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Natural chemistry no longer regarded as a softer option by carrot growers

As the squeeze on pesticides continues, and increases, as a problem for all crop sectors, perhaps the combinable sector can learn something from the integrated pest management techniques being developed by speciality fresh produce growers.

As the squeeze on pesticides continues, and increases, as a problem for all crop sectors, perhaps the combinable sector can learn something from the integrated pest management techniques being developed by speciality fresh produce growers.

Yorkshire grower and NFU chairman of the horticultural and potato board, Guy Poskitt, believes that while pesticides are a necessity in modern agriculture, “We can’t just ignore what our customers want, and that’s less pesticide in their food.

“The world has a growing population and the consumer demands continuity of supply and value. The bottom line is that we can’t feed the world organically,” he says. “We have to remember that retailer demands are the customer’s demands and they want more food safety and lower residues in the crop. It’s a difficult balance for growers to achieve.”

Growers of fresh produce need to produce a quality product at a reasonable cost to the consumer. They’re at the sharp end of crop production with an ever increasing demand for cheap food and an ever decreasing number of pesticides available to help achieve this.

As a result, many growers of speciality crops are leading innovation and helping develop the use of products in the mainstream which at one time would have been considered a softer ‘alternative’ option, left to the realms of organic farming. 

Guy Poskitt has been developing the use of NEMguard on his farm for the last ten years. Now marketed by Certis, NEMguard is an approved nematicide which has a natural substance, garlic extract, as its active ingredient.

As a grower of 1,400 acres of carrots and parsnips, Mr Poskitt supplies fresh produce to retailers for 48 weeks of the year. Carrots were recently identified as a crop where production is in danger of becoming unsustainable due to the impact of pesticide loss on food production, in a report commissioned by the NFU, AIC and CPA and produced by independent farm business consultants Andersons. 

“Since we’re losing more and more pesticides, we’re looking at other ways of attacking our problems,” explains Mr Poskitt. “NEMguard seems to work as a nematicide at a similar level to Vydate (oxamyl) for nematode control, but has the major bonus of not having a MRL or harvest interval which means we can use NEMguard on early crops without a worry.”

Since 2001 UK farmers have lost more than half of the active pesticide substances approved for use in the EU. The Andersons report suggests that 87 of the remaining 250 pesticides currently approved for use in the UK are threatened by a combination of EU directives.

“We need to have access to a range of products as part of our crop protection toolbox, and any pesticide development, including the introduction of natural products, is welcome and possibly where the future lies,” stresses Mr Poskitt.  An attitude echoed by supermarket Tesco, who have embraced NEMguard in their Nature’s Choice grower protocol for carrot and parsnip growers.

Howard Hinds, Root Crop Consultancy Ltd., a specialist in carrot, parsnip and potato agronomy, explains that free-living nematodes are one of the major problems facing carrot and parsnip growers because the fanging and forking they cause makes affected roots unmarketable. 

Reliably identifying the fields where nematodes are likely to cause damage is a big problem, explains Mr Hinds. “Nematodes tend to be clustered in areas within fields, so accurate sampling is unlikely and it is very difficult to predict probable plant damage from sampling results.

“As a result fields are treated with a precautionary nematicide which, in spite of being the second most costly input, is better than risking damage at levels of 30% in untreated fields,” he believes. He adds that an additional benefit of nematicide application is improved crop establishment, even where nematodes are present at low levels. 

Howard Hinds has been involved with the use of NEMguard commercially for the last two years. “NEMguard is recommended to be applied at 20kg/ha and if no rain occurs after drilling, 20mm of irrigation is needed and soil moisture then maintained for the first 4 to 6 weeks of growth to ensure maximum nematicidal activity,” he explains.

A new granular formulation is now approved which has a carrier system based on diatomaceous earth (DE), which further improves the product’s release characteristics and ease of application, he adds.

The science behind garlic

Garlic is well known for its culinary and medicinal uses but how does it work as a crop protection product? 

Dr Chris Hamilton reads Medicinal Chemistry at the University of East Anglia and is at the forefront of research in sulfur chemistry and biochemistry. He explains how garlic extract (the active substance in NEMguard) work to control nematode populations.

“Garlic contains large quantities of a sulfur containing substance called Alliin. When garlic is crushed an enzyme is released which converts the Alliin to Allicin,” explains Dr Hamilton. “If the garlic is then heated (by steam distillation) a garlic extract is obtained where the allicin has been converted to a complex cocktail of molecules called diallyl polysulfides with water.”

This unusual collection of polysulfide molecules contain different length sulfur chains and are one of the few molecules found in nature that contain three or more organic sulfurs in a row within a single molecule. Their biological activity directly correlates with their number of sulfur atoms.” explains Dr Hamilton. “For example, we have shown how the pentasulfides (with 5 sulfurs in a row) are a thousand times more reactive than the trisulfides (with just 3 sulfurs).”

So how does the garlic extract in NEMguard control nematodes? The answer is in several ways and at multiple sites within the organism. This is very different to the way other nematicides work (single site activity) and means that it is virtually impossible for nematodes to develop resistance to the multiple modes of action of the polysulfide cocktail which makes up NEMguard, comments Dr Hamilton.

“A key way these garlic-derived polysulfides work is by creating oxidative stress in the target nematodes by reacting with and reducing the levels of important cellular thiols such as glutathione” explains Dr Hamilton. “These thiol biomolecules play an important role as antioxidants in the nematode’s biochemistry so, when they are depleted, cellular damage occurs.”

A second mode of action involves the polysulfides’ ability to react with and modify many of the nematode’s proteins to disrupt their cellular metabolic functions. 

 “Unfortunately for the nematodes, the polysulfides in the garlic extract are very fat soluble. This means they are also able to embed and accumulate in the waxy cuticle of the nematode, which may further exacerbate their effects” comments Dr Hamilton. 

The mode of action of NEMguard is well researched and its efficacy is backed by rigorous scientific enquiry, adds Robert Lidstone, Certis’ marketing and business development manager.

“In achieving the official EU pesticide approval, the nematicidal activity of NEMguard has had to be demonstrated to CRD. There are also extensive quality controls in place to make sure NEMguard contains the ‘right’ mix of polysulfides to maintain its efficacy and ensure a long-term release.” 

The unseen enemy

Dr Colin Fleming, principal scientific officer at the Agri-Food and Biosciences Institute in Belfast, warns that climate change is affecting the dynamics of nematode populations and that there is a ‘new nematode on the block’ growers need to be aware of.

“Nematodes are plant parasites and generally attack the root systems of plants,” says Dr Fleming. “Growers will be familiar with the free-living needle (Longidorus spp.) and stubby-root nematodes (Trichodorus spp.). These are ectoparasites and feed on the root surface.”

The most damaging nematodes are endoparasites that feed within the plant root system. Globally a different species of nematode – the root knot nematode (Meloidogyne spp.) – is the most significant pest of a wide range of crops and its numbers are thought to be rising drastically in the UK and Ireland.

“No extensive surveys of nematodes has been carried out in the UK since the 1970’s but a recent survey in Northern Ireland has indicated that numbers of root knot nematodes has increased by 300% in the last 30 years,” comments Dr Fleming. 

“We believe root knot nematode (Meloidogyne minor) could become the number one problem in the UK over the next decade in a broad range of crops. Lesion nematodes (Pratylenchus) and potato cyst nematode (Globodera) are also increasing.”

So why are the populations of nematodes increasing? Several factors are probably significant, believes Dr Fleming.

“We are seeing warmer, wetter weather patterns and these favour nematodes and are encouraging species that prefer warmth, like the root knot, to thrive,” he says. “The reduction in the number of active substances available for use in is also having an effect on populations.”