Around this time each year, as the combines are parked up for another season, cereal farmers turn their attention to the next year’s crop. Undoubtedly, the biggest farming decision they will make every year is what crop to grow. As most now utilise a crop rotation, this decision can be relatively easy to make. Following this, they must however decide on what variety of their chosen crop they should plant. The correct decision can leave the agronomy for the following season easy; the wrong decision and they could have a lot of sleepless nights from February through to August. Thankfully, to aid these decisions, evaluation trials are annually conducted to identify varietal strengths and weaknesses.
As disease, in particular septoria tritici blotch (STB), is a key constraint to winter wheat production in North Western Europe; the ability of wheat varieties to resist STB is an extremely important trait under evaluation each season. Due to the nature of Zymoseptoria tritici, the fungal pathogen responsible for STB, breeding for durable STB resistance is an extremely difficult task. Thankfully, over the past two decades, improvements have been made with a combination of minor and major sources of resistances identified and eventually making their way into the varieties now grown on farm. However, as with all living things, the interactions between the plant (wheat) and the pathogen (Z. tritici) is dynamic, and subtle changes to either can tip the balance in favour of either.
Unfortunately, in July 2020 there was an indication that such changes may have occurred in the Irish Z. tritici population. In evaluation trials conducted close to the South coast of Ireland, foci of infection were observed in plots of a number of close-to-market varieties. Although it is not unusual to see STB in plots of wheat in Ireland, these foci stood out. Equally, they appeared to be most dramatic in varieties that had a similar pedigree, specifically; the presence of the variety Cougar, either as a direct parent or as grandparent. A quick look at the different varieties currently under evaluation and their pedigree quickly highlighted that this needed further investigation and quick!
Unfortunately, analysis under controlled conditions confirmed the virulence of these strains of Z. tritici, isolated from Cougar in the field, to Cougar in the glasshouse and to a panel of varieties bred from it. The breakdown of STB resistance in Cougar is not a new finding; strains of Z. tritici virulent to it were identified in the U.K. population following its commercialisation less than a decade ago. Fortunately, at the time (2015-2017), the virulence exhibited by these strains was specific to Cougar and they posed no greater threat to the majority of varieties grown around the same period, which did not have Cougar in their background (Caiazzo et al. 2019).
Whilst all indications are that a similar virulence profile exists for the strains now being detected, we now have an increasing number of varieties, either on the market or coming to market, relying on or depending heavily on the STB resistance conferred by Cougar. How each of these reacts to these strains will be determined, not just by the presence of Cougar, but also by what levels of STB resistance the other parent(s) may bring. For those varieties now available, the recommendation from an Irish perspective is that: in high disease pressure environments, such as the Southern half of Ireland, the disease risk posed may be too high and farmers should consider alternative varieties. In lower disease pressure environments farmers must be aware of the risk and adjust their agronomy accordingly, including ensuring all measures of integrated pest management for STB are adhered to; delaying sowing where possible, monitoring disease over the winter months, and ensuring correct fungicide application timings in spring – early summer.
Even though the detection of virulence to Cougar and the range of varieties recently bred from it in field populations of Z. tritici is unfortunate, and will cause headaches in the short term, we can have confidence that systems of evaluation are performing and can help farmer’s decisions on what varieties to grow. It equally reinforces the need to ensure a diversity of resistance sources are utilised against the ever-evasive pathogen that is Z. tritici. The debate on how this can be achieved, whether by stacking sources or mixing varieties will of course be the logical next step.
Steven Kildea, Liam Sheppard, Mladen Cucak and Fiona Hutton published this study in Plant Pathology:
Detection of virulence to septoria tritici blotch (STB) resistance conferred by the winter wheat cultivar Cougar in the Irish Zymoseptoria tritici population and potential implications for STB control.
TITLE IMAGE: Wheat trials at Teagasc, Oak Park, Ireland, July 2021. All images used with permission of the authors.