The stewardship measures introduced with the SDHI fungicides must be observed if the chemistry is to remain active against key pathogens, according to two national fungicide experts.
There’s no doubt that lessons have been learned since the loss of the strobilurins, agree Bill Clark, commercial technical director of NIAB TAG, and Fiona Burnett of SAC.
“The strobiliurins were used alone, not in mixtures, as soon as they were available,” Mr Clark points out. “And although they were very good for a while, such exposure meant their activity was lost very quickly. First resistance developed in mildew, then septoria.”
They did come with use guidelines, says Dr Burnett, but these were largely flouted.
“The advice that accompanied the strobilurins wasn’t strong enough or emphasised enough,” she says. “And, at that time, growers didn’t have enough knowledge about the risks they were taking.”
That has certainly changed with the advent of the SDHIs. Not only are the three manufacturers being highly responsible about their stewardship, there are restrictions on the number of applications that can made to any crop and a requirement to use the new fungicides in mixtures with other chemistry.
What’s more, there’s clear guidance on rates of partner products, so the chance of leaving the SDHI component with too much to do has been minimised. There’s also the benefit of hindsight, with the fate of the strobilurins remembered by most growers and their agronomists.
“The SDHIs are more eradicant in nature than we first thought,” says Mr Clark. “They are very good fungicides, but they’re also high risk. If they had been introduced as straights, their life wouldn’t have gone beyond three years.”
It’s the first time a new chemical group for cereals has been launched with known resistance in other crops, he adds. “So it’s not as though we haven’t been given a clear enough warning. Everyone is aware of what could happen.”
His main concern is the rate of the partner products, particularly triazoles. “We’ve seen a gradual decline in triazole performance over the years, both in protectant and eradicant mode, and it’s clear the days of half-rate azoles are over.
“More than anything, we’ve got to get growers out of their rate cutting habit. At the flag leaf timing, everyone should be using a three-quarter rate of triazole with an SDHI. We need the persistence that gives to protect the SDHI.”
Dr Burnett believes cereal growers are more receptive to using higher rates of fungicide, both in terms of yield responses and other benefits. “Higher cereal prices have helped. It’s easier for them to justify the investment in their crops.”
Pre-formulated mixes, such as Aviator Xpro, help too, she says. “You can’t change it and there is clear advice on minimum rates.”
A new statement from FRAC on SDHI stewardship for the 2012 season is imminent, she says. “The number of SDHI applications that can be made to a crop won’t change, but it will be really pushing the mixtures message.”
That’s different to other European countries, some of which are likely to limit SDHI use to just one application a crop, she says.
Both Dr Burnett and Mr Clark are clear that growers mustn’t plan their spray programmes on the curativity of the SDHIs. “There’s been a lot of noise about it, but it’s not the way these fungicides should be used.”
What about straights?
Bayer CropScience has no plans to introduce bixafen as a straight product, showing the company’s commitment to SDHI stewardship, confirms Andrew Flind.
“With co-formulated fungicides, you can control the ratio of products and minimise the use of sub-optimal rates,” he explains. “We’ve been clear to growers that they shouldn’t use less than 1 litre/ha of Aviator Xpro.”
Even in low disease pressure years, growers will see good yield benefits from the use of SDHIs, he says, providing rates are maintained. “Having a three-quarter rate of triazole in with bixafen is important. If you aren’t prepared to use that amount, you should consider an alternative solution.”
But BASF will be introducing Imtrex, which is straight Xemium, confirms Peter Hughes. “It will only be available through two distributors and it has to be sold for use with a BASF triazole.”
He also stresses that the 1 litre/ha rate of Adexar is only for protectant situations. “If you need curative activity you must use a minimum of 1.25 litres/ha.”
Mr Clark’s concern is that any straights might not be used properly. “It is a worry. We need as many tools as possible.”