Mixing wheat cultivars: An eco-friendly way to curb the spread of disease

Adrien Vial, a doctoral student at the Department of Plant Breeding, has spent many hours outdoors observing wheat infected with tan spot disease in spring wheat. It is a fungal disease that appears as brown or black spots on the leaves.

– Spores land on the leaf, infect it, reproduce, and send out more spores to colonize the entire field. The disease reduces yields because it impairs the plant's ability to photosynthesize, says Adrien Vial.

During a field trial, he studied how the disease spread from plant to plant, row by row, for several weeks. A positive memory, he says, that made the theory behind the project concrete.

– There are things you learn in the classroom, like growth stages and disease development. But when you're standing out there, you become more curious, more engaged, he says.

Cultivar mixtures as protection against tan spot

The disease is prevalent in Europe, and outbreaks have been reported in Scandinavia.

– There is concern that it may become more common here in the future, he says.

Today, the disease is mostly managed with fungicides. Adrien Vial's project explores an alternative method based on ecological principles: cultivar mixtures, growing several varieties of wheat together in the same field.

– The basic idea is to use within-field wheat diversity to reduce disease spread. Cultivars' diversity leads to trait diversity, complementing one another to improve the resilience and productivity of the system.

The complementarity is important because it is practically impossible to develop a single cultivar that is durably resistant to every disease. 

– Most cultivars can effectively resist one or maybe a few diseases. Mixing cultivars that each specialise in resistance to a specific disease minimises the farmer’s risk of losing their entire harvest, he says.

Investigates how the method works

Cultivar mixtures have already proven effective against foliar diseases, but researchers struggle to accurately predict how it works in terms of plant-to-plant interactions. Many different mechanisms come into play.

One mechanism is that resistant cultivars act as a barrier, preventing spores from reaching non-resistant cultivars.

– Another way plants interact in mixtures is through their hormonal signaling. If a plant gets infected, it triggers its immune system and sends signals that spread to other plants, telling them to switch on their immune defenses. The leading theory is that this happens through volatile hormones and root exudates, a kind of secretion from the roots.

Together with his colleagues, Adrien Vial is working to better describe the mechanisms. The group combines quantitative genetics, plant pathology, microbiology, and ecological theory. 

The goal is to be able to give clear recommendations to farmers and contribute to knowledge development within plant breeding for disease resistance. But he is careful to maintain a critical approach to the method.

– It may turn out that we understand how cultivar mixture works, but that the impact on disease and yield is so small that it isn't reasonable to recommend it to farmers.

Hopes for a larger experiment

The field trial Adrien Vial oversaw consisted of small one-square-meter plots. His dream scenario would be to design a much larger experiment. Disease spread dynamics are better seen at larger scales, and genetic diversity is better explored with large populations.

– There's one question I would love to find the answer to, he says.

He wants to know which parts of the plant's genome are responsible for the synergies of cultivars in mixtures, or, what genetically makes a good mix.

– An experiment like that doesn't seem feasible at the moment. But if I could answer that question, I would be able to tell farmers which varieties to mix based on their genomes. That would be really interesting to know, says Adrien Vial.