Look after leaf layers to build final yield
- Target different leaf layers
- Crucial to identify which leaf layer is emerging
- Robust rates will pay for normal and backward crops
This season’s T2 fungicide application will see the added challenge of tailoring different strategies for normal and very backward crops, but experts agree that effective disease control will be critical for both.
And the key to successful control is to target individual leaf layers, maintain tight spray intervals and employ multiple chemistries.
“In later-drilled crops, leaf layers are going to be worth a different proportion of the final yield and need to be targeted accordingly,” says Agrii’s Chris Bean.
Historic ADAS work shows that the contribution to yield from the flag leaf can drop from 50% to 35-38% on a later-drilled crop, with leaf two and three picking up this yield input.
With thin crops being less tolerant of disease and having a need to maintain green leaves right down the canopy, robust attention at T2 will be required. “So growers should not be tempted to cut back in terms of rates and inputs,” says Mr Bean.
“The more normal September-drilled wheat crops, which are relatively high risk in terms of septoria, will certainly pay back the investment and also warrant significant looking after,” he says.
Workload is likely to affect the ability to get through the crops and high-risk varieties such as KWS Santiago and Oakley need to be treated before more resistant varieties such as Relay and Cougar.
Multiple chemistries
SDHI chemistry will play an important role at T2 in both late and early drilled crops, explains Mr Bean. “The important point to remember is that SDHIs aid triazoles in terms of septoria activity, but you have to get your triazole right.”
If growers are opting for a pre-mix, they must go for the correct dose. That would be around 1-1.25 litres/ha Aviator, 1.2 litres/ha Boogie or 1.2 litres/ha Adexar. Another option would be Seguris, but it has been the least impressive of the main three actives, he adds.
With a tank mix, Mr Bean urges growers to get the right dose of each partner and the correct partner. The major SDHI tank mix product would be Imtrex with a triazole mixture such as Brutus.
“The benefit of a triazole mix is that the septoria population is made up of different subspecies susceptible to different triazoles, and a mix of triazoles has shown to give consistently better results compared with single products.”
It is also becoming much more obvious that the better results are coming out of the higher rates, he adds.
Including a strobilurin at T2 is also a consideration, especially where crops are poorly rooted. In addition to maintaining green leaf and yield benefit, protein increases of up to 1% are achievable with milling varieties, says Mr Bean.
He also reminds growers to consider yellow and brown rust. “While SDHIs do a reasonable job on yellow rust, in the past couple of years where we have used an SDHI at T2 in a rust-sensitive variety, we’ve had rust up on the glumes within a couple of weeks. But where we used a strobilurin, we haven’t.”
Pyraclostrobin, which continues to give 35-40% control of septoria, will be his choice of product this season, and when combined with fenpropimrph in Jenton, it also delivers increased rust and mildew control. Mildew is a consideration this season with poorer rooted crops, which could suffer moisture stress later in the season.
Yield potential
While growers may perceive there is a lower yield potential in some crops, NIAB TAG’s Bill Clark reminds growers that although potential ear numbers are down because of less tillering, yield is determined after flowering, with up to 0.25t/ha a day produced in bright sunny conditions.
“Thin crops are less tolerant of disease, and loss of green leaf on a thin crop is more damaging because the crop can’t compensate. So there is an argument for being very robust with your fungicide programmes on thin crops because you need every scrap of green area you’ve got.”
Work in the late 1990s showed how thin crops responded better to strobilurin use through maintaining green leaf in the lower leaves. And this can be extrapolated to the SHDIs which deliver physiological greening, albeit in a different manner to the strobilurins, he adds.
Timing it right
As the length of eradicant “kickback” activity of triazoles declines, the T2 timing, which only came about because of this class of chemistry, becomes a compromise between control on the flag and leaf two, says Mr Clark.
Traditional flag leaf timing is GS39, when the flag leaf is fully emerged on the main stem. But last season where the interval between T1 and T2 approached four weeks, leaf two was exposed for a long time as cool conditions slowed growth.
“The danger is that if you are delayed with the flag leaf spray, there comes a point where you can’t overcome it with increased dose. Even full dose epoxiconazole or prothioconazole is unable to eradicate disease from leaf two and in that situation a pre-flag leaf spray is needed.”
Crop structure this year is very different and it is unlikely that the same situation will happen, unless we experience a cool period in April that slows growth again, explains Mr Clark.
“But what might happen is because late-drilled crops are likely to produce fewer leaves, it is possible that leaf three and leaf two will emerge earlier in terms of nodal growth stages than normal.”
Flag leaf emergence, dictated by day length, will be around 20 May depending upon the region. Therefore, growers must identify exactly which leaf layer is emerging when planning applications, so intervals don’t become stretched.
Before applying a T2, growers will also need to know if the T1 spray worked, by looking for septoria on leaf three and four before the flag leaf emerges at GS37.
“The presence of septoria tells you that the T1 was not good enough, either timing, dose or product choice was wrong and that should ring alarm bells. Seeing the disease on leaf three, usually on the tip, indicates that a spray should be put on early at GS33-GS37, so when flag leaves are not fully out.”
Should I opt for a T1.5?
A T1.5 application will be justified if there is more than 10 days between leaf two and the flag leaf emerging, says Mr Clark.
This T1.5 application would be a triazole/Bravo mix, with half dose probably sufficient, he adds.
“In these situations chlorothalonil becomes very useful as it will redistribute on the flag leaf as it is emerging, whereas if the flag leaf is fully out there is less reason to include it.
If a T1.5 is applied, then the T2 can be delayed to booting or ears out, which, depending upon the season, might enable combining the T2 and ear spray,” he concludes.
Actives
- Adexar – fluxapyroxad + epoxiconazole
- Aviator- bixafen + prothioconazole
- Boogie – bixafen + prothioconazole + spiroxamine
- Bravo – chlorothalonil
- Brutus – epoxiconazole + metconazole
- Imtrex – fluxapyroxad
- Jenton – pyraclostrobin + epoxiconazole
- Kestrel – prothioconazole + tebuconazole
- Prosaro – prothioconazole + tebuconazole
- Seguris – epoxiconazole + isopyrazam
