Questionmark remains over strobs

Four years on from Septoria tritici resistance becoming established in UK crops, growers are still facing the strobilurin fungicide conundrum.

Are they worth spraying, and if so, when and where?

Unfortunately, despite a second season of trials in which septoria resistance to the group was virtually 100%, the picture remains murky, although strobilurin manufacturer Syngenta has tried to provide some guidelines for growers.

Last season, growers reacted by virtually ignoring strobilurins at T1, apart from in specific take-all situations.

It resulted in strobilurins only accounting for 40% of the value of the fungicide market in 2005, compared with 50% in 2005.

The big question this season is whether to use strobilurins on the flag leaf, says ADAS plant pathologist Bill Clark.

“It is a complicated and difficult decision, but I think there is still a place for strobilurins.”

That is despite last season not being ideal for getting yield responses from the group.

“To get the best response you need perfect grain filling weather – bright days and cool nights.”

low light

But the lowest level of radiation, or light intensity, since strobilurins were introduced, coupled with high temperatures and an early harvest, meant high yield responses to strobilurins were always unlikely in 2005.

“I was surprised by some of the responses we did get, which is a good reason to be cautious about dropping them.”

BASF’s farm-scale trials across 13 farms, where the only difference between two 5ha blocks was the addition of 0.5 litre/ha of Comet 200 (pyraclostrobin) to the three-spray fungicide programme, illustrate responses are possible, says the firm’s Rachel Mee.

“Twelve out of the 13 gave an economic response.

The most responsive was 1.5t/ha – the average 0.62t/ha.”

Similarly in 33 BASF, distributor and independent small-plot trials the average yield response from adding 0.5 litres/ha of Comet 200 to Opus (epoxiconazole) on the flag leaf was 0.4t/ha.

Both sets of trials discount the Bravo (chlorothalonil) effect, however.

“It is fair to say most growers should be adding Bravo to their triazole first before thinking about whether to add a strobilurin,” Mr Clark notes.

“The response from strobilurins over triazole plus Bravo is smaller.”

Even so, adding in a strobilurin is still worthwhile based on the evidence from HGCA Appropriate Fungicide Dose 2 trials.

The trials investigate the effect of triazole dose, and the addition of strobilurins and chlorothalonil in commercial programmes.

At Rosemaund there was a benefit from adding a strobilurin, depending on triazole dose, Mr Clark says.

“Once you put on 1.5 litres/ha total dose of Opus the effect of the strobilurin all but disappears.

Most growers, though, are not putting on that much.”

Results are variable and difficult to predict, he admits.

At Morley the yield response was much smaller.

“You might get half a tonne or you might get nothing.

“Unfortunately, we’ve never been able to identify with any real certainty where you are going to get a response.”

But Amistar manufacturer Syngenta has taken up the challenge, coming up with some guidelines for growers to use to help decide whether a second strobilurin spray can be justified.

Its analysis is based on six regularly recorded factors – variety, drilling date, soil type, quality requirements, diseases other than septoria, and application timeliness – in 70 trials over the past two seasons.

“Most of those factors could also be useful in helping growers decide whether to spray a strobilurin in the first place,” Syngenta’s David Ranner says.

“But based on our information one strobilurin is well-justified – typically you get a two for one return on investment from a half rate Amistar.

It is the second strobilurin that needs to be justified.”

Where other diseases, such as take-all, rust or ear infections, are present growers get twice as high a response from strobilurins, Mr Ranner says.


But it is the variety information that will most intrigue growers and advisers.

“Trials have shown there is a clear difference in response between varieties, but it has never really been explained just by disease susceptibility.”

For example, over the past two seasons in Syngenta trials, relatively disease-resistant Claire responds well to strobilurins, while Access, relatively susceptible to Septoria tritici and yellow rust, doesn’t.

“Over the two seasons the overall ranking of the varieties was very similar.”

Based on that evidence Syngenta has placed varieties into three categories – high, medium and low responders (see table, below right).

For the high responders margins from applying 1.25 litres/ha Amistar Opti (azoxystrobin + chlorothalonil) on the flag leaf and a 0.3 litres/ha Amistar (azoxystrobin) ear spray on top of a triazole + Bravo base range from £3.30 to £29.30/ha across a range of trials, justifying the two strobilurins, says Mr Ranner.

The case for a second strobilurin in the medium response category is much tighter, he admits.

For those varieties, the margin ranges from a loss of 55p/ha to a profit of £3.35/ha.

“It depends on what other favourable factors are present – the more there are, the more likely there will be a positive response.”

On low-responding varieties, such as Hereward, a second strobilurin was not justified – at least not for yield.

“It you are worried about specific weight, it maybe justified.”

The variety categories have been made based on analysis of all Syngenta’s trials over the past two seasons, but the firm also did some trials last season where all the varieties were on the same site.

Generally, varietal yield responses matched their respective category placement, Mr Ranner says.

The reason could be due to a differential greening response to strobilurins, he suggests.

“On some varieties the difference was quite big between strob and triazole treatments, for others it was quite small.”

One theory Syngenta has is around a variety’s senescence speed.

“It is only a theory, but it is well known some varieties, such as Malacca and Claire, lose green leaf quite quickly during grain fill, while others, classically Robigus, hold onto it.”

The theory is on the faster finishing varieties that lose green leaf quickly applying a strobilurin improves green leaf retention improving yield, he explains.

Conversely it is more difficult to improve green leaf area on varieties that already holding on well, and therefore strobilurin responses will be less.

Syngenta trials have previously shown how applying a strobilurin and maintaining green leaf area during grain filling for a few days extra can improve yield, Mr Ranner says.

“Each additional day can potentially deliver an extra 0.15t/ha, but there is also a finite point where it stops.”

To test their theory Syngenta asked breeders how quickly their varieties senesced.

“It wasn’t something they recorded as a rule, but their general feeling matched our categories reasonably well.

“Malacca, Claire and Tanker are all said to senesce quickly and are in the high response category, while Hereward and Solstice are slower, and are low responders.”

The exceptions are Robigus and Consort, which are slower to finish but are categorised as medium responders.

“Obviously other factors will play a part.

For example with Consort disease is a key factor.

If you can’t keep septoria out you will get a low response, if you can, you can get a high response.”

The new variety information has been included in Syngenta’s updated strobilurin justification table.

“It is not an exact science, but it includes factors growers need to relate to.

A score of three high responses means a good chance of a strobilurin response, while if you have two high factors the response may be more borderline.”

Syngenta tested the decision table on fields where strobilurins did and didn’t respond last season, the firm’s Matt Pickard says.

“Where we had the 10 best responses, there was a minimum of three high factors, while the ten worst responses didn’t have any high factors.

It gives us some confidence we’re on the right lines.”