The harvest is almost behind us and many will have had good experiences with most crops, in terms of yield and quality. This also means a new planting season is just around the corner so it’s time to plan again. What crops, what proportions etc.

All in all, 2017 was not a bad year. It was relatively easy to get work done at critical times and yields were good. But price is the killer. Not being able to convert good yields and quality into profit remains a threat to the future of the industry. But this is not the only threat.

Late in the season many growers spoke about the speed at which septoria came into some crops. Others report the presence of big 2-3m diameter circles in crops when harvest arrived, not having seen any evidence of these earlier in the year.

Signs since summer

At Crops & Cultivation in late June, many growers saw cause for concern in wheat plots at Teagasc that used straight SDHI actives to control septoria. At the Oak Park site, previously robust products showed disappointing septoria control. Population diagnosis indicated that strains of the fungus were present which showed reduced sensitivity to the SDHI actives. Many of you may have seen these plots and witnessed the rapid development of septoria.

At subsequent workshops conducted by Steven Kildea of Teagasc, it was obvious that SDHI products, used alone or in combinations, at different timings displayed poor septoria control late in the season. Their efficacy in plots was noticeably down on their performance in previous years.

Stephen said that the septoria population at these sites contained a high level of a moderately resistant isolate. This may have been the first time control was visually lower in a field situation.

Up to that point, no one was certain whether this specific isolate would affect field performance. But the answer was clear late in the season. The presence of high levels of septoria verified the reduced field efficacy of the SDHI actives alone. At that time, however, the performance of mixtures in well-timed programmes seemed to be doing a good job in commercial crops.

It would be expected that programmes which utilised alternation of triazole efficacy classes (epoxiconazole/propiconazole versus tebuconazole/metconazole) and included a contact active with an SDHI would have performed adequately.

The fact that the level of this moderately resistant septoria isolate has been increasing around the country is worrying. At the time Steven emphasised that good use of sensible mixtures with chlorothalonil were still performing well and commercial yields seemed to verify this at farm level.

But we must still ask what happened in some fields at the end of the 2017 season? Were we witnessing SDHI resistance and what might it mean for 2018?

A few days after I visited the wheat trials in Knockbeg I visited wheat trials in Rathangan with Tom McCabe of UCD. This trial had different treatments and possibly a different starting population of septoria.

The response to treatments there was considerably different to Oak Park and generally along the lines of what one might have expected. But even in early July spots or circles were obvious within many treated plots which looked to have quite poor control.

Field resistance

Steven emphasised that the results from all the 2017 trials will be scrutinised to formulate sound fungicide advice for 2018. But experience from trials would suggest that the game is changing, or it has changed. Was it SDHI resistance that caused either the circles or the late-season septoria control failures? The question must be asked if we have witnessed SDHI resistance, and could it mean control failure at field level?

If that was resistance at the end of 2017, it means additional risk for 2018. We do not know if these resistant types are likely to decrease in the total population over winter but it is reasonable to assume that they will not as they have been building for a few years already. But just because there was no problem in 2017 does not mean that there will not be problems in 2018. The aerial spread of septoria via ascospores post-emergence will tend to spread all elements of a population through the country and beyond.

So we must acknowledge the additional challenges that seem likely to lie ahead for the coming season. This additional risk necessitates an even greater reliance on husbandry and cultural factors to help slow disease build-up in the autumn/winter, followed by adherence to the best fungicide advice use in 2018. The main factors to consider here are:

Variety: the starting point in resistance management, but our options are limited. Rockefeller and Torp are probably our two most septoria resistant varieties but these are still only rated 7 rather than 8 or 9. However, these levels will potentially slow any initial epidemic.

Sowing date: perhaps the most influential factor for autumn infection. Crops that emerge early are more likely to be infected by the clouds of ascospores that are emitted in autumn. These can travel considerable distances to both cause infection and spread resistance. While later-sown and emerging crops will still get infected, infections that take place earlier allow additional cycles of the disease to occur in autumn to increase the physical numbers of resistant spores.

Stubbles: are the main source of ascospore production. While burying all previous trash would not necessarily prevent ascospore production or arrival, it might reduce the numbers in the air and infection is a numbers game. The more spores that arrive, the more individual infections that will occur and this may speed up the development of leaf symptoms to produce the pycnidiospores, which are spread by rain splash. The burial of all wheat stubbles in early autumn may reduce infection pressure, especially on early sown crops.

Caution is urged

The possible arrival of widespread resistance to SDHI fungicides forces real concern at farm level. While field control is still likely to be possible, we must do all we can to reduce the pressure and risk associated with big acreages of wheat on any farm.

There may be relatively little that individual growers can do except take every action to minimise disease pressure through some combination of variety resistance and later planting.

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