To achieve high quality produce and maximum yields of cereals under UK conditions, a range of foliar fungal diseases must be controlled.

For diseases like mildew and rusts, good sources of variety resistance are available, says NIAB disease control programme leader Bart Fraaije, who is leading an AHDB project* monitoring the effectiveness of fungicides.

But control of septoria leaf blotch, ramularia and net blotch has proved more difficult and relies heavily on chemical control, he says.

The development of resistance to various single site acting fungicides means the chemical approach has become compromised.

So, up-to-date knowledge on fungicide efficacy and varietal resistance is needed to achieve the best disease control while minimising the risk of resistance developing.

The main target for this project is zymoseptoria tritici which causes septoria.

It has developed resistance to quinone outside inhibitors or strobilurins for example, azoxystrobin and pyraclostrobin and different levels of insensitivity to azoles and succinate dehydrogenase inhibitors the SDHIs, says Dr Fraaije.

Resistant varieties

After the withdrawal of chlorothalonil, control of septoria was initially based on employing the most resistant varieties and applying fungicide mixtures of azoles and SDHIs.

Other multisite inhibitors can be added to reduce selection for fungicide resistance further.

A new azole, mefentriflucona zole, and a new single-site acting inhibitor, the quinone inside inhibitor fenpicoxamid, were added to the weaponry to combat septoria last year.

This project, linked to the AHDB winter wheat Fungicide Performance trials, aims to find the ongoing Z.tritici fungicide sensitivity for all key actives representing different modes of action (MOAs).

Knowing how fungicide insensitive genotypes evolve and accumulate within populations will help show how fungicides can be best used when it comes to choice, timing, dose and MOA partners, adds Dr Fraaije.

Early season monitoring involves sampling 50 leaves with septoria symptoms per field in winter/early spring.

After isolating up to 50 Z.tritici strains per sample, the corresponding population is then assessed for its fungicide sensitivity using 96-well microplate assays.

This is to see if there are any sensitivity shifts for existing fungicides between seasons and to determine the baseline sensitivity for new actives likely to enter the market, says Dr Fraaije.

We also isolate populations from infected leaves from commercial fields and the Fungicide Performance trials after treatment to determine whether new fungicide-resistant phenotypes have been selected.

A selection of insensitive isolates is further characterised each year to understand the underlying resistance mecha nisms, he adds.

This involves DNA extrac tion, polymerase chain reaction and sequencing.

We now have a much better understanding of why the septoria control efficacy of some fungicides is declining.

For example, the decline in efficacy of azole and SDHI fungicides is often due to different and/or multiple fungicide target protein changes caused by various mutations.

Most of these field mutants are controlled at the recommended dose, but low doses dont provide sufficient control.

With strobilurins just one mutation cytochrome b G143A conferred high levels of resistance, and this accumulated quickly in UK field populations, most isolates within populations carrying it within two years of its detection, he says.

Genotypes

By contrast, several genotypes (for example, SdhB H267L, SdhC H152R and SdhB N225T + SdhC N86S) have been found to confer high levels of resistance to SDHI fungicides; but in UK field populations only one (C-H152R) is often detected, albeit at low frequencies.

This is indicative of a very slow accumulation, in contrast with common Sdh variants C-T79N and C-N86S that confer low levels of SDHI insensitivity.

C-H152R is associated with a fitness cost and, in the absence of SDHI selection pressure, these mutants cannot compete well with other Sdh variants and/or strains without mutations, says Dr Fraaije.

With regard to azoles, mefentrifluconazole seems to select differently for CYP51 variants in comparison with prothioconazole, he adds.

BASF continually monitors disease populations across Europe, says the companys business development manager cereal fungicides, Dr Jon Helliwell.

Historic data offers insights for future resistance development and can be built into product development, he says.

However, confusion can arise in relating lab-based resistance results to field performance, particularly when many actives in the same group of chemistries have different intrinsic efficacies.

So, its important to always review resistance data within the wider context and scope of product use.

Patterns of resistance development depend on the mode of action, he adds.

For example, quinone outside inhibitor resistance developed over only a few years from about 2004, due to a single target-site mutation reducing field efficacy from high to low in a single step; and the G143A mutation, now widespread across the UK, affects all strobilurins.

But published literature shows that pyraclostrobin still has a positive impact in reducing septoria spore efficiency, even in the presence of that mutation, something not seen with azoxystrobin in the same study.

Development Newer modes of action (such as quinone inside inhibitors), designated moderate-high risk of resistance developing by the Fungicide Resistance Action Group, are also likely to follow a ‘single step resistance route, he believes.

Using mixtures with other effective alternative modes of action and limiting the number of applications is important to slow resistance development.

Triazoles have formed the backbone for UK cereal fungicide programmes for two decades.

However, there are clear differences in sensitivity and intrinsic activity affecting efficacy and ability to protect other modes of action between conventional and new azole introductions, such as Revysol.

• *AHDB project 21120018a Monitoring resistance to foliar fungicides in cereal pathogens
• April 1, 2019 April 1, 2023
• Funding: AHDB £126,000; industry in-kind £79,500
• Collaborators: NIAB, ADAS, SRUC, Teagasc
• Industry partners: BASF, Bayer CropScience, Corteva, Syngenta