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Perhaps one of the most recognizable scenes on the Canadian Prairies is fields upon fields of golden wheat. Long a symbol of agriculture in the Prairies, wheat is Canada’s largest field crop, with an average of about 30 million tonnes harvested annually. That’s about seven times what Canadians typically consume, and much of our wheat is destined to feed families around the world. Highly valued worldwide for its high quality and protein content, our wheat is exported to approximately 80 countries, making it an important economic crop. So, we must ensure wheat crops in Canada continue to thrive.
Like any type of plant, wheat faces harm from disease. As one of our earliest and most iconic cereal crops, researchers have long been exploring new breeds and ways to protect this important food staple. In fact, Canada has a rich history of studying wheat, all the way back to the late 1800s.
Spotting the problem: the worldwide emergence of tan spot disease
One of the more recently discovered diseases that threaten our wheat crops is a fungal infection called tan spot disease. In infected crops, if you look closely at the green leaves of wheat, you may see tan-coloured lesions, or spots, covering the leaf surface. These lesions are symptoms of infection by the fungus Pyrenophora tritici-repentis. The fungus is a specialized necrotroph (a parasite that rapidly kills its host and feeds on the dead tissues), and since its emergence about 50 years ago it has become one of the most destructive leaf diseases of wheat worldwide.
Like most fungi, the fungus that causes tan spot thrives in wet conditions. While spring rain helps wheat grow, it may also be creating the right conditions for infection. The fungus will release its spores, spreading the disease across fields, infecting crops and causing significant damage.
If left unmanaged, severe tan spot infection can drastically damage the foliage (leaves) of the wheat crop. As in most plants, the leaves on wheat capture light and convert it into the chemical energy that helps the plant grow. It sounds simple, but for those who remember science class in school, you may recall that it’s actually a very complex process called photosynthesis. The leaf damage that tan spot causes impacts the wheat plant’s ability to photosynthesize, stunting its growth and decreasing yield and grain quality. If the damage occurs on the upper leaves, it has the potential to reduce yield by 60% or more. In addition, if the pathogen progresses from the leaf to the wheat head, it can cause further damage known as red smudge – a reddish discolouration of the grain which can reduce its grade quality and value. This makes tan spot a serious economic threat to wheat production, with potential losses estimated at millions of dollars annually.
Most growers use fungicides to control tan spot, but given the danger this disease poses, more work must be done to identify effective management strategies and cultivate new, resistant wheat varieties.
A team of researchers at AAFC is leading a national and international collaboration (with Europe, North and South America, North Africa, India, Japan, and Australia) to understand how the disease has evolved globally – and how we might be able to slow its progression. This diverse team of experts is lead by Dr. Reem Aboukhaddour of the Lethbridge Research and Development Centre, also collaborating locally with Dr. Kelly Turkington of the Lacombe Research and Development Centre and Dr. Myriam Fernandez of the Swift Current Research and Development Centre.
Getting to the (genetic) root of the issue
The team is analyzing the pathogen’s entire genetic makeup and changes in its chromosomes using multiple areas of science including: pathology, the study of disease; genomics, the study of an organism's genetic material; and bioinformatics tools to help understand the complex biological data sets. The focus is understanding how disease-causing genes have shifted and moved across various strains of the pathogen. The team is also looking at how these genes vary in different populations globally, and which local variables or conditions can cause the fungus to survive and reproduce.
“It’s so important to get a clear picture of how the pathogen that causes tan spot disease is evolving. This will help us spot trends, environmental conditions and other factors that enable the disease to thrive, and hopefully discover solutions that help manage it effectively,” says Dr. Aboukhaddour.
Researchers have identified mutations, duplications, and transposons as key factors driving the evolution of this fungus. Transposons – sometimes called “jumping genes” – are pieces of DNA that readily move around within the genome or across different genomes, carrying disease-causing genes as they go. Finding a novel class of transposons from which this fungus spreads its virulence was an important discovery and may help researchers unlock the key to controlling the disease. The team also found unusual new forms of the pathogen in various places around the world – including Canada – that cause increased damage to durum wheat. Ongoing research aims to identify virulence factors in these new forms, helping researchers find ways to effectively control the disease in wheat crops.
Laying the path forward
While research continues, the team has already provided invaluable resources to the research community, such as fully sequenced genomic data of this pathogen using a diverse set of P. tritici-repentis fungus from Canadian and worldwide sources. Scientists hope that this AAFC research will one day lead to solutions that could reduce the incidence of tan spot in wheat fields around the world – ultimately helping farmers protect their fields from the damaging disease and severe economic threat.
Key facts
- Canadian wheat is highly valued worldwide for its high quality and protein content. It’s also exported to about 80 countries, making it an important economic crop.
- Tan spot is a fungal infection that damages the leaves of wheat, affecting the plant’s ability to photosynthesize, stunting its growth and decreasing yield and grain quality.
- Since its emergence about 50 years ago, tan spot disease has become a serious economic threat to wheat production, with potential losses estimated at millions of dollars annually.
- AAFC researchers, lead by Dr. Reem Aboukhaddour, are conducting a national and international collaboration to understand how the disease has evolved globally – and how we might be able to slow its progression.
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