Continual changes in yellow rust pathogenicity over the past season has meant that almost two-thirds of mainstream winter wheats have a lower resistance to the disease than their current Recommended List ratings suggest.

According to the latest information from husbandry specialist, Agrii’s national cereal disease survey – which has monitored an extensive series of national variety trials and tussock plots at its Technology Centres and iFarms throughout the country – the data revealed that no fewer than 23 of today’s 36 main winter wheat varieties had a lower rust resistance than their 2017/18 RL ratings. Fifteen of those were at least a point lower.

It also showed that half of 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.

“A number of varieties are holding their resistance ratings well, with 11 still scoring 8.0 or more in our national survey,” reported Agrii’s northern R and D manager, Jim Carswell. “However, our monitoring also shows many have become noticeably more susceptible to yellow rust over the past season, several seeing their resistance ratings fall by two or more full points.

“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 and five at less than 3.0.

“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, as well as several candidates with yellow rust resistance scores at least a point lower than their official ratings suggest,” he pointed out.

“Later sowing will do much to reduce the yellow rust pressure on varieties. 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 and how devastating losses can be when conditions are in its favour or the weather gets in the way of the most timely spraying. So, it’s especially important to go into the season knowing exactly the level of risk the varieties we’re growing present.

“That’s especially so with such a large area of wheat almost certain to go into varieties with real yellow rust resistance levels of 6.0 or less this autumn.”

He said the extra intelligence Agrii’s own list can provide should give growers an added decision-making tool. “Developed with the national authority on the disease, Dr Rosemary Bayles, our agronomists and growers can use the list to make the best-informed choices for their main sowing slots,” Mr Carswell concluded.