The risk of free-living nematode damage in sugar beet crops is increasingly being influenced by changes in climate and farm management practices, which could strengthen the case for using a bionematicide where risk is identified. CPM finds out more.
“Growers can no longer rely on environmental conditions to naturally reduce FLN field populations.” – TOM PRIOR
By Rob Jones and Janine Adamson
Docking disorder – a condition caused by free-living nematode (FLN) feeding damage on the roots of young sugar beet plants – should be a familiar problem to growers on light, sandy soils.
In a bad year, it can cause losses of up to 50% through the characteristic ‘fanging’ of roots, which are fragile and often break off during the mechanical harvesting process.
Unlike beet cyst nematode (BCN) – the other major nematode problem affecting sugar beet – there are no genetic traits which can help crops to tolerate attack and manage populations, so growers are left with few control options.
Currently, the only approved chemical treatment is biorational nematicide NEMGuard DE, a garlic extract-based granule applied in-furrow at drilling, and the case for its use in risky situations continues to build, according to experts.
The symptoms which characterise docking disorder are caused by two key FLN species: stubby-root nematodes (Trichodorus and Paratrichodorus spp.) and/or needle nematodes (Longidorus spp.).
As generalist feeders, they attack and feed on a range of plant species, swimming through moist soil to a suitable plant root before stabbing it with a stylet. The turgor pressure then releases sugars and water for the nematode to feed.
GENERALIST FEEDING
The generalist nature of nematode species can make it difficult to manage populations in the soil because they’ll likely have a food source to feed on consistently throughout the rotation. This is particularly true now cover crops are becoming more common ahead of spring-sown crops like sugar beet, with the recently published British Beet Research Organisation (BBRO) cover crop guide shedding some light on such green bridge risks.
Aside from his role as a BBRO crop protection scientist, Alistair Wright is also experienced in growing sugar beet on his family’s light land farm in North Norfolk, which has a history of FLN.
He says all of the cover crop species listed are also FLN hosts and the only cultural means of reducing risk would be a sterile fallow leading into the sugar beet crop, along with vigorous cultivation before drilling.
However, because cover crops have other agronomic and environmental benefits, such as improving soil structure and capturing nutrients, he believes growers may continue to accept the trade-offs.
“Destruction timing ahead of sugar beet is important and we’d recommend that they’re killed at least six weeks before drilling. Free-living nematodes will still be there, but they’ll be below that level of destruction caused by primary cultivation,” he explains.
In addition to agronomic practices like cover crops influencing nematode risk, climate is a significant factor according to FERA’s senior nematologist, Tom Prior. He says free-living nematode populations have historically been reduced over winter, by extended periods of low soil temperatures and hard frosts, which aren’t conducive to breeding.
With the UK now experiencing warmer and wetter winters than reported 50 years ago, FLN testing data shows that in general, populations of nematodes are exhibiting a lower natural decline during the winter months.
“Growers can no longer rely on environmental conditions to naturally reduce FLN field populations and this could increase the burden of plant parasitic nematodes on young seedlings,” notes Tom.
As cultural interventions for FLN are limited, growers may consider using a nematicide to protect crops from feeding damage in the weeks after drilling. But there’s now only one option – NEMGuard DE – which root crop growers have been steadily building confidence in since carbamate nematicide Vydate (oxamyl) was withdrawn in 2021.
Alistair says studies carried out at the University of East Anglia have proven its mode of action and efficacy against the nematodes which cause docking disorder. When applied in-furrow at drilling, NEMGuard DE granules absorb moisture and release diallyl polysulfides, the active substances present in garlic extract.
These then penetrate the nematode’s skin – a semi-permeable membrane which allows it to breathe and absorb water – leading to a chemical reaction that causes death by oxidative stress.
“It’s very hard to do replicated small plot trials because the nematodes that cause docking disorder are such patchy pests. However, from strip trial work, we’ve seen improvements in root shape and quality, and improvements in yield using NEMGuard.
“This was particularly evident in the high-risk year of 2021, when sugar beet plants were much greener and taller in season and root quality much better in the treated at lifting. I could see the benefits that season,” says Alistair.
Association of Independent Crop Consultants (AICC) member Penny Oakes oversees sugar beet crops on some high-risk land across Norfolk and has been advocating the use of NEMGuard DE in recent seasons.
With the sugar beet price dropping slightly she wanted to revisit the return on investment from NEMGuard DE when conditions are conducive to FLN attack. To do this, she set up on-farm trials for the 2024-25 campaign, filling half of the applicator units across a 12-row drill with the product, creating 12-row treated and untreated strips across a high-risk field.
A wet and mild winter and a raised water table created seemingly ideal conditions for FLN and Penny expected to see differences above ground in the untreated strips.
As the summer remained relatively wet and warm, she says plants established and grew away quickly, and weren’t put under any significant drought stress so little impact of FLN, such as stunted tops, was observed.
However, when doing hand digs ahead of harvest, there were visible differences in root quality between treated and untreated (see photo).
IMPACT OF CONDITIONS
Although this didn’t translate into a significant yield increase in 2024-25, both Penny and Alistair agree that if it had been a dry summer limiting water availability, fanging symptoms and losses would have been much worse.
“Where we’ve historically had a problem, I’ll definitely be recommending NEMGuard again and will keep encouraging growers to do more strip trials,” highlights Penny.
“Growers want to be able to justify its use. I think it would only take one bad year where we have FLN damage followed by a drought and they’ll never consider not using it again.”
She adds that evidence on cover crops potentially increasing risk, presented in recent media coverage, could make treatment a good insurance policy.
While such on-farm trials are important in gaining grower confidence in a product, Alistair says BBRO will be looking at expanding FLN control options for sugar beet growers in its own work during the 2025-26 campaign.
The work includes assessing the impact of endophyte grasses, brown mustard, and vetch and rye cover crops on FLN populations. These cultural measures will then be overlayed with a NEMGuard DE treatment in the following sugar beet crop to look for any complimentary or additive effects, he says.
“We don’t want to rely on NEMGuard completely and the work should give us a good idea of what else works, and what doesn’t,” concludes Alistair.
This article was taken from the latest issue of CPM. Read the article in full here.
For more articles like this, subscribe here.
Sign up for Crop Production Magazine’s FREE e-newsletter here.
