The extra cost of the new generation of SDHI fungicides is well worth the investment, even in a low-disease-pressure year, according to wheat disease experts. Adam Clarke reports.

Wheat growers could be risking a hit to their yields and profit margins if they avoid using the new generation SDHI fungicides in their winter wheat crops this spring.

Managing risk is the name of the game when formulating a fungicide programme, and using the best products will provide insurance if disease pressure escalates.

Last season saw relatively low levels of the main winter wheat disease, septoria tritici, in many parts of Britain .

Despite these low levels, Bill Clark, disease expert at crop consultants NIAB TAG, said that using the leading SDHI-triazole formulations such as BASF’s Adexar and Bayer CropScience’s Aviator still gave a good return on investment.

Further to that, during the past five years of trials, the average yield response from good SDHI programmes has been 1.29t/ha, highlighting their worth to the grower.

“Predicting disease risk is very difficult as conditions can change so quickly, and if our assessment is wrong it can be very bad news,” he said.

“How much risk are growers prepared to take? I would advise against tweaking programmes to low in an attempt to make savings,” Mr Clark added.

With 2014 shaping up to be a high-pressure year due to an increased wheat area, lush early-drilled crops and a mild winter, relying on just triazoles would not be sufficient.

Prothioconazole and epoxiconazole remain the two leading triazoles in spring fungicide programmes, but their curative effect on septoria has been declining due to resistance.

Speaking at last month’s Association of Independent Crop Consultants (AICC) conference, Mr Clark used recent data to demonstrate that where disease levels are high, prothioconazole and epoxiconazole gave very little control of septoria.

In contrast, applications of Adexar provided robust disease control and, at current wheat prices, a margin-over-fungicide cost of between £260/ha and £410/ha, with the margin increasing with dose rate.

“An SDHI is a must, and even with a low wheat price of £100/t, you would still not be losing any money by investing in these products,” he said.

Mr Clark added that the highest-yielding varieties showed the best fungicide responses.

Product choice

Jonathan Blake, fungicide expert at crop scientists group ADAS, also said the SDHI products showed their worth again in last year’s HGCA trials.

With an average yield response of 0.37t/ha for using an SDHI product at T1, Mr Blake said the data justifies considering that strategy.

“Seguris and Vertisan would be good products to use at T1, but where there are high levels of eyespot this season, the boscalid-based Tracker remains a strong option,” he explained.

At T2, Adexar and Aviator have been joined by DuPont’s latest release, Vertisan, on to the SDHI market as the most effective septoria-controlling fungicide product.

Mr Blake believes a mix of Vertisan and the triazole Ignite is as good and is comparable to the other two when considering overall levels of control.

“Growers must remember not to rely on the SDHI products for yellow and brown rust, with the triazoles still key to controlling those two diseases,” he added.

Mr Blake also warned that brown rust could be a factor for the coming season, with many varieties in the ground with a resistance score of 5 or less.

“There is already brown rust present in crops and the weather pattern is very similar to the last bad brown rust year in 2006-07,” he added.

Resisting resistance

Scientists have seen mutations in the barley disease net blotch that are been resistant to SDHI fungicides.

The mutation has been discovered for the first time in UK field samples – not manufactured in the laboratory – and NIAB TAG’s Bill Clark (pictured) said it is inevitable that septoria will also become resistant to SDHIs.

Genetic mutations in the pathogen prevent the fungicide molecule from binding to its target site, rendering them ineffective.

“These nasty mutations can dramatically reduce field efficacy, and although SDHIs are currently holding up septoria control, they are very vulnerable, so must be protected,” he said.

Mr Clark warned that four-way mixtures of SDHIs, triazoles, strobilurins and multi-site protectants such as chlorothalonil are something that growers must consider in the future to slow down the inevitable.

“We have seen how little the triazoles are now adding when on their own, so we have to do all we can to protect the SDHIs,” Mr Clark concluded.

See also: Yellow rust on course for worst ever season