Yellow rust in winter wheat© Tim Scrivener

About two-thirds of wheat varieties have a lower resistance to yellow rust than their official AHDB Recommended List rating, according to a new cereal disease survey.

Yellow rust is a devastating disease that can result in wheat yield losses of about 40-50% in susceptible varieties.

Varietal resistance has a role to play in helping to manage the disease risk, along with fungicides. However, the pathogen continues to mutate, leading to lower varietal resistance.

See also: Advice on getting ready for the yellow rust onslaught

Monitoring across Agrii’s national variety trials and tussock plots at its technology centres and iFarms throughout the country in 2017 revealed that 23 of the 36 varieties trialled have a lower yellow rust resistance than their rating on the 2017/18 Recommended List (RL). Fifteen are at least a point lower.

It also showed that half the six main candidate varieties are noticeably more susceptible to the disease than official figures indicate, providing a timely early warning ahead of 2017/18 planting, said the company.

Holding up

“A number of varieties are holding their resistance ratings well, with 11 still scoring 8.0 or more in our national survey, including Siskin, Graham and Dickens,” reported Agrii R&D manager Jim Carswell.

However, Agrii monitoring also showed that many have become noticeably more susceptible to yellow rust over the past season, with several seeing their resistance ratings fall by two or more full points.

Mr Carswell said: “Overall, the latest Agrii Advisory List we produced to complement the RL now rates nine of the mainstream wheat varieties at less than 5.0 for yellow rust resistance, including Spyder, Moulton and Leeds. Five are at less than 3.0, including Myriad, Diego and Reflection.”

Mr Carswell said that a progressive fall-off in resistance is only to be expected with varieties that have been widely grown for a number of years.

“More worryingly, though, we’ve recorded seven of the 11 new varieties on the RL, including Savello, Hardwicke and Frieston, as well as several candidates with yellow rust resistance scores at least a point lower than their official ratings suggest.”

Reducing risk

To help mitigate the fall in resistance, later sowing will do much to reduce the yellow rust pressure on varieties, Mr Carswell continued.

“As will well-balanced micro-nutrition to promote the healthiest crops. And, despite the withdrawal of the fluquinconazole seed treatment that was such a good early defence, in the majority of cases infections remain eminently controllable with the in-crop chemistry still available,” he said.

“We know how rapidly the disease can take off, though, and how devastating losses can be when conditions are in its favour or the weather gets in the way of the most timely spraying.” 

Therefore, he concluded that it is especially important for farmers to go into the season knowing exactly the level of risk the varieties they are planning to drill offer.

He said: “Farmers can keep a particularly close eye on varieties in their planting mixes posing a markedly greater risk than their RL yellow rust ratings suggest, and better prioritise their most susceptible crops for robust treatment should conditions prove favourable for disease development in 2018.”