Breeding the Best OSR Disease Resistance

13-Nov-2014

Advanced hybrid breeding is allowing the most complete and robust disease resistance to be built into modern oilseed rape production by Europe’s foremost plant breeding programme. And it is doing so far more rapidly than has ever been possible with pure-line breeding.
 

“Growers tell us the single most important thing for them is yield stability,” stressed Dekalb OSR breeder, Laurent Verdier at a specialist briefing at the company’s OSR breeding headquarters at Boissay in the Paris basin. “So, our efforts are directed as much to safeguarding oilseed rape yields in practice as to increasing overall yield potential.

“Alongside vigorous establishment, winter hardiness and pod shatter resistance, we prioritise comprehensive resistance to the most damaging OSR diseases in everything we do. Phoma stem canker is our Number One priority across Europe, with light leaf spot an additional focus in the UK and verticillium in Germany and Scandinavia.

 

“More than 25 years of breeding and selection in areas of high disease incidence across Europe with no fungicide treatment whatsoever gives our hybrids a unique advantage.  It means we only ever develop parents and hybrids from lines that can withstand the greatest disease pressures.

“At the same time, we’re harnessing the in-depth experience of our breeders and the latest marker technologies to unravel and take full advantage of the genetics of disease resistance and the many genetic and environmental interactions that determine its expression and robustness.”

A total of nine major RLM (Resistance to Laeptosphaeria Maculans) genes to phoma stem canker have, for instance, been identified in oilseed rape to date.  Many of these are known to exist in different forms – related to the specific shape of the proteins involved in the ‘lock and key’
gene-for-gene resistance mechanisms with which they are involved.

 

While almost all the major genes have been relatively rapidly overcome by the constant mutation of the pathogen, the Dekalb team have concentrated their phoma stem canker resistance work on one specific form of Rlm 7 which has proved remarkably robust despite its widespread commercial use across Europe since 2004.
“Research suggests it only takes a single mutation in the corresponding avirulence gene of the pathogen to render major resistance genes like Rlm 4 and Rlm 9 ineffective,” he explains. “So there’s a high risk of breakdown in the field.  In contrast, the form of Rlm 7 we’ve been working with seems to require far more than a simple mutation in the pathogen to overcome.

“It also appears that any such change has a severe fitness cost to the pathogen. Which may be why, although we can sometimes observe phoma lesions on our Rlm7 material, they are restricted to old and senescing tissue.  This means any breakdown will have only minor consequences in practice.”

 

Even so, with varietal stability their over-riding aim, Laurent Verdier and his team have been taking the greatest possible advantage of their hybrid breeding resources to reinforce Rlm7 protection with polygenic resistance from the large number of minor genes they also know have an affect on other aspects of phoma stem canker growth and development.
 

He likens building minor gene resistance to steadily increasing the height and strength of the embankments along an estuary while major resistance genes lower the water level. That way any unforeseen tidal surges won’t breach the defences.

 

Just as the stronger embankments of polygenic resistance protect against tidal surges, of course, lowering the water level with robust major gene resistance also give the embankments extra protection by reducing the pressure placed upon them.

 

“Adding the RLM7 and polygenic resistance together to make the double phoma resistance we have in our ‘DK’ varieties provides the highest level of yield protection,” stresses Laurent Verdier. “What’s more it can only really be achieved through modern hybrid breeding. In pure lines it remains almost impossible to know what strength in depth you actually have behind any major gene resistance because the effect is total, preventing you screening for under-lying polygenic resistance.