Initial impressions in early June suggested that triazoles had lost their efficacy as the main ingredient in wheat fungicide programmes, which has now been confirmed in the same Velcourt trials.
“In the 23 years I have been working with Velcourt, I have never seen such a black and white trial,” says technical director Keith Norman.
“There have been some big questions raised by what we have seen this year. The biggest one being ‘why are the stalwarts of our fungicide programmes – Opus and Proline – failing to the extent they have,” he says.
The trial, in Kent, shows that where the plots received two full-rate applications of Opus or Proline at T1 and T2 respectively, the crop is completely defoliated almost to the same level as the untreated.
“Is it just the sheer pressure of the disease levels this season or is insensitivity building?” questions Mr Norman. “It must be said though, that where Bravo was used in conjunction with the triazoles, particularly at T1, disease control was improved.”
Velcourt as a company is strategically sampling areas where triazoles have not been performing as well as they have done in the past. “This will differentiate between the septoria strains and look for the presence of resistance in those strains,” says Mr Norman.
It is this sampling that Mr Norman believes will provide the facts that will allow Velcourt’s future fungicide strategy to be based upon. “At present there are just too many unknowns,” he adds.
Complementing the Velcourt trials at the site in Kent is a LINK-funded project, which is also looking into insensitivity in SDHI and triazole chemistry. It is believed that this will provide the categorical evidence of what is happening in the field, with results available in December.
The trial also compared the old chemistry with the three commercially available SDHI actives, at various rates from quarter up to double. “There was a clear difference, as expected, when an SDHI is included in the programme,” says Mr Norman. “It leads to greener crops, with leaves photosynthesising and filling grains.”
There were also differences between the three products, with Xemium the most effective at the site, but jostling it out for top spot was Aviator when it comes to overall efficacy across the country.
“It does depend on where you are in the country, as we have seen a role reversal in the South-West,” points out Mr Norman.
Cut rates at your peril
The SDHI rate comparisons shed some light on the efficacy of the three products, and Mr Norman concludes that where rates were cut, the SDHI’s become much less effective.
“The recommended rates are on a cliff edge and any reduction results in efficacy dropping off that cliff. A robust dose must be used, where as in the past people have looked at rates and thought ‘I’ll cut it in half and it will be fine’,” he says.
“That approach cannot be adopted with this chemistry,” he adds. “It is vital to keep rates up or you lose any advantage you gain from using the SDHI in the first place.
“A minimum of three-quarter rate must be maintained and the addition of Bravo or Phoenix will help, but there are antagonism issues with Bravo and Aviator. These multi-site actives will also help with resistance management, should resistance be developing,” says Mr Norman.
This year the extreme disease pressure has led Mr Norman to believe that the SDHI’s should have been used in sequence at T1 and T2. “Hindsight is a wonderful thing, though.
“All the historic data we had showed little benefit of an extra application; however, the data had been gathered in dry, low disease years. The only problem with such a sequence though is that resistance may be accelerated, so perhaps an alternative may have to be sought,” he concludes.