. thornei is present. It is common to find up to three times as many P. thornei remaining after growing these varieties, compared to a less susceptible wheat such as EGA GregoryPBR logo.”

Recent field trials have also shown similar differences within chickpea varieties. While all varieties of chickpea are susceptible to P. thornei, some are more susceptible than others. “Older varieties such as Jimbour and KyabraPBR logo, for example, produce significantly larger P. thornei populations than PBA HatTrickPBR logo and PBA BoundaryPBR logo,” Mr Daniel says.

Over a series of nine trials conducted by the New South Wales Department of Primary Industries, Queensland DAFF and the NGA, the mean P. thornei population remaining was more than twice as high following Jimbour and KyabraPBR logo rotations.

“Unless there are other major agronomic advantages to growing these varieties,” Mr Daniel says, “growers should be avoiding them where P. thornei is known to be an issue.”

Rotations reduce nematodes

“The message we’ve been trying to get out there is that it’s about more than just choosing the right variety,” Dr Owen says.

“It can take two, three or more rotations of resistant crops to decrease RLN populations.”

Mr Daniel agrees: “Even when growing resistant crops, such as sorghum, sunflowers, maize or canary seed in winter, it can be a slow process to reduce P. thornei populations. Growers may need to consider including as many as three crops with high resistance ratings in a five-year rotation.”

“It’s certainly not a simple situation,” Dr Owen says. “You can’t get rid of RLN. Their prevalence is so widespread we don’t ever expect to eradicate them, only manage them.

“Choosing varieties that are less susceptible to nematodes,” she says, “is definitely the first step in managing their populations.”

More information:

Dr Kirsty Owen
kirsty.owen@daff.qld.gov.au

Richard Daniel
0428 657 782
richard.daniel@nga.org.au

Growers can access up-to-date wheat varietal resistance and tolerance information on the National Variety Trials website (www.nvtonline.com.au).

Information on resistance to RLN is available on the Queensland DAFF website (www.daff.qld.gov.au).

To organise testing and sending of soil samples, visit the PreDicta B website (www.sardi.sa.gov.au/pestsdiseases/diagnostic_service/predicta_b). For testing and interpretation of results for agronomists who have not been trained in the use of PreDicta B, contact:

Rob Long, Crown Analytical Services
0437 996 678
crownanalytical@bigpond.com

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