Researchers at the John Innes Centre in Norwich have solved a decades-old that has confounded wheat breeders.

A newly published paper from JIC researchers Professor James Brown and Dr Lia Arraign reveals why breeders have found it difficult to produce wheat varieties which combine high yield and good resistance to Septoria, a major disease in wheat which can cut yield losses by up to 50%. the paper traces the problem back to decisions made nearly sixty years ago.

Septoria infection is characterized by the growth of necrotic brown lesions on wheat leaves, which lead to a significant reduction in crop yield and quality. The fungal infection is a major crop disease in temperate climates throughout the world, so much so that resistance to Septoria is now categorized as being of very high importance as a trait on the AHDB Recommended List of Wheat Varieties. In fact, it’s judged to be the most destructive disease affecting wheat in the UK.

Septoria emerged as a serious threat to wheat in the 1970s. Since then, the fungus has become insensitive to most fungicides, meaning demand for new Septoria resistant wheat varieties has risen sharply, but progress in breeding new varieties which combine high yields with resistance to Septoria has been slow.

For their paper, Professor Brown and Dr Arraiano analysed resistance and susceptibility to Septoria in wheat varieties grown in the UK between 1860 and 2000. Using a technique called association genetics, they found that the gene with the biggest effect on increasing susceptibility to Septoria is very closely linked to one that increases yield and grain size.

Professor Brown said: "As we studied a historical set of varieties covering more than a century of wheat breeding, we discovered where the small region of the genome that increases both Septoria and yield came from. We traced it back to a variety called Heines Peko, which was used to breed for yield and rust resistance in the late 1950s."

Heines Peko was crossed with Cappelle Desprez, the major wheat variety in Britain at the time. This cross was so influential that all modern wheats bred in Britain are descended from it. Professor Brown suggests that as wheat breeders selected ever more strongly for higher yield, susceptibility to Septoria hitch-hiked along with it.

"My group is now trying to find out if the connection between the two traits can be broken" he added. “It's surprising that a decision made so long ago has had such a long-lasting effect.”

Increased yield is the main driver of the market for new varieties, but Professor Brown points out that although Septoria resistance has improved in recommended varieties over the last ten years, the lack of knowledge about the relationship between yield and susceptibility to Septoria has hindered progress.

Groups contributing to maintaining seed banks and food policy experts have warned of the dangers presented by the modern food system, in which most major crops are genetically very similar. They warn that more attention needs to be paid to crop wild relatives (wild plants from which modern crops were domesticated, and which in many cases could hold the key to providing resistance to pests and disease, changing climates or improving nutrition), as many of these are threatened in their natural habitats.

Commenting on the breakthrough, Ed Flatman, Head of European Wheat Breeding at Limagrain said, "Professor Brown and Dr Arraiano's work was one of the very first applications of association genetics in wheat and it has helped us to understand the past history of breeding for resistance to Septoria. We have now taken James' results and built on them by identifying novel Septoria resistances in current, high yielding elite varieties."

Dr Arraiano and Professor Brown's research points to a way of rising to the challenge of combining yield and Septoria resistance. They found ten other genes scattered throughout the genome with smaller effects on Septoria. They also found that nearly half the variation in Septoria was controlled by genes with effects that were too small to identify individually. "We know the genes are there," said Professor Brown, "but we don't know where they are."

Nevertheless, Prof Brown is confident that breeders can make advances in Septoria resistance with the knowledge they have. "When breeders make crosses between diverse varieties, they produce new combinations of genes with small effects," he said. "Then if they run field trials at sites where Septoria is really rampant, they can spot the most resistant lines to commercialise and to use in the next generation of breeding."

He believes this approach will enable breeders to improve Septoria resistance, while minimising undesirable side-effects, such as reductions in yield.

Professor Brown added, "With the pressures of restrictions on pesticide use added to Septoria becoming insensitive to most fungicides, farmers need varieties which combine yield and quality with resistance to Septoria and other diseases. Our breakthrough should accelerate progress in developing these new varieties."