The use of two SDHI fungicide applications for septoria control in one season should not become the norm if resistance is to be avoided, especially where wheat varieties with high disease resistance are grown.

Concern about the pathogen developing resistance to fungicides prompted Adas’ head of crop protection Neil Paveley to question the need for two SDHI sprays, although permitted by industry guidelines.

“If the disease pressure and weather conditions are such that you need two SDHIs, then fine. 

“But it would be wrong for that approach to be standard practice, as we know that an increase in the number of sprays increases the selection for resistance,” he says.

See also: Septoria disease in wheat worst for 20 years in the East

Both increasing the dose and splitting it pushes the pathogen population towards resistance, whereas adding a partner such as an azole and some chlorothalonil is a good way of slowing selection,” adds Dr Paveley.

Septoria is becoming more sophisticated and complex, with no sign of its evolution slowing or stopping, he warns.

“We’ve seen the loss of sensitivity that has happened with repeated use of azoles, with further movement recorded again this year.

“Now we are just starting to find the first signs with SDHIs, with insensitive isolates being detected at very low frequencies at a few sites,” says Dr Paveley.

Highly insensitive strains are very rare and there haven’t been any control failures in the field, and monitoring carried out over the past two years by Adas and Rothamsted Research shows plots which received SDHIs were the ones where insensitive septoria was picked up.

Low and moderately insensitive strains have been detected, as well as one highly insensitive strain known as C-H152R.

“This particular strain can withstand whatever dose of SDHI is applied to it, so it mustn’t be allowed to dominate.

“At this stage, both the low and moderately resistant strains can still be controlled with SDHIs,” he says.

Fungicide programmes for 2017 can be tweaked if a resistant variety is being grown, such as taking out the azole at T0 if a multi-site is sufficient.

“Look hard at whether you really need an SDHI at T1 on a more robust variety and if it’s really necessary to use as azole at T3. If it’s dry by that stage, the fusarium threat will be reduced,” he says.

Dr Paveley highlights that margins over fungicide costs differ little in Adas trials.

Three wheat varieties were grown – resistant, moderate and susceptible to disease – while three different fungicide intensity programmes were applied – low (no SDHI), moderate (1 x SDHI) and high (2 x SDHI).

“What we found was the margin was very similar across the three regimes.

“So it’s very difficult to conclude that two SDHIs are required from a financial perspective as well as a resistance one,” he says.

These fungicides experts were speaking at the AHDB Agronomists Conference, which was held in Peterborough in early December