When considering which fungicide products to use this spring, this year’s crop might not be the only one that reaps the benefit from an application of a particular SDHI active ingredient. Adam Clarke reports.

Research looking at how a leading SDHI fungicide works has revealed it reduces septoria disease pressure in both current and following wheat crops in the rotation.

See also: Farmers urged to re-think fungicide plans to fight resistance

New generation SDHI fungicides, such as BASF’s fluxapyroxad – contained in products Adexar and Imtrex, are now an established part of UK fungicide programmes.

But up until now, little has been known about how SDHIs are working to reduce septoria epidemics – the ever-present wheat disease that can cause untreated yield losses in excess of 50%.

To explore the effects of using SDHIs on all phases of the septoria lifecycle, Adas crop pathologist Julie Smith and her team compared wheat plots treated with a straight azole and an azole/SDHI co-formulation.

“Following treatments of Adexar, there was a greater reduction in spores entering the following crop, compared with other treatments.”
Julie Smith, Adas

Sprays of 0.5litre/ha of Opus at T1 and T2 and 1litre/ha of Adexar at the same timings were applied and plants assessed for disease after both the T1 and T2 spray.

“It was unsurprising to see that there was a greater reduction in septoria disease severity when using two applications of Adexar over two Opus during the asexual phase.

“On closer examination, the Adexar treatments significantly reduced the number of lesions on the leaves, showing a reduction in the infection efficiency,” explains Ms Smith.

Fewer spores

In addition, the lesions themselves were smaller and contained fewer pycnidia – or fruiting bodies that produce spores. Subsequently, the number of spores a leaf were also reduced following application with the SDHI containing product.

Researchers also examined spores under a microscope and observed a decrease in the size of spores, which could indicate a fitness cost.

 The Adas team went on to test the effect of reduced spore production in subsequent cycles of infection spreading up the crop canopy.

Inoculation studies in spring wheat plots in Wales showed a reduction of infection pressure produced less disease at the next cycle and also meant it took longer for the pathogen to produce lesions from new infection.

“This effect provides greater flexibility for the timing of subsequent sprays and could help to widen the spray window,” adds Ms Smith.

Latent period

An influential factor in resistance development is the number of latent periods there are each year, with shorter latent periods associated with faster cycling of the disease.

It is believed if you can extend the latent period it may help to slow the development of fungicide resistance and help extend the life of the available chemistry.

“The benefit of early inoculum reduction was demonstrated in 2014 – people who hadn’t applied an SDHI at T1 were chasing disease all season,” says Ms Smith.

Carryover

Adas crop pathologist Julie Smith.

With overall inoculum reduced in the asexual phase by the addition of fluxapyroxad in the programme, the effect on the carryover of septoria to next years crop has also been assessed.

Large wheat plots were established using min-till and isolated as much as possible by surrounding the plots with winter barley.

In the first year, commercial fungicides were applied at T1 and T2 and the second year all plots were left untreated, with a spore trap monitoring the amount of winter inoculum released from trash into the surrounding crop.

“Following treatments of Adexar, there was a greater reduction in spores entering the following crop, compared with other treatments.

“We backed this up by taking trash into the lab and inducing ascospore release and the debris collected following the Adexar treatment produced fewer spores,” adds Ms Smith.

The winter ascospore release was directly related to the amount of disease seen in the following crop at early stem extension, showing that benefits from T1 and T2 applications of Adexar can persist for longer than originally thought, demonstrating how valuable the SDHI fungicides are growers.

“We did compare fluxapyroxad with older SDHI chemistry (boscalid) and didn’t always see effects, so we can’t assume that all SDHI’s are acting on the same lifecycle parameters, in the same way,” concludes Ms Smith.