A deadly, mutating wheat fungus is spreading – here’s how to track it

Yellow Rust spores can be seen bursting out of a wheat leaf from the inside, tearing their way through the epidermis. Kim Findlay/John Innes Centre, CC BY-NC-SA

One of the major diseases of wheat is caused by the yellow rust fungus, Puccinia striiformis, which threatens all major wheat-producing areas of the world. Ominously, we have discovered that the UK population of this pathogen is shifting dramatically, with the emergence of new strains which can overcome some of our most important wheat varieties.

We have developed a genetic technique that helps us characterise the pathogen, allowing farmers to make informed decisions about which wheat varieties to plant.

Wheat is a critical staple crop, providing 20% of the calories and 25% of the protein consumed globally by humankind. Despite modern agricultural practices, diseases of major food crops can cause pre-harvest yield losses of up to 15%.

In 2013, I joined forces with colleagues at the John Innes Centre and The Sainsbury Laboratory in Norwich with the National Institute of Agricultural Botany in Cambridge to develop a new genomics-driven surveillance method to track the devastating yellow rust fungus and investigate the genetic basis of the new pathogen population.

Our new “field pathogenomics” method is a fast way to analyse fungal diseases from field samples and pinpoint the exact genotype. Current techniques rely on time-consuming phenotypic characterisation – checking the response of different plant varieties to infection by the pathogen – or costly in-lab processes. These methods can only sample a relatively small proportion of the fungal population.

With help from contributors to the UK Cereal Pathogen Virulence Survey, we collected wheat samples infected with the wheat yellow rust pathogen from 17 different counties across the UK. We then used our newly developed “field pathogenomics” method to characterise the genotypes of the samples. As each field sample consists of both the pathogen and its host plant, we were able to analyse both the pathogen and the susceptible host. In the future, this will provide a rapid means for confirming the presence of disease on wheat varieties that may have previously been resistant to disease.

Under siege: wheat is a major global crop, but threatened by fungal pathogens. Takkk, CC BY-SA

Invasive pathogens

We found that the wheat yellow rust pathogen population has undergone a major shift in recent years. Interestingly, the yellow rust population detected in the UK in 2013 was completely different at the genetic level to previous UK populations. This difference seems to represent a number of recent exotic introductions into the UK and could have serious implications for wheat production in the UK.

A subset of the new pathogen population was able to infect the same wheat varieties as a subset of the older UK pathogen population. Because the same varieties are infected, this new pathogen population would have been missed if analysis were based on traditional phenotypic characterisation alone. Spotting this new pathogen population is important, because even if it infects the same wheat varieties it could still have serious implications for disease incidence. The new pathogen population may have other important traits or infect other wheat varieties not included in our test set.

As we move forward, “field pathogenomics” could be applied to the surveillance of many pathogens besides wheat yellow rust pathogens, and could contribute to addressing human, animal and plant health issues. Such detailed knowledge of shifts in pathogen populations is important for both understanding and managing emerging diseases. For wheat yellow rust, our new technology could provide farmers with early indications of changes in the pathogen population, and have a positive impact on decisions regarding which varieties to plant in the field.