Knowing a wheat variety’s weakness is essential when considering fungicide strategy, especially as some popular varieties are proving to be noticeably weaker against rusts than their AHDB Recommended List ratings would suggest.

A relatively mild winter means even late-drilled wheat crops are currently looking “fairly frothy”, says Agrii head of agronomy Colin Lloyd.

“Septoria tritici is present, but disease levels are not too bad at the moment,” he says.

However, it could still prove a tricky spring because of the unseasonably warm spell in February. Many crops received very early fertiliser and the resultant lush growth may bring an increased risk of disease and lodging.

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So how do growers manage these crops in the coming weeks to maximise their potential while watching their costs and without greatly adding risk?

Mr Lloyd believes the varietal data in the Agrii Advisory List, published to complement the AHDB Recommended List, can help farmers manage this risk when planning their fungicides. For example, the risk of dropping a T0 will vary greatly between varieties.

Seven varieties

Based on certified seed sales, just seven varieties account for half of the crop currently in the ground (see box). What’s important is that they have very different fungicide responses.

Looking at last year’s data from Agrii’s national and regional variety trials, there was an average response of 1.7t/ha to the standard four-spray programme. This compares with 3.4t/ha the season before.

Interestingly, though, within the seven most widely grown varieties, Agrii saw responses ranging from 0.75t/ha for Siskin to 1.5t/ha for Gravity.

Mr Lloyd highlights that it was a lower-pressure year, which meant the weaker varieties got away with it. “In a higher-pressure year, the differences would be even greater.”

There are also regional differences and interactions with varieties. For example, in the East, Siskin and Gravity gave a similar yield response of about 1.3t/ha. However, in the west, there was a 0.8t/ha difference in their responses.

He says the reason is that in the east, rusts are driving responses while in the west it is Septoria tritici and Gravity is weaker on this (AHDB score of 5.0 versus 6.7).

So armed with variety data, farmers can tailor their inputs. For example, Graham can cope without a T0 in terms of Septoria, but is weaker on brown rust so care is needed in higher risk areas.

Mr Lloyd also warns that not all yield responses are down to disease control, so farmers could lose out on other benefits such physiological greening.

Beyond the Recommended List

Mr Lloyd points out that while the AHDB Recommended List data is useful, it does not tell the full agronomic story. That’s why he likes to view them alongside results from Agrii screens which push varieties harder under more commercial conditions and assess disease, in particular, on the most recent season’s performance (see panel below).

Both the AHDB and Agrii data agree on septoria resistance ratings for the seven varieties.

However, for yellow rust, three of the seven are more than 1.0 lower, according to Agrii data. For brown rust, there even is more disagreement with five out of the seven being more than 1.0 lower. With lodging, four varieties are more than 1.0 weaker on straw strength.

Brown rust

Crusoe is known to be weaker on brown rust, with an AHDB rating of 3.0, and growers have still been able to achieve high yields with the right fungicide programme.

But what the AHDB list doesn’t tell farmers is the risk with a variety such as Kerrin, which according Agrii data is nearer 4.2 than the 7 on the AHDB list.

For varieties with a brown rust weakness, he suggests using an SDHI that is strong on the disease, like Solatenol (benzovindifupyr), at T2. Also be careful when thinking about cutting out the T0.

“While brown rust is a late-season disease, if it gets in early it can be difficult to manage,” he says. “Brown rust infects deep in leaf axils and is difficult to get at with sprays.”

But this weakness can easily be managed and Kerrin will perform. “It is good second wheat in trials and yields well. Grain quality is fine and the disease profile is pretty good.

Yellow rust

A good example here is Skyfall, rated on the RL as a 5 for yellow rust, although Agrii data suggests it is a 3.4.

Yields are okay and it has good grain quality and a premium market. It also has orange wheat blossom midge resistance, can be drilled later and for those with blackgrass, it is very competitive.

However, yellow rust is not so good and this can be managed by using an azole at T0 and following up with good rust fungicides at T1 and T2. The key here is to “mind the gap” with a maximum of four weeks between each spray timing.

Lodging

Four out of the seven varieties are noticeably weaker in the stem in Agrii trials (more than 1.0) than the RL suggests. One example is Siskin untreated – 5.0 versus 6.2.

Mr Lloyd’s advice is that if you don’t drill it early and have a robust plant growth regulator programme to help keep it standing, it will perform well

In fact, drilling date can also help bolster disease ratings. “Drilling four weeks later equates to increasing the disease score by one point,” he says.