Rhizomania strain Anglicised earlier than thought
After re-testing historic soil samples, scientists have uncovered the presence of the so-called “new” rhizomania strain years before it was officially discovered.
It was thought the more aggressive AYPR strain of the devastating sugar beet disease hadn’t been present in the UK much before its discovery in 2007.
However, after mapping the genome of the A, B and P types of beet necrotic yellow vein virus (BNYVV), diagnostics have been developed to detect the genetic variation between strains.
This allowed scientists to revisit the historical samples of the previously notifiable disease that were taken since rhizomania was first discovery in the UK during 1987.
The Food and Environmental Research Agency (FERA) archive consisted of 33 samples from 18 different farm cases where the disease symptoms were seen.
Lead scientist at the British Beet Research Organisation (BBRO) Mark Stevens described it as a “Richard III moment”, alluding to the recent discovery of England’s notorious fifteenth century monarch.
“The samples stored by FERA at York turned out to contain a field with AYPR, in the Suffolk coastal area, from 1994,” says Dr Stevens.
“So it’s been here for at least 19 years, or possibly longer,” he adds.
Genetic resistance is the only known control method of the virus carried by a soil borne vector, polymyxa betae, which can persist in the soil for many decades.
The resting spores are stimulated by the presence of a potential host, such as sugar beet, when soils warm in the spring.
“The economics must be considered, so until the AYPR strain is confirmed on your farm there is no need to use the solutions, but it’s reassuring the tools are there to manage the disease.”
SESVanderHave general manager Ian Munnery
Symptoms usually appear in patches and include, amongst others, pale leaves and the notorious “bearding” of the root giving rise to the diease’s name after growers noted infected crops’ “crazy roots”.
Rhizomania can cause significant yield losses, and the AYPR strain is known to be devastating and can reduce root weight by up to 70% and decrease sugar content by as much as 22%.
“The AYPR stain of rhizomania is not unique to the UK and it’s probably been imported on plant material from the Continent,” Dr Stevens explains.
“The good news from this is it has been here for much longer than we thought and is still confined to a small area of the sugar beet growing regions,” he adds.
Resistance
The 2013 BBRO Recommended list was the first year that all varieties included the resistance gene, nicknamed “Holly” or Rz1, which slows the build up of A and B types effectively.
There is still a slight increase in the virus in the presence of such varieties, but the genetics allow the crop to tolerate BNYVV and produce an economically sustainable yield.
However, the AYPR strain can break Rz1 resistance and still cause the dramatic yield losses associated with the virus.
Breeders have now managed to introduce a second resistance gene, Rz2, which can be used to counter the threat of AYPR to sugar beet yields where the strain is present.
“The virus is very clever and will subtly adapt over time to overcome these resistances, so it is essential that we stay one step ahead,” notes Dr Stevens.
He advises that the first sign of any symptoms should be reported to a British Sugar area manager or the BBRO to allow the new diagnostics to identify quickly the rhizomania strain present.
Understanding the populations and where they are will assist in controlling the spread of the virus, allowing growers to move to resistant varieties.
On 2013 Recommended list there was just one variety that contained the Rz2 gene – KWS Isabella – that yielded 99.6% of control in national trials.
Breeder KWS’s sugar beet product manager Simon Witheford points out that although it isn’t the highest yielding variety, it is right in the mix with other top performers.
“It didn’t have official recognition for AYPR resistance ahead of last season, but it has been proven in tests to reduce its spread and multiplication,” he explains.
He advises growers that might be suffering a subtle yield loss from rhizomania to trial a unit (100,000) of seed amongst a conventional Rz1 variety and carry out a visual or yield assessment.
SESVanderHave general manager Ian Munnery agrees, but urges caution before adopting any double resistant varieties, as yield should be the number one priority.
“The economics must be considered, so until the AYPR strain is confirmed on your farm there is no need to use the solutions, but it’s reassuring the tools are there to manage the disease,” he adds.
Dr Stevens concludes that aside from varietal resistance, good farm hygiene is also vital, with soil and trash transfer seen as the main cause of its spread.
“It will ensure that we minimise the spread of this important disease as much as possible,” he adds.
