Specific recommendations support innovation in the Recommended Lists (RL)

Friday, 6 December 2024

Sacha White provides an overview of Barley yellow dwarf virus (BYDV) tolerance and resistance in cereal varieties, which can add value to disease management strategies.

We have adapted the processes that underpin the RL in recent years.

This allows us, for example, to list varieties even when headline yields do not appear to merit it. We now consider a wider range of characteristics, and recommend varieties that may provide a consistent economic benefit in specific situations.

One important innovation is resistance or tolerance to BYDV in cereals, which we added to the RL as a specific recommendation a few years ago.

Earlier in December, we released the latest iteration of the RL online (RL 2025/26), which saw a big influx of varieties with BYDV resistance or tolerance: one new winter wheat variety and five new winter barley varieties.

BYDV genetics

The use of genetics to battle BYDV has become more appealing, especially since the loss of seed treatment options for autumn aphid control, concerns about resistance to pyrethroid sprays in aphids, and the impact of these sprays on beneficials and the wider environment.

In the RL tables, varieties with BYDV resistance or tolerance are indicated as a specialist (Sp) trait in the ‘Scope of recommendation’ line.

The characteristic is presented as a breeders’ claim and, as such, is not verified in RL trials. Typically, such claims are associated with clear genetic markers that breeders use to verify whether a variety has inherited the trait.

Although we indicate whether a variety possesses resistance or tolerance to BYDV, it does not influence the recommendation process. In fact, our trial operators are not required to make routine assessments of BYDV (although they may note the presence of symptoms).

Resistance v tolerance

Resistant variety: Typically shows no or few symptoms (because it prevents virus replication), with no significant yield loss.

Tolerant variety: May show substantial symptoms, but it will not suffer significant yield loss as a result.

Although unlikely to be grown widely, resistant or tolerant varieties are likely to appeal to farmers growing cereals in high-risk BYDV situations or to those taking a more regenerative approach to farming.

BYDV management in RL trials

This growing season, we are bringing the RL processes to life through the lens of a single winter wheat harvest 2025 fungicide treated trial in Norfolk.

In this trial, you will find the newly recommended UKFM Group 2 winter wheat variety RGT Goldfinch, with its specific recommendation for BYDV resistance, alongside 49 other winter wheat varieties.

As per the fungicide programme, we follow an insecticide protocol to minimise the impact of disease. In practice, this means targeting the main BYDV aphid vectors (English grain aphid and bird cherry-oat aphid).

Because each plot is treated equally, the potential benefit of BYDV resistance or tolerance is not often seen in RL trial yields (which is why we use breeders’ claims).

The Norfolk trial was sown on 7 October 2024. This is neither particularly early nor late, so the sowing-date risk can be described as moderate for BYDV. An earlier sow date would have increased the risk, because it provides more opportunities for aphids to arrive, develop and spread.

At the site, potatoes were grown as the previous crop and the field was cultivated before drilling, so we were not too concerned about a green bridge (living plant material) giving aphids a head start.

The AHDB BYDV management (T-Sum) tool uses sow-date and spray-date information and accumulated daily air temperatures to calculate when the second wingless generation of aphids is likely to be present. As this generation tends to move away from the plant originally colonised, it is the recommended spray target.

The winter wheat on trial site received a pyrethroid spray at the full label rate at the beginning of November (consistent with the alert in the BYDV management tool).

Although some English grain aphids are resistant to pyrethroids in the UK it is only partial resistance, which means a well-targeted spray at the full label rate should control them.

As temperatures cool during the autumn and winter, aphid flights reduce. However, activity tends to continue in crops, as long as temperatures are above 3ºC (before any significant frosts). This is why our trial operators regularly monitor the plots for pest activity to judge the need for any follow-up sprays.

Aphid monitoring

This autumn, the number of BYDV vectors has been relatively low (much lower than the 10-year averages), according to aphid counts from local suction traps (operated by Rothamsted Research).

Interestingly, monitoring on a subset of these traps shows that most aphids do not carry BYDV. For example, less than a fifth of aphids tested positive for BYDV in the closest suction trap to our winter wheat trial site (Broom’s Barn in Suffolk) this autumn. This result is consistent with data from recent autumns.

Autumn 2024 BYDV/CYDV monitoring overview

Overall, 1,048 aphids were tested in autumn 2024, with about a fifth of aphids testing positive for virus
The final proportions with virus (both aphids combined) at the specific suction trap sites were: York (24%), Hereford (23%), Starcross (18%) and Broom's Barn (17%)
As usual, far more bird cherry-oat aphids were trapped and tested (960), with 20% testing positive for virus (ranging from 24% to 16%)
Relatively few grain aphids were trapped and tested (88), with 26% testing positive for virus (ranging from 38% to 15%)
Most of the grain aphids were tested during the early autumn
BYDV variants were far more commonly detected (82%) than the CYDV (18%) variant

We are working on an enhanced BYDV tool that accounts for aphid numbers and the proportion carrying virus, in addition to several other parameters, to improve the accuracy of spray decisions. We hope to publish the new tool towards the end of 2025.