Wheat stem rust recorded for the first time in decades in Ireland

A portable, nanopore-based genotyping platform for near real-time detection of Puccinia graminis f. sp. tritici lineages and fungicide sensitivity

BACKGROUND

Fungal plant disease outbreaks are increasing in both scale and frequency, posing severe threats to agroecosystem stability, native biodiversity and food security. Among these, the notorious wheat stem rust fungus, Puccinia graminis f.sp. tritici (Pgt), has threatened wheat production since the earliest days of agriculture. New Pgt strains continue to emerge and quickly spread over vast distances through the airborne dispersal of asexual urediniospores, triggering extensive disease outbreaks as these exotic Pgt strains often overcome resistance in dominant crop varieties of newly affected regions. This highlights the urgent need for a point-of-care, real-time Pgt genotyping platform to facilitate early detection of emerging Pgt strains.

RESULTS

In this study, we developed a simple amplicon-based re-sequencing platform for rapid genotyping of Pgt isolates. This system is built around a core set of 276 Pgt genes that we found are highly polymorphic between Pgt isolates and showed that the sequence of these genes alone could be used to accurately type Pgt strains to particular lineages. We also developed a simplistic DNA preparation method and an automated bioinformatic pipeline, to enable these Pgt gene markers to be sequenced and analysed rapidly using the MinION nanopore sequencing device. This approach successfully enabled the typing of Pgt strains within approximately 48 h of collecting Pgt-infected wheat samples, even in resource-limited locations in Kenya and Ethiopia. In addition, we incorporated monitoring capabilities for sequence variations in Pgt genes that encode targets of the azole and succinate dehydrogenase inhibitor fungicides, enabling real-time tracking of potential shifts in fungicide sensitivity.

CONCLUSION

The newly developed Pgt Mobile And Real-time, PLant disEase (MARPLE) diagnostics platform we established, now allows precise typing of individual Pgt strains while simultaneously tracking changes in fungicide sensitivity, providing an early warning system for potential indicators of changes in the Pgt population and emerging fungicide resistance. Further integration of this Pgt MARPLE diagnostics platform into national surveillance programmes will support more informed management decisions and timely responses to Pgt disease outbreaks, helping reduce the devastating crop losses currently caused by this 'cereal killer'.

The threat from ozone to vegetation in Ireland

Ozone is the most damaging air pollutant to vegetation globally. Metrics of accumulated ozone above a concentration threshold (e.g. AOT40, ppb·h) have been widely used to assess ozone risk. However, there is growing consensus that accumulated Phytotoxic Ozone Dose (POD) above a receptor-specific critical stomatal flux threshold (y; nmol O3 m-2 s-1), expressed per unit of projected leaf area, provides a more reliable risk assessment, as it considers ozone entering the leaf (PODy, mmol m-2 leaf area). Few studies have assessed both concentration- and flux-based metrics using site-specific observations of ozone and meteorology. In this study we assessed the risk that ozone poses to five vegetation types across eight sites in Ireland during 2005-2021, using AOT40 and PODy risk metrics, and we predicted impacts using dose-response relationships. Long-term trends in both metrics were also assessed. The PODy critical level for vegetation protection was exceeded for all vegetation types, with exceedances most common at Atlantic coastal sites, and for tree species (beech POD1 15.7-25.7 mmol/m2 PLA). When PODy and AOT40 results were normalised based on their respective critical levels, predicted impacts were higher for PODy. There were significant increases in PODy for three vegetation types at rural sites during the study period, which also experienced increases in temperature and global solar radiation. The long-term trends were consistent with other European studies that show decreases in AOT40 and increases in PODy. While ozone concentrations in Ireland are relatively low (39-75 μg/m3 five-year average range), the humid climate and longer growing season may lead to elevated stomatal ozone uptake and thereby a risk to vegetation.

Mapping rust resistance in European winter wheat: many QTLs for yellow rust resistance, but only a few well characterized genes for stem rust resistance

KEY MESSAGE

Stem rust resistance was mainly based on a few, already known resistance genes; for yellow rust resistance there was a combination of designated genes and minor QTLs. Yellow rust (YR) caused by Puccinia striiformis f. sp. tritici (Pst) and stem rust (SR) caused by Puccinia graminis f. sp. tritici (Pgt) are among the most damaging wheat diseases. Although, yellow rust has occurred regularly in Europe since the advent of the Warrior race in 2011, damaging stem rust epidemics are still unusual. We analyzed the resistance of seven segregating populations at the adult growth stage with the parents being selected for YR and SR resistances across three to six environments (location-year combinations) following inoculation with defined Pst and Pgt races. In total, 600 progenies were phenotyped and 563 were genotyped with a 25k SNP array. For SR resistance, three major resistance genes (Sr24, Sr31, Sr38/Yr17) were detected in different combinations. Additional QTLs provided much smaller effects except for a gene on chromosome 4B that explained much of the genetic variance. For YR resistance, ten loci with highly varying percentages of explained genetic variance (pG, 6-99%) were mapped. Our results imply that introgression of new SR resistances will be necessary for breeding future rust resistant cultivars, whereas YR resistance can be achieved by genomic selection of many of the detected QTLs.

Resurgence of wheat stem rust infections in western Europe: causes and how to curtail them

Ten years ago, (black) stem rust - the most damaging of wheat (Triticum aestivum) rusts - re-emerged in western Europe. Disease incidences have since increased in scale and frequency. Here, we investigated the likely underlying causes and used those to propose urgently needed mitigating actions. We report that the first large-scale UK outbreak of the wheat stem rust fungus, Puccinia graminis f. sp. tritici (Pgt), in 2022 may have been caused by timely arrival of airborne urediniospores from southwest Europe. The drive towards later-maturing wheat varieties in the UK may be exacerbating Pgt incidences, which could have disastrous consequences. Indeed, infection assays showed that two UK Pgt isolates from 2022 could infect over 96% of current UK wheat varieties. We determined that the temperature response data in current disease risk simulation models are outdated. Analysis of germination rates for three current UK Pgt isolates showed substantial variation in temperature response functions, suggesting that the accuracy of disease risk simulations would be substantially enhanced by incorporating data from prevailing Pgt isolates. As Pgt incidences continue to accelerate in western Europe, we advocate for urgent action to curtail Pgt losses and help safeguard future wheat production across the region.

Wheat Stem Rust Back in Europe: Diversity, Prevalence and Impact on Host Resistance

The objective of this study was to investigate the re-emergence of a previously important crop pathogen in Europe, Puccinia graminis f.sp. tritici, causing wheat stem rust. The pathogen has been insignificant in Europe for more than 60 years, but since 2016 it has caused epidemics on both durum wheat and bread wheat in local areas in southern Europe, and additional outbreaks in Central- and West Europe. The prevalence of three distinct genotypes/races in many areas, Clade III-B (TTRTF), Clade IV-B (TKTTF) and Clade IV-F (TKKTF), suggested clonal reproduction and evolution by mutation within these. None of these genetic groups and races, which likely originated from exotic incursions, were detected in Europe prior to 2016. A fourth genetic group, Clade VIII, detected in Germany (2013), was observed in several years in Central- and East Europe. Tests of representative European wheat varieties with prevalent races revealed high level of susceptibility. In contrast, high diversity with respect to virulence and Simple Sequence Repeat (SSR) markers were detected in local populations on cereals and grasses in proximity to Berberis species in Spain and Sweden, indicating that the alternate host may return as functional component of the epidemiology of wheat stem rust in Europe. A geographically distant population from Omsk and Novosibirsk in western Siberia (Russia) also revealed high genetic diversity, but clearly different from current European populations. The presence of Sr31-virulence in multiple and highly diverse races in local populations in Spain and Siberia stress that virulence may emerge independently when large geographical areas and time spans are considered and that Sr31-virulence is not unique to Ug99. All isolates of the Spanish populations, collected from wheat, rye and grass species, were succesfully recovered on wheat, which underline the plasticity of host barriers within P. graminis. The study demonstrated successful alignment of two genotyping approaches and race phenotyping methodologies employed by different laboratories, which also allowed us to line up with previous European and international studies of wheat stem rust. Our results suggest new initiatives within disease surveillance, epidemiological research and resistance breeding to meet current and future challenges by wheat stem rust in Europe and beyond.