Pests, weeds and diseases are frequently in the farming news but last year there were several developments reported, with a particular focus on specific varieties and their genetic resistance to disease.

These included the launch of the Yellow Rust Watch List; the breakdown in resistance to septoria of winter wheat varieties with the variety Cougar in their parentage; the arrival on the AHDB Recommended List for 2021/22 of a winter wheat variety with barley yellow dwarf virus (BYDV) resistance and the addition last November to the list for 2022/23 of a winter barley variety with BYDV tolerance.

Last winter saw the low-key launch of the AHDB wheat Yellow Rust Watch List.

The list is based on RL data and aims to highlight varieties which may be vulnerable to falls in resistance in the coming season.

RL senior manager Dr Paul Gosling says: The ratings tell you what happened last season, this is an attempt to try and tell you what is going to happen in the coming season.

Resistance

We know there is an issue with yellow rust in wheat.

We know the yellow rust population is very diverse and that causes a problem; it means the genetic resistance the breeders incorporate into their varieties can be overcome quickly and, for the Recommended List, keeping resistance ratings up with what is going on can be difficult.

We know in past years our ratings havent reflected what is happening in the field, which isnt very helpful.

The premise of the Watch List is that by looking at disease levels in the three worst trials for each variety relative to the performance of the other varieties, a ‘rating can be produced based on these three trials.

Results from the three worst trials are used to produce a new rating, which is then used to produce a scale of the ‘most resistant varieties those resistant to yellow rust in all trials to ‘least resistant ones, those which are highly susceptible to yellow rust in at least some trials.

There are still those varieties that are susceptible to yellow rust everywhere but there are also some varieties that are generally pretty resistant but starting to look a little bit susceptible in some areas and some trials, says Dr Gosling.

One year on, the view is the Watch List would seem to be a ‘reasonable way of predicting varieties that may be vulnerable to seeing their ratings fall.

Although that conclusion comes with a caveat, in that there is only one years data available so far.

A comparison of changes in variety yellow rust ratings between the 2021/22 and 2022/23 Recommended Lists reveals that in most varieties classified as ‘most resistant, ratings were stable or fell slightly.

The largest falls in ratings were seen in those varieties classified as least resistant, but not all those classified as least resistant saw falls, adds Dr Gosling.

Septoria

Summer 2021 brought evidence that winter wheat varieties with the variety Cougar in their background were suffering from higher levels of septoria than expected from their resistance ratings.

That there are septoria isolates virulent on Cougar around in the environment is nothing new; they were previously collected in the UK in 2016-17, when Cougar broke down to septoria and in Ireland in 2020, when some current varieties started to break down.

Dr Gosling says: We know these isolates are widespread and genetically diverse; it is not a single isolate that has evolved resistance and spread.

We also know these Cougar isolates do not appear to be very common in Scotland.

In Scotland, the resistance profile of the Cougar isolates is similar to the general septoria population; in trials in 2021, no breakdown was seen in key varieties.

In most of the trials where we did see it, it certainly wasnt to the extent we saw the breakdown in the South, says Dr Gosling.

One piece of good news is the isolates virulent on these Cougarrelated varieties have a similar resistance profile in terms of fungicides, so when you treat them they will behave just as you would expect septoria to behave in terms of fungicides.

What does this all mean for the coming season? First, not all Cougar-derived varieties are equally affected by the fall in ratings.

Some varieties have seen their new ratings on the RL fall by up to two points, while other varieties have seen their ratings hardly fall at all.

Dr Gosling says: We think that disease this year will most likely follow the one-year ratings we published in the new Recommended List, rather than the three-year ratings.

It is not clear whether these new isolates will become widespread in Scotland; it may be that they simply do not like cold, wet conditions.

Variety ratings have not crashed because most varieties have multiple sources of resistance, he adds.

Breeders have done a very good job of not relying on single major genes and are incorporating lots of minor genes, which individually dont do much, but do add to give good background resistance.

Dr Gosling confirms AHDB will raise an alert if further significant shifts in the susceptibility of Cougar-derived varieties in Recommended List trials are seen during the 2022 growing season.

Unfortunately, last year because the septoria epidemic was very late, by the time we realised something was going on, most people had put their T2s on, he says.

After a mild autumn and winter, wheat and barley crops are expected to come under significant BYDV pressure this season, with symptoms appearing in the coming weeks.

With the arrival of RGT Wolverine on the winter wheat Recommended List and KWS Feeris on the winter barley list, growers now have access to genetics to combat the disease, but what difference will these varieties make to performance in the field?

Genetics

Control of BYDV through genetics is becoming more appealing following the loss of seed treatment options for autumn aphid control, particularly to those concerned about resistance in aphids to pyrethroid sprays and especially in high-risk BYDV situations.

Dr Gosling says: We know pyrethroids are cheap and, generally speaking, we dont have much of a resistance issue in cereal aphids, so is it a useful trait or is it just a gimmick? By chance, two RL barley variety trials conducted in Dorset during 2020/2021 provided an insight into the influence of crop genetics.

Not by design, the trials ended up being only 1.5km apart something which is normally avoided where possible.

They were drilled 10 days apart on September 15 and 25.

The earlier-drilled trial received a pyrethroid (lambda-cyhalothrin) on November 11, the other did not.

The trial drilled on September 25 developed BYDV symptoms in spring but as of April 26, 2021, there was little or no BYDV in KWS Feeris and a second variety claimed to have BYDV tolerance.

Symptoms-wise, it looked like the tolerance was doing something useful, says Dr Gosling.

And harvest results showed the BYDV tolerance in the two varieties gave yield increases of 23% and 21% (see panel, above).

Clearly, these results will not be replicated everywhere.

But certainly, where BYDV is a threat, it does look as though this trait is useful, adds Dr Gosling.