EU blackleg becomes harder to manage as genetic tools fail

Georgia Mitrousia of the University of Hertfordshire has shown that a major gene resistance, resistant gene Rlm7, is breaking down for U.K. blackleg in oilseed rape.  |  Chris McCullough photo

Farmers in the United Kingdom could be facing trouble with rapeseed as new research warns of a resistance gene at risk in the crop.

Oilseed rape is the third most important crop in the U.K., with a total of 2.2 million tonnes harvested in 2017. It is the third largest source of vegetable oil worldwide and with meal being widely used as animal feed.

However, there are three main diseases, all with a worldwide distribution, which affect oilseed rape, or canola, which cause around 15 per cent crop yield losses worldwide each year. One of these, phoma stem canker, known in North America as blackleg, causes losses to U.K. farmers of more than C$150 million annually.

Researchers at the University of Hertfordshire in the U.K. have discovered that an important source of gene resistance against one of the leading pathogens of oilseed rape, namely phoma stem canker, is becoming less effective, which could lead to substantial losses to the oilseed rape breeding industry and to arable farmers in the U.K.

One of the lead researchers in this project, Georgia Mitrousia, said, “Oilseed rape cultivation has been increasing in the U.K. in recent years and is of vital importance for the U.K.’s agriculture industry, and the wider U.K. economy.

This is the first study showing that the host resistant gene Rlm7 has become less effective at controlling the phoma stem canker pathogen Leptosphaeria maculans in the U.K. and is of great importance to plant breeders and farmers.

When host resistance is rendered ineffective by changes in the pathogen population there is an estimated cost of $9 million per year to the breeding industry through the loss of commercially available cultivars.

As there has been a withdrawal of the effective fungicides used to control phoma stem canker, farmers have been relying more on cultivars that have good resistance against the pathogen causing the disease.

Phoma stem canker is caused by two closely related fungal plant pathogens, Leptosphaeria maculans and L. biglobosa. L. maculans is generally considered to be more aggressive. Host resistant genes in the cultivars (Rlm genes) operate against specific pathogen races and one important gene that has been bred into many commercial oilseed rape cultivars in the U.K. is the Rlm7 gene.

“This study acts as a warning to the oilseed rape industry and they will hopefully develop strategies to prevent the loss of commercially available cultivars.

“This includes combining Rlm gene resistance with quantitative resistance in the same cultivar and developing strategies to rotate development of different Rlm genes to ensure consistent harvest of oilseed rape for many years to come,” she said.

Blackleg as North American farmers know it, phoma stem canker in Europe, defence with a major gene is breaking down and could cost farmers there significantly. | Chris McCullough photo

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