Caution Urged Over Using Two SDHI Sprays In Wheat

(Farmers Weekly) – The use of two SDHI applications fungicides 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.

Septoria control and low selection

Minimum SDHI dose and number of treatments

Limit number of azole treatments

Robust azole dose at T1/T2 when mixing with SDHIs

Maximise use of multi-sites

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.

Fungicide results

Wheat

New fungicide Ascra from Bayer gave improved septoria control over its stablemate Aviator in the AHDB-funded fungicide performance trials, reports Stuart Knight, director of crops and agronomy at Niab.

Ascra contains two SDHI bixafen and fluopyram, together with the azole prothioconazole, and will be available for 2017.

The SDHIs all gave good septoria control and the SDHI/azole mixes also  performed well. Differences between the azoles on protectant septoria activity were evident, with prothioconazole performing better than epoxiconazole.

In contrast, prothioconazole was less effective than either epoxiconazole or metconazole on brown rust.

“However, prothioconazole is the best product for reducing fusarium. So it’s clear that growers have a good selection of fungicides to choose from to tackle the various diseases,” he says.

Barley

The performance of SDHIs against net blotch in barley was disappointing in 2016,  and researchers are looking at the reasons, says Mr  Knight.

“All of the SDHIs gave results below our expectations, while prothioconazole maintained its very good performance on net blotch,” he says.

The SDHIs gave varying performance on rhynchosporium. Imtrex (fluxapyroxad) was robust on this disease and equalled prothioconazole.

Overall, the SDHI/azole mix Siltra Xpro gave good control of all the barley diseases tested, including mildew.

“However, it was good to see the non-azole option Priaxor (fluxapyroxad+pyraclostrobin) performing well, offering good rhynchsporium control, and giving growers some choice,” he says.