New, possibly more aggressive, strains of yellow rust are adding to the complexity of the disease in UK wheat crops.
The recently-identified Kranich yellow rust race pinpointed as responsible for unusual 2014 disease outbreaks in some winter wheat crops may not have been the cause of further unexpected cases in 2015, suggesting the already diverse UK yellow rust pathogen population is becoming more complex.
This is the primary conclusion from the UK Cereal Pathogen Virulence Survey (UKCPVS), which has presented its 2014/2015 monitoring results.
The arrival in the UK during 2014 of the Kranich race, named after the Scandinavian variety in which it was first detected, was recently confirmed following the collection of a disease isolate from a 2014 crop which was subsequently tested in the field in 2015, with some Recommended List and candidate varieties showing a slight sensitivity to the new race.
But results from the survey saw no further Kranich race isolates identified from yellow rust-infected wheat samples analysed from 2015 crops.
NIAB TAG’s Sarah Holdgate, UKCPVS project manager, says: “That suggests it’s highly unlikely it was Kranich which caused last season’s unusual rust observations – although it could yet be found again here.
“But it also means the actual risk posed by the new race is unknown, so we are urging wheat growers to monitor all winter wheat varieties, including those with high resistance ratings, and report abnormal amounts of yellow rust to the UKCPVS.”
During 2014-15 UKCPVS received reports of higher than expected yellow rust levels from sites in Essex, Lincolnshire, Yorkshire and Scotland, on varieties with high Recommended List (RL) disease ratings.
Dr Holdgate says: “Initially, it was a sample from a site in Scotland that showed a causal isolate carrying a combination of virulence factors never seen before by the UKCPVS, The same pathotype was subsequently identified in samples from Essex and North Yorks – although not from the Lincolnshire site.
“It’s too early to tell if this new pathotype caused the high yellow rust levels, but our adult plant tests this summer will help provide an answer and indicate whether it will have a significant impact on UK varieties.”
Almost three-quarters of the yellow rust samples received by the UKCPVS in 2015 were of Warrior isolates, and the Warrior race appears to be continuing to evolve. To account for this, a revised classification system based on three pathotype groups within the race – Warrior 1, Warrior 3 and Warrior 4 – has been put in place.
“When the Warrior race was detected in 2011, Warrior 1 was dominant. Since 2012, though, the Warrior 4 group has increased in frequency. Meanwhile, another group of isolates, sampled in 2015, appeared to be similar to the Solstice race, based on their reaction to a selection of varieties in UKCPVS tests,” says Dr Holdgate.
However, the severity of symptoms observed at some of the sites from which these isolates were sourced suggests a more complex picture, she adds.
“This group of isolates may also be behind some of the unusual yellow rust findings in 2015. Again, we’ll be investigating this in 2016 to provide a clearer picture.”