By Professor Fiona Burnett,

SRUC for the Farm Advisory Service

Resistance to some of our key fungicides has been of concern for a few years, but now monitoring data from 2018 shows that the position for septoria in wheat has continued to worsen and there were also new and unwelcome developments on the barley front.

The latest data, presented at the Fungicide Resistance Action Group UK meeting, last week, did show, luckily, that field control in 2018 was broadly acceptable – but that had more to do with the heat which helped to dry up any early disease.

Septoria management has been getting harder, with azole chemistry declining in efficacy. Data from 2018 suggested that there has been a further reduction in efficacy, following some hope in the 2016 data that it had stabilised. However, the 2017 and 2018 data confirmed that the septoria population is still adapting.

The azoles have thus far been ‘propped up’ by the use of SDHI fungicides, but an unwelcome surprise in the 2018 monitoring data has been the jump in the frequency of mutations which can affect SDHI efficacy in the septoria population.

We’ve seen emerging issues for the last couple of seasons, but still at very low levels in the population. It now appears as if up to 40% of the populations is now affected with at least one mutation.

The better news is that is that many of these mutations have a very low or negligible affect of on efficacy. But, there is no denying it is an unwelcome development.

There are several mechanisms that allow fungal pathogens to develop resistance. One is mutations that affect the shape of the target site where the fungicide locks into the fungus. A second way that fungal pathogens develop resistance is that they adjust to pump fungicides out of their cells faster.

AHDB project data, incorporating SRUC trial sites, shows that Scottish sites tend to have more of that latter type of mutation compared to English sites. It can affect more than one type of fungicide, but fortunately tends to incur large fitness costs for the fungus so target site mutations are still the principle worry.

This year's field trial data demonstrated yet again that the septoria population can shift very rapidly – after just a single spray of an SDHI or azole the population shifted just slightly to the less sensitive end of the spectrum.

This all points to the fact that stewardship is vital because that shift was reduced where products were used in mixture (as is the case in real life) and not seen where chlorothalonil was used in the mixture. It remains more important than ever that actives are mixed and alternated where possible and it is crucial that multi-sites are used.

Net blotch is another example of a pathogen where SDHI mutations have crept in over recent seasons and this moved up the spectrum again in 2018 and 75%t of all net blotch populations now carry some form of mutation which could affect SDHI efficacy. France and Germany seem to be the countries most affected, with the UK not far behind on this.

Net blotch is less of an issue for Scottish barley fungicides and historically we have done much to manage the disease through certified seed schemes and saving seed of high health status.

Fungicide performance data from 2018 also confirmed that the efficacy of strobilurins to net blotch is slightly compromised by the presence of partial resistance in a portion of the populations – probably around 30% – but in mixtures and programmes, control is still broadly acceptable.

Ramularia remains a major concern in Scotland as we are left with only chlorothalonil as an effective option because of widespread resistance to azoles, strobilurins and SDHIs.