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Warning to avoid over-reliance on Rlm7 phoma resistance gene

A new study by University of Hertfordshire researchers has discovered an important source of gene resistance against one of the leading pathogens of oilseed rape, phoma stem canker, is becoming less effective, which could lead to substantial losses to the OSR breeding industry and to arable farmers in the UK.


Marianne   Curtis

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Marianne   Curtis
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Results from a research project looking at host resistance to phoma (L. maculans) in OSR have led to recommendations important genes conferring resistance to the disease, such as Rlm7, should be rotated in terms of time and geographical location to prolong their use.

 

Project leader Prof Bruce Fitt, University of Hertfordshire, says: “If you put the same gene in all the varieties you put extra pressure on pathogens to become virulent.”

 

In the project, 3% of phoma isolates were found to be virulent against Rlm7 in the 2012/13 cropping season at a time when OSR varieties with Rlm7 represented about 5% of the UK OSR area. This was the first report of phoma populations virulent against the Rlm7 gene in the UK.

 

Breeding companies estimated the use of the Rlm7 gene in commercial crops in the UK had increased to 20% of the OSR area by 2016/17, according to the report.


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Gene loss risk

 

The risk of losing the use of this important resistance gene is highlighted by the situation in France. Currently, 50% of the French OSR area is sown with Rlm7 varieties and 20% of the phoma population is now virulent against this gene after 10 years of its use in commercial varieties there, says the report.

Resistance breakdowns such as this have led to organisations equivalent to AHDB in France, Australia and Canada to publish schemes for how resistance genes are deployed, says Prof Fitt.

 

“I recommend it is done [in the UK]. It would not be difficult. AHDB could test all varieties on the RL for what resistance genes they have in them,” he says.

One reason the UK has not experienced such a significant host resistance breakdown as some other countries could be due to quantitative resistance (QR) in the background, usually controlled by several genes which contribute, each with variable effect, to resistance to a pathogen.

Recommendation

 

“I would recommend continuing to breed QR into varieties, this would make them more durable. A combination of Rlm gene resistance and QR in OSR is a primary objective for the breeding industry and should be the basis for disease management strategies.”

 

The researchers also found that environmental factors, such as temperature and rainfall, can affect the severity of phoma stem canker on OSR and have shown, for the first time, that commercial cultivars with combined resistance (qualitative R gene-mediated and quantitative resistance) are less sensitive to a changing environment and therefore better at protecting the oilseed rape plants from phoma stem canker.

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