August 19, 2015
Bolster 2015/16 yellow rust defences to counter growing variety concerns
Yellow rust has become more problematic on a wider range of varieties over the past season, according to the latest Agrii National Cereals Disease Survey (ANCDS) monitoring; and in the north and west as well as the traditional eastern hotspots. So much so that growers have been urged to bolster their agronomic defences in the coming year.
“We’ve seen a worrying number of RL candidates as well as established varieties showing more yellow rust than we would expect from their official resistance ratings in our network of untreated tussock and variety trials across the country,” reported Agrii seed trials manager, Colin Patrick who oversees the surveillance programme.
“Some highly resistant varieties have been holding-up well to the high disease pressures we’ve seen on a number of sites this season while others with solid 9 ratings should really fall to 8s as a warning.
“Although more data is needed to confirm their position, a relatively large number of varieties rated 8 on the current RL now look like being 5s to 7s. Several current 7s should probably be lower too. And at the susceptible end of the spectrum, yellow rust scores of 4 seem decidedly over-optimistic in a number of cases.
“The situation varies with location. Across eastern England our tussock trial work with benchmark varieties carrying known sources of resistance suggests noticeable changes in susceptibility in Cordiale derivatives based on the Yr7 resistance gene. In Scotland the situation is different, with varieties based on Claire and Alchemy taking more yellow rust than in the past. We’re also seeing higher levels of the disease in our untreated western trial sites as well as in some spring wheats.
“On the evidence so far, Rosemary Bayles, Bill Angus and I consider we’re probably seeing a general erosion of adult resistance accompanied by a change in the balance of existing rust races rather than the national emergence of any new race,” he suggested.
“However, it certainly looks like being a more significant shift in varietal susceptibility than we’ve seen in recent years; and one which could shorten the utility and lifespan of several promising current candidates for Recommendation while hastening the end for a number of established varieties.”
Colin Patrick stresses that treated Agrii trials show the infections continue to be well controlled with current chemistry. However, he warns that the greater degree of yellow rust susceptibility inevitable in the 2016 wheat crop means particular agronomic care and attention will be important for growers in the year ahead.
“We know just how devastating yellow rust can be if not adequately controlled,” he said. “And we know that the greater yellow rust susceptibility we have in the national crop the bigger the danger of an epidemic and the greater the pressure it will put on crop protection should conditions favour disease development.
“So no one should just sit back and wait for the official ratings to come out in the new RL towards the end of the year. Instead they should heed the timely early warning the 2015 season is clearly delivering and be especially careful in their agronomy from T(-1) this autumn right through to T3 next summer.”
In particular, Colin Patrick advises growers to:
- Select varieties with strong resistance ratings based on the most recent season;
- Treat seed of varieties with questionable resistance with fluquinconazole at T(-1);
- Monitor yellow rust levels carefully in all varieties, including spring wheats.
- Employ a yellow rust-active T0 spray where early spring risk levels are high; and,
- Include extra strobilurins in the main spray programme if yellow rust becomes evident.