Strobilurin resistance continues to spread

By Andy Doyle

Hopes that 2003 might unravel the resistance riddle would appear to have been unfounded. In as much as we can measure these things, the general level of resistance in Ireland seems to have increased since this time last year but maybe not significantly. And despite one of the driest and lowest septoria pressure years for some time elsewhere in the EU, the severity of strobilurin resistance in septoria tritici has doubled in the UK and become more widespread in most other wheat-growing EU countries.

At a recent BASF information session, Kristin Stoermer from the company stated that the strobilurin resistance level in the UK had virtually doubled during the year. This was according to results from international fungal monitoring company EpiLogic. The speed of the spread and severity of resistance leaves the main question unanswered - how could the disease spread so rapidly in a year which appeared to show so little active disease pressure across the EU?

Importance of ascospores

Any attempt to answer this question is purely speculative. However, one probable theory is of interest. Septoria tritici reproduces both asexually (fungal growth within the plant and pycnidiaspores from pycnidia spread by rain splash) and sexually (ascospores produced by sexual recombination are much smaller and can be blown in the wind to spread across wider areas).

The importance of ascospores and their significance in the spread and distribution of the disease are not well understood. However, the question is now being asked if, in a UK context in 2003, the production of ascospores was increased considerably when asexual propagation was limited by environment? This is not at all an improbable theory as reproduction is an inevitable urge for any species under threat.

Could it be that the low septoria pressure year of 2003 led to significantly increased ascospore production, which spread small pockets of resistance over a much wider area? Maybe we will never know for certain, but the possibility of ascospores being a significant factor in the rapid development of widespread resistance cannot be discounted.

A more local reason for considering this theory is that the occurrence of relatively widespread strobilurin resistance here in 2002 may well have been a consequence of ascospore spread during the relatively low septoria pressure year here in 2001. In such seasons, strobilurin use is likely to be less well defended by triazoles, thus adding pressure for selection. At this point, no one was looking for resistance. Following 2003, everyone is now searching and so other areas may be spared the lack of field efficacy found here in some instances in 2002.

Selection pressure

There would now appear to be a growing acceptance that resistance is a result of the ongoing selection of the G143A mutant, which resided in the natural population to begin with. There is still no indication that resistance was a result of the production of a mutant. Scientists had always based their views that septoria resistance was unlikely on the fact that they could not produce a mutant. The natural ‘mutant' now seems to be the explanation and this now raises concerns for the presence of this mutant in other diseases such as net blotch, rusts, etc. and maybe even Microdochium nivale.

It now seems likely that the possibility of resistance management has passed us by. For reasons still not understood, the build-up of resistance has been very rapid. During 2003 there was a considerable increase in the incidence and severity of strobilurin resistance in regions like northern France, southern Denmark and northern and other parts of Germany.

Indeed, it would appear that wherever septoria tritici is a formidable disease in wheat, and where strobilurins are used, resistance is now prominent. It now seems that the pace of resistance spread and prominence is an indication of how relatively little science knows about this disease.

Monitoring in 2003 showed that samples with zero-resistance are now relatively rare in countries where wheat is produced. But still, Kristin Stoermer does not believe that the production of ascospores per se could spread the disease resistance as rapidly as mildew resistance moved. Still, this is what happened.

Part of the reason for this belief is that ascospores are not thought to move nearly as far as mildew spores in the wind. A second part is the relatively slow turnaround post-infection compared with mildew. But there is also a lack of understanding as to how a mitochondria-based allele behaves during standard cell reproduction and multiplication.

Resistance measurement

With so much importance now placed on the new genetic fingerprinting QPCR technology for resistance detection, it is interesting to note that there are now many questions arising. Scientists believe that there is a high degree of variability associated with the QPCR technology. This may not be a result of the technology itself but rather of the methodology used.

Questions like how many leaves, which leaves, how many pycnidia, the consequence of disease pressure, etc. According to Kristin, variability is highly significant when resistance levels in samples are greater than 50%. However, at resistance levels of 0-20%, the technology gave very repeatable results. This is probably not surprising as the technology was really designed to help detect very low levels of resistance. It was designed to find the needle in the haystack, not the metal in a scrap yard.

The challenge now is to find the relevance between QPCR results and field performance of strobilurins. It is generally accepted that at greater than 30% resistance the strobilurins are of little practical benefit for septoria control. We are at this level. But before this, it is important to acknowledge that individual field resistance results are of little value in decision making. The feeling is that a very large sample is needed - a country or a regional result - to get a handle on practical resistance level.

A transition period

The past year has been very much about learning to live with strobilurin resistance. In this respect, we are now very much in a transition period as we go back in time to meet our future techniques. The main response to strobilurin resistance was to depend more on triazoles. Amongst the lessons of 2003 was the fact that triazoles seem quite rate-responsive and that mixtures of triazoles appeared to perform better where the relative total dose was increased. Use of chlorothalonil also increased in 2003.

But now we may well have a new dilemma. Serious concern is being expressed with regard to the possibility of altered sensitivity to triazole fungicides.

If this happens, it could herald the end of grain production in this country. So we have a vital interest at stake and we must begin to wake up to possibility of serious triazole resistance.

Individual triazole actives have slightly different modes of action and in the past it was often suggested that mixtures or sequences of different triazoles would help protect the family. However, Kristin Stoermer disputes this, stating that mixtures of triazoles do not constitute an anti-resistance strategy. While all commentators suggest that any change in the sensitivity of triazoles is likely to be much slower, one must remember that the scientists got it wrong in the past and very recently also.

Triazole efficacy

There is some evidence about that disease resistance is becoming an issue for some triazole actives. One company told us earlier in the year that its triazole was now much more sensitive in Ireland following the intensive use of strobilurins. Limited work from France suggested that products such as difenconazole and triticonazole had shown considerable shifts also.

However, BASF monitoring in many countries, including Ireland, suggests that there has not been any lessening in the efficacy of epoxiconazole since its introduction. Monitoring at Oak Park has confirmed this. This may have been helped in the past by the use of strobilurins, but now it is much more significant in the market once again. This potential threat is made all the more real by the introduction of even more effective triazoles to the market.

This comment is more related to rhyncho, which is another very serious disease for Irish producers. If we introduce new products with high curative activity it seems inevitable that we will exert more selection pressure on any fungus.

The effect of product application rate on the build-up of strobilurin resistance remains unknown. However, there seems to be general agreement that reduced rates of triazole fungicides would increase the likelihood of a population shift.