Crop protection - Control strategies

Ongoing restrictions on conventional crop-protection products are leading to a range of alternative approaches, Gavin McEwan finds.

Crop protection: current lack of effective solutions cited as having a significant impact on UK growers - image: USDAGOV
Crop protection: current lack of effective solutions cited as having a significant impact on UK growers - image: USDAGOV

Not since the DDT controversy of the 1970s has pesticide use been so much in the headlines.

The use of neonicotinoid insecticides in particular has been the subject of fierce debate, culminating in the EU-wide ban of three such com­pounds, used as seed treatments, from the start of last month due to their alleged impact on pollinating insects.

Suppliers of the chemicals, Bayer and Syngenta, have begun legal action to overturn the ban — a move backed by the NFU. Meanwhile, a Bayer representative stresses: "Vegetable growers can continue to use Biscaya and fruit growers can continue to use Calypso, both products being formulations of thiacloprid for use as foliar sprays."

But last month the European Food Safety Auth­ority also claimed that the chemicals could harm the developing brains of unborn babies and called for maximum exposure limits to be cut.

The neonicotinoids case is only the most contentious within ongoing moves to limit crop-protection products within the EU. Independent consultant Cathy Knott points out: "The herbicide Ioxynil will not be supported — onion and other growers can use it until August 2017.

Linuron will not be supported either, but no date has been given yet. This affects carrots, parsnip, celeriac, celery, parsley, coriander and others."

She continues: "In the UK, crop-protection companies are now developing co-formulations rather than straights. However, this does not help horticultural crops."

Countering the trend

West Midlands MEP Anthea McIntyre is attempt­ing to counter this trend at policy level. Having published a report on possible strategies to boost EU horticulture, she spoke at a seminar in Brussels last month in support of approvals for so-called minor-use crops.

Citing soft fruit in particular, she says: "The range and complexity of pests and diseases has significant economic impact and means we need effective and sophisticated control measures. Without these, the production of some soft fruit will become uneconomic.

"We very much need a minor-use fund because manufacturers do not support use on these crops, because of the relatively low returns on their investment gathering the necessary data."

On asparagus, meanwhile, she says: "The lack of crop-protection solutions is having a significant impact on all UK growers. The economics of asparagus production in the UK are already difficult due to the very restricted range of chemistry available. Further reductions to the list of permitted active ingredients will jeopardise current British asparagus production and curtail the expansion, which is urgently needed to replace imports from Peru."

The loss of Linuron is a particular worry for asparagus growers, she adds. "It is not commercially viable to grow asparagus as an outdoor crop in the UK without effective residual herbicides."

Meanwhile, a neonicotinoid insecticide is "crucial" for control of the common asparagus beetle, the crop’s most serious pest in the UK, she adds.

Against this background, McIntyre welcomed the launch in November of a Centre for Integrated Pest Manage­ment (CIPM) at Shropshire’s Harper Adams University, saying: "IPM is absolutely the way forward, but we need to base what we do on hard scientific evidence, for which Harper Adams is in the lead."

The CIPM will undertake research in all areas of crop protection, while also providing advice and training, and is seeking research partners in the EU’s Horizon 2020 Framework Programme.

Already the large suppliers of conventional crop-protection products are seeking to capitalise on the growing interest in alternative products, says biopesticide consultant Dr Roma Gwynn. "IPM, including the use of biological control agents such as macro-organisms, micro-organisms, botanicals and semiochemicals, will play an important part in future crop protection, and it is encouraging that demand for and availability of these technologies is increasing rapidly," she maintains.

"There has been lots of movement in the market, with larger companies buying up specialist companies, so those specialist products are now being sold through existing networks. So far it hasn’t changed the actual products on the market, but that process will start to speed up."

She adds: "In 2009 there were around 60 agents, now there are around 100 in the EU, with new ones being added all the time. But the whole thing still takes around five years — it could all be moved faster."

To help growers keep pace with these developments, Gwynn edits The Manual of Biocontrol Agents, published by the British Crop Protection Council.

IPM is not simply a matter of swapping one treatment for another, she stresses. "You aren’t trying to kill them all, but to reduce them to below the damage threshold. To be called a ‘control’ of a pathogen, it has to reduce numbers by 80 per cent, and several are on or around that line. But often the efficacy is about 50-60 per cent, so it’s not something you just use once. You need to look carefully at the label."

Meanwhile, the Horticultural Development Company-backed SCEPTRE project, which aims to trail and further the approval of new conventional and biological controls specifically for horticultural crops, "has thrown up a number of things that are looking promising", she adds.

During this final year of the project, SCEPTRE will concentrate on building up a trials dossier for the most promising candidate products, ensuring that growers will have some alternatives to turn to in the years ahead.

Carrots and onions: biological and cultural controls debated

The use of biological and cultural controls of common diseases was discussed at last the latest UK Onion & Carrot Conference in Peterborough
(20-21 November 2013).

Warwick Crop Centre research leader Dr John Clarkson explained that even identifying the cause or causes of cavity spot in carrots remains a challenge. "There is a whole complex of fungi attacking carrot roots, leading to varied symptoms," he said.

The main culprit, Pythium violae, can only be detected in soil tests once the crop is growing, he pointed out. "The preceding crop has some bearing on the incidence of cavity spot, but we don’t fully understand the mechanism."

Biofumigation — incorporating plants such as mustard that contain natural fungal deterrents into the soil — "has shown preliminary success" in cavity spot control, though not of Pythium itself, he added. But he warned: "No one biofumigant will be the answer as they produce different ITCs [isothiocyanates]. You have to watch that you don’t stimulate pathogen production."

The technique also has "potential" in controlling Sclerotinia in carrots, said Clarkson. "We have a prediction model that we think is roughly correct. It could tell you if germination is occurring early, so you could spray early. Clipping the foliage and removing senescent material seems to work quite well as you open the canopy and prevent bed-to-bed spread."

Plant-derived compounds can also help carrot growers control nematodes, according to Geert Jan van Roessel, senior adviser at Dutch research and advice centre DLV Plant. "Green beans and Tagetes work against Meloidogyne chitwoodi and M. fallax," he said. "T. patula is also the most effective control of Pratylenchus penetrans, though it can greatly increase trichodorid nematodes."

ECOspray’s NEMguard, which controls nematodes with compounds derived from garlic, already has 22 per cent of the UK carrot and parsnip nematicide market. "There are not many on the market and some of those have known toxicological side-effects, whereas this is a food-grade product with no harvest interval or minimum residue level," according to technical director Dr Murree Groom.

The company is also conducting field trials to test the product’s efficacy against potato cyst nematodes.

On onions, Washington State University plant pathologist Lindsey du Toit outlined measures to control neck rot caused by various species of Botrytis, advising a combination of cultural and chemical measures tied to times of peak infection risk.

"Infection requires a combination of dead or injured plant tissue, including naturally senescing foliage, and moisture, though high nitrogen levels also contribute," she pointed out. She suggested field rotation and careful disposal of volunteers and culled crops to help prevent buildup of inoculum, while also spraying regularly during field curing, especially in cool, wet periods, with a range of fungicides to avoid resistance buildup.

Groom also recommended treating seed with fungicide, while acknowledging that the relative importance of this remains contentious.

A recently completed Horticultural Development Company-funded study of seed treatments to prevent Botrytis infection in onions concluded that the conventional HY-TL liquid treatment is sufficient to reduce infection levels to below one per cent, according to lead researcher Dr Charles Lane.

His trials also found that the naturally derived fungicide Fludioxonil achieved comparable reductions, along with two experimental products, though the conventional Topsin M (thiophanate-methyl) proved to be less effective.

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