Making strobilurins pay
TARGET STROBILURIN fungicide use to where yield responses are guaranteed. An obvious aim and, in previous seasons, pretty easy to achieve. Not any more. Resistance build-up by Septoria tritici appears to have eroded the large, consistent yield responses previously seen, and with strobilurin prices unlikely to fall, targeting their use effectively is one of key issues for growers in the coming season.
But with last year”s trials results giving anything but clear guidance, just where should growers be targeting? Five situations appear favourite, according to agronomists.
1 Flag leaf sprays – wheats with good yield potential
Trials sites on good wheat land – heavy clays or silts that were not drought-prone – did respond to strobilurins last season, stresses ADAS fungicide expert Bill Clark. “Responses of 0.7t/ha over triazoles were recorded.”
On strong wheat land, with yield potential of over 9t/ha, use a half dose strobilurin for its physiological greening effects, he advises. “Additionally there might be some septoria control, although that is no longer the primary reason for using them.”
Heavier, drought-free soils are more likely to be able to sustain greening effects, agrees Agrovista”s technical manager Mark Hemmant. “Most varieties will respond economically to flag leaf strobilurins.”
But don”t just expect to spray strobilurins and achieve yield responses, warns Banks Cargill”s Bob Mills. “Strob responses are all part a good agronomy package.”
That means getting all other aspects of agronomy correct to allow strobilurins to express themselves fully, he says.
For example nitrogen shouldn”t be limiting. Strob responses over a triazole programme in a trial comparing 20 different varieties with two different nitrogen regimes averaged a disappointing 0.12t/ha when 175kgN/ha was applied. “At a higher nitrogen rate of 225kg/ha the response was 0.75t/ha.
“To get the best out of strobilurins, you need to give it the tools to perform, and be able to assess site potential early to know when they will be able to respond.”
On more marginal land drop the strobilurin dose, suggests Mr Clark. “Marginal wheat land, such as thin, brashy soils are much less likely to respond to strobs. But I wouldn”t necessarily leave it out altogether – just restrict my total dose.”
2 Drought-Prone Soils – T1 Sprays
Flag leaf sprays may be less beneficial on drought-prone soils, says Mr Hemmant. “That may not be the best place to put them on. In very hot temperatures, such as two seasons ago, crops just die off and are not able to benefit from prolonged grain fill.”
Instead consider using strobilurins at T1 to take advantage of their physiological effects in reducing plant stress, he says.
“But it has to be the correct strobilurin. Both kresoxim-methyl and pyraclostrobin inhibit ethylene produced in response to stress.”
SAC”s Keith Dawson is also a T1 advocate for strob use. “At T1 the major advantages of strobilurins are effects on rooting and drought tolerance. Use a low-cost strobilurin before the period of rapid growth at about half rate.”
3 Take-all control in second wheats
Initially somewhat sceptical, most agronomists are now backing the use of strobilurins in second wheats at T1 to help control take-all whole-heartedly.
“Growers can get very worthwhile yield responses from using a take-all strob in second wheat,” notes Mr Hemmant.
But opinion is split whether the best responses come in conjunction with a take-all active seed treatment, such as Latitude or Jockey. According to Mr Hemmant activity from the strobilurin is more reliable following such a seed treatment, but Dr Dawson suggests strobilurins could be used as an alternative. “They may well be more cost-effective than a seed dressing.”
Soil type may also play a part in the degree of response, adds Strutt & Parker agronomist Mark Hall. “Target more take-all prone areas, such as lighter soils. We”ve also seen good responses on heavy clay soils but chalky boulder-clay sites seem less likely to show a benefit.”
The choice for take-all activity within strobilurins has doubled this season with the launch of Bayer CropScience”s fluoxastrobin (available in mixture with prothioconazole as Fandango), joining the established option of Amistar.
“At commercial doses they are fairly similar,” suggests Mr Hemmant. “But Fandango gives very good septoria protection and exceptional stem-based disease control [from prothioconazole] in one package, and will be cheaper than the equivalent Amistar tank-mix.”
Rates need to be kept up to achieve take-all suppression. “Stick with 0.6 litres/ha of Amistar if you”ve used a take-all seed treatment, bump it up slightly if you haven”t,” advises Mr Hall.
But the requirement for high doses is a problem, warns Mr Clark. “The dose is just too high, too costly. You”re not even getting that good take-all suppression. Growers will only get a cost-effective response 30-40% of the time.”
4 Ear disease
Ear wash sprays, provided growers have not already used two strobilurins in the programme, are the obvious place for strobilurins, says Mr Hemmant. “That”s particularly where there is a brown rust risk, locations with longer grain fill such as the north, and on quality wheats.”
Ear sprays, as with the two other main fungicide timings, still need to be based around a triazole, adds Dr Dawson. “But strobilurins will help against ear diseases, particularly cladosporium and sooty moulds in wetter environments, and reducing mycotoxin risk.”
Choice of strobilurin is between Swing Gold, Fandango and Amistar, suggests Mr Hemmant.
5 High yellow rust risk varieties
If yellow rust is clearly established in the crop, then a strobilurin treatment will be merited, says Mr Hemmant. “Pyraclostrobin is the strongest strobilurin against yellow rust.”
But a yellow rust epidemic is unlikely this season, he predicts. “To get an epidemic you need several factors, and while we have a lot of susceptible crops in the ground, and had a mild autumn, there wasn”t a large carryover of inoculum from last season.”
That means while there could be some outbreaks, a major epidemic is unlikely. “I wouldn”t worry too much about yellow rust for this season. Next year might be different.”
However, Mr Hall will be considering using a strobilurin on big blocks of susceptible variety Robigus. “Which one will depend on price. There is a difference in efficacy between the likes of Opera and Amistar, but not so much as the difference in price justifies.”
