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Growers advised to maximise CTL use early and then switch to other multisites

With the backdrop of resistance issues and withdrawal of chlorothalonil (CTL), growers attending the AHDB/SRUC Agronomy 2020 meeting in Inverurie heard how to maximise disease control this season. Jo Learmonth reports.

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Analysis of SRUC’s Adopt a Crop data from across Scotland last year showed the average spring barley growth stage on May 20 was GS17.

 

This year advanced crops may fall within the timeframe for getting CTL on at T1, said SRUC’s Prof Fiona Burnett.

 


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“In winter barley the average growth stage was GS48 on that date so many crops will fall into the range for T2. In winter wheat, there was quite a spread of growth stages. We might be able to add CTL to flag leaf sprays, but you might not, so be super careful.”

However, it is important growers think about their own scenario when designing programmes, Prof Burnett added.

 

“Many late sown crops are at lower risk of over-wintering disease, so can the T0 be removed? The flag leaf spray is where you want to be using your best chemistry. Work back from that T2 timing and see how you could alternate things at T1.

 

"Make it different from what you plan to use at that T2. You certainly want to be using CTL at the early timings to maximise its use while still within the time frame.

 

“Then move forward and do something different at T3 - mix your azoles, add a different multi-site. Folpet is the likely successor to CTL at T2 but consider mancozeb at T3.”

 

Yield

 

In both winter and spring barley crops later sprays, after T2, do not give a yield benefit, she added. “A T2 application at GS49 gives sufficient protection of the canopy after flowering to ensure grains fill to their storage capacity.

 

“In winter barley the T1 spray is important as it is about retaining tillers and grain sites and setting the grain potential. In spring barley, the T2 is proportionally more important, as this spray timing keeps the quality and yield and attempts to manage ramularia.

 

“With the loss of CTL the control of ramularia is at the forefront of grower’s minds,” she added. “Having prothioconazole in the T2 spray and moving to new chemistry, Revystar XE, in the highest risk scenarios may well be sensible.”

 

Use up dates

 

Distribution for all chlorothalonil containing products ended on November 20 2019, and growers have until May 20 2020 for the disposal, storage and use of existing stocks.

Protecting new products

Do not play fast and loose with new fungicides. This was the message from Prof Burnett when she told growers of the new chemistry coming through.

 

She said: “The launch of BASF’s Revystar XE, a new azole mixed with an SDHI, brings an improvement in septoria protection, compared to other SDHI azole mixtures. All our septoria chemistry is best used at optimal timings; by working protectantly, potentially you can use slightly lower doses. That’s not a new message but its where you want to be with septoria.”

 

With Corteva Agriscience’s new chemical active, Inatreq currently in registration, Prof Burnett added: “It will be good to have a new group of chemistry that lets us genuinely mix and alternate chemistry on crops. It will be restricted to a single use per crop, acknowledging the resistance risk and will have to be mixed with other actives, which is the best way of preserving that chemistry.”

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