Minimising residue

Industry trials on providing effective pest control while leaving minimal pesticide residue are yielding positive results, Gavin McEwan finds.

Steve Humphreys
Steve Humphreys

The need to minimise residues of crop-protection products on fresh produce remains pressing. With this in mind, Bayer CropScience conducted extensive crop trials under its Minimizer project - the results of which it has just released - to test for traces of its crop-protection products at harvest.

Last season, the project focused on the fungal pests Sclerotinia and Alternaria in carrots, looking at six crops sown from March to June by growers in Suffolk, Cambridgeshire, Nottinghamshire and Yorkshire. In each case, growers devised their own programme of six-to-eight sprays from June to October, including one or two full-rate sprays of Rudis (prothioconazole) and two or three of Nativo 75WG (tebuconazole + trifloxystrobin).

In September and October, harvested samples were analysed for residues of the three chemicals, which have a Limit of Determination (LoD) of 0.01mg/kg. No residues were found in three of the six crops, while levels in the other three were negligible - the two prothioconazole detects of 0.01mg/kg were at the LoD, a 10th of the Maximum Residue Level (MRL), and the tebuconazole residue of 0.03mg/kg was just a 16th of its MRL. Bayer food industry manager Dr Stephen Humphreys describes this as "a great result".

Rodger Hobson, managing director of Hobson Farming, one of the companies involved in the trials, says: "We aim to avoid any detects and never more than 10 per cent of the MRL, but it's realistic to get very low levels following late treatments. These results give confidence that our programme ticks the technical box for residue minimisation and the agronomy box for effectiveness."

Positive results

Hobson's agronomist Howard Hinds notes that the results were positive despite high Sclerotinia pressure during last year's very wet conditions. "This may have been down to the stronger programme including two Nativo and two Rudis sprays. Since we started using Rudis two years ago, it's proved to be a strong Sclerotinia control product. We haven't had crown rot in carrots or parsnips in storage."

A similar Minimizer study on brassicas by the Allium & Brassica Centre (ABC) for Bayer looked at harvest-time residues following two applications of its aphid and whitefly control Movento (spirotetramat). In 2011, at the trials site in Bedfordshire, mid-season crops of cabbages and Brussels sprouts were treated on 9 August and 7 September and late-season crops on 7 September and 12 October. When mid-season cabbages were harvested on 11 October, the average residue level was 0.05mg/kg. For late-season cabbages harvested on 9 November, it was 0.06mg/kg.

These levels, recorded around a month after the second treatments, were at or very close to spirotetramat's LoD of 0.05mg/kg and a fraction of its good agricultural practice MRL of 2mg/kg. When the mid-season sprouts were harvested on 11 October, and late-season sprouts on 28 November, residue levels averaged 0.06mg/kg - again, very close to the LoD and well short of the MRL of 0.3mg/kg.

ABC agronomist Carl Sharp says the work has gone a long way in supporting brassica growers to minimise residues, adding: "Communicating the results to packers and supermarkets also demonstrates that growers are committed to producing what consumers want." Meanwhile, Humphreys argues that the Minimizer project makes a nil- residues policy unnecessary. "Residue minimisation is the realistic goal, which growers can achieve while pursuing sustainable intensification," he says.

Shallot defence

Bayer has also secured approval for the use of the fungicide Unicur (fluoxastrobin + prothioconazole) on shallots. ABC managing director Andy Richardson says: "Due to their much greater planting density than onions, shallots tend to face greater disease risk. Unicur is one of the few fungicides that controls all of the three main diseases of shallots - downy mildew, leaf spot and leaf blotch." The wet conditions last year made downy mildew control particularly challenging, he adds.

The crop-protection company has also published a guide to aphids to help growers and agronomists easily identify the main aphid pests of arable, field vegetable and fruit crops. Written by Mark Taylor of the Rothamsted Insect Survey, it profiles 21 species, each with illustrations of the aphids' distinguishing features, as well as an explanation of their life cycles and activity patterns.

Aphid numbers have increased in potatoes during the recent hot, dry weather - and last month, winged Myzus persicae were identified in a crop of Maris Piper in south Lincolnshire. Syngenta potato technical manager Stephen Williams says the species remains the biggest threat for transmitting potato viruses and presents the greatest control challenges, but he points out that the company's Plenum product provides effective control even of resistant aphid strains.

"Myzus colonies in particular are often present on the underside of lower leaves, which can be difficult to find," he says. "This poses a real challenge for application to target aphids hidden deep in the crop. Angled sprays, using the Syngenta Potato Nozzle, will help to get spray through the canopy." Williams adds that these sprays also improve application of blight controls Revus and Shirlan to lower leaves and stems. Growers can keep track of the distribution of aphids and other pests via the Horticultural Development Company pest bulletin, available for free from Syngenta's website.

Mildew control

Meanwhile, Certis has launched a fungicide to control powdery mildew in cucumber, courgette and other cucurbits. Takumi SC contains active ingredient cyflufenamid, has a novel mode of action and is approved for use on both indoor and outdoor crops.

The company's technical officer, Alan Horgan, says: "There have been very few options to control powdery mildew over the past couple of years, but grower trials of Takumi SC last autumn have shown it has good efficacy."

He adds that to get the best out of Takumi SC, growers should treat young crops early at the first sign of disease. "As crops become fully established and reach the eaves, the maximum level water volume (1,500litres/ha) may not be adequate to give full canopy coverage," he warns, but adds that in such cases higher water volumes are permitted.

Using bumblebees to deliver bio-fungicide

Scientists at crop research consultancy ADAS and East Malling Research (EMR) have come up with a novel way of using bumblebees to deliver minute quantities of bio-fungicide to strawberry flowers.

Flowers can become infected with grey mould (Botrytis cinerea), which can manifest itself only later as fungal growth on fruit as they ripen and after picking.

As part of an ongoing strawberry HortLINK project funded by Defra, the Horticultural Development Company and the industry, ADAS and EMR developed a technique by which bees pass through a specially designed dispenser in the hive - picking up a minute quantity of a powdered bio-fungicide, containing the biological control fungus Gliocladium catenulatum, as they go.

The bees then transfer the powder directly to flowers during pollination, allowing a very precise application throughout flowering. The Gliocladium then outcompetes the Botrytis on the flower.

ADAS project manager Harriet Roberts says in trials the technique has "regularly achieved levels of grey-mould control equivalent to areas treated with fungicides, while significantly decreasing residue left on the fruit".

This data, and data from ongoing trials in Finland and Belgium where the technique is being adopted, will be used to support an application for UK registration of the fungicide and its unusual mode of delivery.

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