As featured in Arable Farming Magazine
Managing wheat disease threat – learning the key lessons of 2021
by Arable Farming Magazine April issue
When it comes to disease control, no one season is the same but there are always lessons that can be learned from experience. Teresa Rush reports.
Keeping one step ahead of disease in cereals is a perennial challenge.
And as last season so clearly demonstrated, a weather- or variety-induced stumble can have unwelcome consequences.
In wheat, septoria is becoming increasingly difficult to control, for several reasons.
And as Ceres Rural partner Jock Willmott observes, once septoria gets into a crop it is very difficult to get it out.
So, what are the considerations? Hitherto, higher yielding wheat varieties have been those which have produced the biggest response to fungicides.
But times are changing, with the arrival of the likes of KWS Extase, Mayflower, KWS Palladium and Theodore.
In NIAB trials last year and averaged over five years from 2017-2021, the mean response to fungicides in 38 varieties was 2.3 tonnes per hectare.
Mr Willmott says: “We invest £60, £80, £90/ha in fungicides to secure – in this case – an average of about 2.3t/ha of crop back.
And at 2.3t/ ha, at £200/t, for example, fungicides are well worth it”.
But varieties such as Extase and Mayflower are breaking the mould.
“They still yield well but they do not get a lot of disease and therefore they are not responsive to fungicides and this is where you have got to be careful.
“With a variety which only responds with on average 1t/ha to a fungicide, you are spending £80-£90/ha to get £200/ha back – you wind your spend in on these.”
Nonetheless, this type of variety is the way forward, adds Mr Willmott.
“As long as these varieties stay consistent and their type, how they grow and their marketability suit your farm, then these are the varieties for the future.
“We are going to be able to farm with less risk and be able to meaningfully cut back on fungicide spend.
As long as these varieties stay true and there are enough of them that we can get three or four into our cropping, they will work well.
“We don’t want to grow varieties that put our business at risk if we can’t get to spray them because it is either too windy or too wet.
“We can control yellow rust easily with £10/ha of fungicide but if we can’t get to it to physically spray it, then that is when it starts to have an effect on the bottom line.”
He points out that Gleam, which he says has been a ‘tremendous’ variety, has a yield rating of just 2% above Extase, which is within the statistical margin of error.
This equates to around 0.2t/ha yield difference, worth £40/ha at £190/t.
“How much of that £40/ha is going to be eroded in extra fungicide costs? This is what we are trying to price up in our mind.
We still pick varieties on the basis of what we can grow well, sell well and are comfortable with storing but, we have got to start looking at varieties which offer this potential of lower response [to fungicides] but high yields.”
The breakdown of resistance to septoria derived from the variety Cougar has seen the resistance ratings of several varieties revised down on the AHDB Recommended List for 2022/23.
And with Cougar genetics present in the make-up of a number of current varieties, the disease threat associated with this source of resistance will remain this season.
Noting that most of the varieties affected (see box) are soft wheats, Mr Willmott says: “If you have got them on-farm this year it will be interesting to see if there is any further degradation of that Cougar resistance.
We need – we must have – gene resistance because we can’t rely on chemistry.”
But septoria is a shapeshifter in more ways than one.
As well as evolving its virulence, it is additionally becoming increasingly resistant to fungicides.
“These things are acting together and can make septoria difficult to control to the point where full rate Proline, for example, gave only 20% disease control in a curative situation [in AHDB fungicide performance trials in 2021],” says Mr Willmott.
And already there is evidence of strains of septoria resistant to the relatively recent SDHI fungicides bixafen, Imtrex (fluxapyroxad) and Solatenol.
“If you go through your crop three times with products that contain these, there could be strains that slip through at each spray timing.
It is not as simple as that but that is what can happen, so part of your strategy to control disease is to use fungicides with different actives.”
Commenting on the latest products to reach the market, Revystar XE (mefentrifluconazole + fluxapyroxad) and Univoq (fenpicoxamid + prothioconazole), he says on
farm observations last season would suggest Univoq was very persistent, while Revystar was quite eradicant, but ran out of persistence.
“Because we were all looking at the weather and thinking last year was quite benign – there was not a lot of disease up until mid-May when it started to rain – we were pretty skinny on rates and Revystar probably wasn’t applied at high enough rates to give it persistency.
“And in a year where we had a really dull June and a dull start to July and things stayed green, it was a season that favoured a fungicide that was really persistent.”
What then might this season bring? “We’ve got good strong crops and big canopies generally, with growth through December, January and February.
If that carries through, we could see a higher amount of disease than we’ve had for the last several years.”
As was the case last season, the weather will inevitably play its part.
“It was unfortunate that as we got to T2 last year we were in the fifth week of protection by the time we got out to spray and the flag leaves did get rain on them before they got their spray and that is where disease came in.
“There was not enough eradicant activity at a litre [of Revystar] and not enough protectant activity, which is why when it got to mid-June we were starting to see a level of septoria in crops that I hadn’t seen for some years.
“The take-home from that is we have got to be vigilant with our spray timings because even with something that was pretty new, pretty eradicant, it wasn’t strong enough to get disease out of the leaf,” says Mr Willmott.
He urges growers to try in season disease testing diagnostics, such as those available from Bayer and Swift Detect, if possible, to determine what level of disease is in the crop before the sprayer goes into the field.
“It is something we have been crying out for.
Last year, right up until T2, there were no warning signs of disease – yellow rust or septoria – and this was driving the idea that it was quite a low disease year, which it was.
“But what we didn’t get quite right was, when it came wet, just how quickly septoria moved on, and the length of time it took crops to finish meant we were starting to see quite a lot of disease.”
He also highlights some changes in fungicide availability due to take place over the next couple of seasons.
“Tebuconazole is still available, but we have probably got only two years left.
I think when we get to the end of this year if you still had some in the shed it is not going to be a bad decision to keep stock on-farm.”
New developments in the pipeline include Adepidyn from Syngenta, which looks to have good activity on septoria, as well as ramularia in barley; isoflucypram from Bayer (available outside the UK as Iblon) and metyltetraprole from BASF (available in other markets as Pavecto), he adds.
Influence of Cougar strain
- First noticed in 2016 – put down as an anomaly
- Varieties with Cougar in their breeding affected but not all varieties affected the same
- Other resistant varieties without Cougar in the parentage were not impacted
- There are hundreds of Cougar isolates – not just one strain – all mixing in the soup of septoria strains
- Cougar isolates not known to be more resistant to fungicides
- No evidence Cougar isolates have a shorter latent period
- Varieties with a Cougar parent: Swallow, KWS Firefly, Merit, RGT Saki, LG Quasar, RGT Bairstow, RGT Rashid.
- Varieties with a Cougar grandparent: LG Illuminate, LG Prince, LG Astronomer, RGT Stokes
Fungicide use spring 2022
- Clear rate response with Revystar – important if late application
- Univoq gave best green leaf retention and yield response in 2021, perhaps more dose flexibility
- NIAB trials suggest it is difficult to get consistent significant responses with folpet but it has a value in higher pressure situations – go twice at full dose of 1.5 litres per hectare
Example of a robust fungicide programme for disease susceptible wheat:
- T0 rust spray, possibly take all/eyespot – 125g of tebuconazole
- T1 – Ascra or Revystar @ 1-1.2 litres/ha +/- Arizona – will cover most bases
- T2 – Univoq @ 1.1-1.2 litres/ha +/- Arizona if wet
- Feed wheat T3 – cheap rust top-up + ear disease
- Milling wheats – more robust and include prothioconazole
Jock Willmott was speaking at a Ceres Rural On the Farm day at Law Farming’s Thrift Farm, Royston, Hertfordshire