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Independent.ie

Thursday 8 December 2016

Take steps to minimise the rise of septoria resistance

Published 20/04/2016 | 02:30

New chemicals to tackle septoria are a long way off so timing of application is vital in getting the most from existing fungicides.
New chemicals to tackle septoria are a long way off so timing of application is vital in getting the most from existing fungicides.

Most wheat growers are coming off the back of record farm yields and can rightly pat themselves on the back for this achievement. It's obvious weather had a role but without the correct agronomy, high yields would not have been achieved.

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However, during the 2015 season, septoria populations changed.

The protocols of septoria control are well established and are unlikely to change this year but are we on the cusp of more significant changes?

Nobody can say whether farmer actions this year will have an ultimate influence how fast septoria populations will change but collectively we must try to minimise changes.

Where we are now?

Going back a little in history septoria has been controlled predominantly by triazoles (Tilt, Alto, Opus, Proline, etc) and with the help of chlorothalonil (Bravo), strobilurin (Amistar, Comet, etc) and more recently SDHI (Bixafen, IZM, etc).

Septoria population changes have resulted in the loss of strobilurin about 10 years ago, the reduced effectiveness of triazoles and we have recently discovered resistance populations in the SDHI fungicides.

More worryingly, the fungicides we have now are the ones we have to work with as new chemistry is a long way off.

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It's worth examining each component and understanding what fungicides we are most reliant on for control.

Chlorothalonil

This is a multi-site fungicide meaning it works to control septoria at different points of the fungus growth stages. This means resistance is unlikely with this fungicide.

Chlorothalonil is a vital component of the mix at the third last leaf emerged and the flag leaf emerged with trials showing good yield responses from this relatively low-cost fungicide. The fungicide also provides good protection for the triazoles and SDHI.

Triazoles

These fungicides (Opus, Proline, Caramba, etc) work on a couple of different fungus pathways to achieve control. In the past 10 years control from these triazoles has been slipping due to changes in the septoria populations.

Practically this has resulted in the reduction of curatively activity combined with reduced persistency. Results from the trials are showing triazoles have reduced their contribution to septoria control to about 30pc.

SDHIs

This group of fungicides (Fluxapyroxad, Bixafen, etc.) controls septoria in a very specific area of septoria's growth.

Due to this control mechanism, the group is referred to single site fungicide. This puts the fungicide group at high risk of quicker breakdown in response to changing populations of septoria. To date these fungicides have worked extremely well to control septoria with the contribution to disease control close to 70pc.

However, recent resistance results are worrying.

Septoria population changes update

Plant Pathologist, Dr Stephen Kildea, and his team have been assessing septoria population changes in the field.

Samples from around the country and subsequent analysis have shown there has been a shift in septoria sensitivity to the SDHIs.

This shift is in two parts: firstly, there were very small numbers of the septoria population with effective resistance to SDHI and; secondly, small but significant parts of the septoria population are showing reduced sensitivity to SDHIs. This work also showed this sensitivity is across all the SDHIs.

The team retested populations from around the country this spring and found similar results. The underlining concern is to keep SDHI fungicides as active as possible for as long as possible. This can be achieved by either not using the fungicide or using the fungicide less.

The FRAC guidelines in place for the past number of years recommended the SDHI should not be used more than twice per year.

Teagasc recommends only using SDHI where needed. There is a case for leaving out the SDHI from a disease control program in later sown crops where it is reasonably dry.

The other major recommendation is to avoid using mixes of triazoles with SDHI (eg Gleam + Imtrex) as this will increase selection putting increased selection pressure on the SDHI in the mix.

The main fungicide timings in winter wheat

As mentioned previously timing is the most important factor in getting the most from fungicides in 2016. The following are the timings which will achieve the best returns:

First major application is at leaf three fully emerged (T1) stage. This will protect the entire leaf 3 and half of leaf 2, if timed correctly.

Returns of 0.5t/ha can be expected from this timing. Options include Bravo + Adexar/Aviator/Librax/Treoris + triazole. On later sown crops Bravo + Gleam/Prosaro may be sufficient.

The second fungicide (T2) is the most important and is capable of delivering on average 1.7 t/ha (in trials).

The key is to apply the fungicide as soon as the flag leaf is fully emerged. Options include Bravo plus a high rate of Adexar/Aviator/Librax/Treoris and a triazole.

The final application (T3), at the flowering stage, will help to top up septoria control and and also help to control fusarium. Options include Gleam and Prosaro, or in low pressure sites Caramba or Folicur.

Wheat growers enjoyed record yields last year but septoria still poses significant risks, writes Michael Hennessy

New chemicals to tackle septoria are a long way off so timing of application is vital in getting the most from existing fungicides

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