Use disease impact as much as resistance scores to choose and manage wheat varieties, plant pathologists are urging growers.

It pays to employ all available Recommended List information, stresses NIAB’s Rosemary Bayles.

“This includes untreated yield as well as disease resistance ratings.”

The RL ratings are a good measure of the extent to which varieties are affected by the most important foliar, stem and ear diseases.

But they tell growers little about the true impact of disease on yield, says RAGT Seeds’ Bill Hollins.

“This depends as much on the variety’s ability to tolerate infection which depends on the scale of challenge and the robustness of its metabolism.

“That’s why the RL untreated yield trials are so valuable.”

Assessed over several years they show how varieties perform under the disease pressures commonly encountered in the field, he explains.

In this context the pathologists suggest the best measure of disease impact, particularly of Septoria tritici, is the difference between untreated and treated yield as a proportion of treated yield.

Overall, there is a reasonable correlation between septoria resistance rating and yield losses.

Varieties such as Robigus with a rating of 7 lose up to 15% when untreated while those such as Wizard, rated 4 or less, shed upwards of 20%.

But there is marked variation – especially among varieties rated 4, 5 or 6.

Even though all five breadwheats on the 2006/07 list rate 5, yield losses in the field differ by more than 3% – from 19% in Xi19 and Malacca to just over 16% in Mascot and Hereward (table 1).

Likewise while the three feed wheats, Ambrosia, Gladiator and Glasgow are rated 4, 5 and 6 respectively, all lose almost exactly 18% of their yield to disease.

It all underlines the value of using disease impact measurements alongside resistance ratings to show varieties’ true strengths.

“Untreated grain yields haven’t traditionally been a primary consideration for most growers,” says Dr Bayles.

“Like resistance to lodging without PGR, it’s easy to assume they aren’t very relevant if you’re using a full spraying programme.

“Yet the difference between a variety’s treated and untreated yield rating is an excellent indication of the overall level of risk you run if pressures of work or weather prevent you getting your fungicides applied at precisely the right time.

Or, for that matter, the risk involved in economising on rates, mixes or applications.

“I’d go so far as to say disease impact is just as important as the resistance ratings in assessing the risks in growing a particular variety.

“You should always look closely at the resistance ratings as well, of course, to assess specific risks both from regularly occurring diseases and those like yellow or brown rust which, though less common, can be devastating on occasion.”

For most value in variety selection and management, Dr Bayles and Mr Hollins advise growers to base decision-making by assessing key disease risks (table 2).

Complementing local experience is DEFRA’s Crop Monitor service www.cropmonitor.co.uk which provides data on disease levels and trends by region and offers practical disease management tools, they note.

“Knowing which diseases your crops are really at risk from allows you to focus on the variety data that matters to you,” says Mr Hollins.

“Most wheat growers in the west don’t really need to worry about brown rust ratings, for instance.

“At the same time, while the disease has been quite prevalent in other parts of the country of late, it has far less effect than yellow rust simply because it develops later in the season.

“Mildew is another good example.

It’s really widespread, but it’s nowhere near the yield robber that septoria is, as well as being relatively easy to control.”

andrew.blake@rbi.co.uk