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Disease Management

Bayer Crop Science

6 steps to managing the threat of virus in crops

Article overview

Here, with the assistance of Eric Anderson, senior agronomist at Scottish Agronomy, we outline six steps to follow in managing the impact virus can have on crops.


Aphid-borne viruses are a threat to all crops, but the impact is perhaps felt most severely where infection leads to significant loss of income through reduced yield or quality. The extent of any losses often depends on a range of factors but what unites all crops is the limited means of protection.

The peach-potato aphid (Myzus persicae) is the principal virus vector. As a single species it is responsible for most of the virus transmission in potatosugar beet, field vegetables and oilseed rape.

Other aphid species are also important. The willow-carrot aphid (Cavariella aegopodii) is the primary vector of a range of damaging viruses in carrots, parsnips and celery while the bird-cherry aphid (Rhopalsiphum padi) and the grain aphid (Sitobion avenae) are responsible for transmitting Barley yellow dwarf virus (BYDV) in cereals.

In all crops, an integrated approach involving effective cultural measures along with a targeted spray programme that utilises conventional and, where appropriate, biological products to manage populations throughout the season is regarded as the basis of control.

Here, with the assistance of Eric Anderson, senior agronomist at Scottish Agronomy, we outline six steps to follow in managing the impact virus can have on crops.

 

1. Prioritise higher value crops

It will be near impossible to protect all crops across the rotation, so prioritise those of higher value or susceptible cultivars where infection has a greater impact on yield or quality.

“Where possible higher value crops should be isolated and situated away from reservoirs of infection. Cereal crops around the field margins can also be worthwhile, especially with potato crops, as they serve to remove virus particles from the aphid mouth parts,” Eric says.

In crops grown from certified seed such as sugar beet, oilseed rape and field vegetables, growers can be confident that the seed won’t be harbouring virus infection. In potato crops, however, there is no such certainty, so it is often worthwhile using seed of a known quality to minimise the risk.

­­­­­“Minimising the potential for infection should be the first course of action. If this means using high-grade seed of known provenance, then so be it. Likewise, control groundkeepers or other volunteers in near-by crops and avoid cover-crop species such as brassicas that harbour virus and provide a refuge for aphids to over-winter,” says Eric.

 

2. Control reservoirs of infection

Many crops, or in some cases volunteers, serve as a potent source of infection therefore it is often prudent to take them out before virus can be transmitted to a crop of commercial significance.

“Barley is often under-sown in sugar beet to protect soils and it may be beneficial in keeping aphids out of neighbouring potato crops by serving as a distraction, but these benefits need to be balanced with its potential to serve as a reservoir of infection for BYDV in near-by cereal crops,” says Eric.

“Similarly, mustard is a common game cover crop while stubble turnips are popular as a source of winter forage for sheep, but both have the potential to serve as a reservoir of infection for Turnip Yellows Virus (TuYV) in oilseed rape, cabbage and other brassica crops, so should be destroyed before spring crops are established,” says Eric.

 

3. Virus and crop interactions

Some varieties have resistance that can be utilised to good effect while in other crops, such as sugar beet, protection needs to be focussed on the first 12-13 weeks after which time adult plant resistance will develop.

“In all situations the first course of action should be to quantify the impact that virus can have on the crop and the propensity of that crop to suffer infection. From here, those crops and varieties most at risk should be prioritised for protection,” says Eric.

“In such situations, understanding the risk is the basis of a programme but it may be that the options for managing it (the risk) are limited. Where it is judged to be higher than is tolerable, alternative crops should be considered,” adds Eric.

 

4. Use decision support systems

The Rothamsted Insect Survey is a national capabilitythat has been developed over more than 50 years and it supports the valuable Aphid Forecasts produced by AHDB.

While both Rothamsted Insect Survey and the AHDB aphid forecasts provide a useful assessment at the national level, they should not be used as the sole basis for decision making.

“Accurate information is the basis for all good decisions. In much the same way that in-field weather stations inform our understanding of the late blight pressure facing potato crops during the season, placing yellow water traps in and around crops before emergence means sprays can be accurately timed to deliver the greatest protection,” says Eric.

“Once the crop is established, information from aphid bulletins and yellow water traps should be considered alongside the assessed activity seen in crop from regular inspections. The success of any intervention is often dependent on timing, so accurate assessment of the threat will support appropriate scheduling of any sprays,” adds Eric.

 

5. Targeted use of insecticides

Once aphids are flying, it is time to spray crops. The question to be considered is, with what? Roughly 98% of the peach-potato aphid population demonstrates resistance to pyrethroid insecticides (known as ‘knockdown resistance’) and the majority are resistant to pirimicarb (known as ‘MACE’).  MACE resistance is now only of relevance to pulse crops as Aphox (pirimicarb) is now only approved for application in peas and beans.

Other insect pests, notably the grain aphid, diamond-back moth, cabbage whitefly, onion thrips, pea and bean weevil and pollen beetle are similarly resistant to pyrethroid insecticides.

In early 2020 peach-potato aphids resistant to the neonicotinoid class of foliar sprays were found in Belgium. These resistant types were previously thought to be restricted to southern France and north east Spain but have since spread further afield. Should they spread to the UK, it would limit the effectiveness of the only neonicotinoid spray still approved for use in oilseed rape and potato crops, and in sugar beet under an emergency authorisation, InSyst (acetamiprid).

“Teppeki (flonicamid) is still effective against aphids in spring barley and oats, winter wheat, brassica vegetables, oilseed rape, potatoes and sugar beet, but is set to be withdrawn from sale on 30th April 2022. There is a minimum interval of 21 days that must be observed in potatoes which means growers will need to alternate with a different product and it must not be mixed with an adjuvant oil if the crop is to enter the human food chain,” says Eric. This may call in to question the validity of so-called ‘dual-purpose’ crops in the case of seed potatoes, he adds.

In sugar beet the choice of insecticides for the control of peach-potato aphid is even more limited with just one application of Teppeki authorised, however, this can be followed by a spray of InSyst, but Teppeki must always be applied as the first spray,” he adds.

In potato, growers have the option of applying Movento (spirotetramat) up to four times, but not before the end of flowering (where this applies). The use of mineral oils has been found to reduce virus acquisition and transmission when applied early and frequently.

“Mineral oils in a programme including systemic insecticides is now the standard approach, but is worth noting that while systemic insecticides such as Movento are effective against the transmission of Potato Leaf Roll Virus (PLRV) they do not work against Potato Virus Y (PVY) due to the rapid nature by which the virus is transmitted,” adds Eric.

In field vegetables, product options are almost greater than in broadacre crops such as cereals and potatoes, as there are several biological control agents authorised for use.

Up to two applications of Movento are allowed in brassica vegetables, lettuce, carrot, parsnip and swede while four sprays can be made in onions and shallots. InSyst can be applied under an Extension of authorisation for minor use (EAMU) in Brussels sprouts.

Biological insecticides such as Flipper (fatty acids C7-C20) offer a novel means of managing aphids over a long period without fear of product residues or harvest intervals to observe. Under an EAMU Flipper can be applied to outdoor field vegetables up to nine times and eight times to protected field vegetables.

Application timing, water volume and accuracy of placement are, however, essential to performance. Due to their contact-only activity it is advisable to use products such as Flipper in the early part of the programme when open canopies make it easier to hit the colonising aphid.

Commercial trials with three applications seven days apart can be followed with some confidence. As the crop develops and the canopy becomes dense, ensuring contact with the pest becomes more challenging.

At this point it makes sense to use a lifecycle breaker such as Movento to give lasting control. Flipper can then be used again towards the end of the season to help clean up crops ahead of harvest with no pre-harvest intervals or residues to worry about. Under the specific restrictions of the EAMU Flipper applications must only be made between 1 March and 30 August when applying to crops grown outdoors.

 

6. Protect crops through until harvest

Season-long protection can be hard to achieve but doing so requires a level of vigilance that will test any grower.

“All insecticide programmes have their limitations. PVY, for example, can be transmitted (to potato plants) in about a minute so any gaps in a programme will be quickly exposed,” warns Eric.

“Although mature plant resistance can make a useful contribution to the control of PVY°, it does not apply to PVY n which is typically without visual symptoms, so roguers and crop inspectors will not see the virus and thereby enabling it to spread to other crops.

“Although PVY n spreads less readily to neighbouring plants after flowering, it is more likely that infection will be spread to daughter tubers: precisely the opposite effect to mature plant resistance,” he adds.


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