Look across be new AHDB Recommended List of winter wheat varieties and you’ll see quite an impressive line-up of disease scores for Septoria tritici, especially among the clutch of new Group 3 winter wheats.
But there’s some essential information missing from the RL, according to independent plant breeder Bill Angus, that he feels might make you want to question how much you’re going to rely on those healthy scores.
“It’s their pedigree,” he says. “Six out of the eight Group 3 varieties are derivatives of Cougar as is Group 4 (soft) variety RGT Saki. That’s a lot of wheats that depend on one set of genetics for their resistance to septoria.”
What’s more, I suspect that Cougar itself relies on major gene resistance, he points out. “The problem with major genes is that when they capitulate, they go b1g time. As a grower, you’re then left relying on the variety’s background resistance. But since the major gene has always masked what the minor genes are contributing, you have no idea whether your variety will then be moderately resistant or completely exposed.”
Bill is concerned that in just the same way as growers are losing their choice of chemistry to fight septoria, they’re losing the diversity in their genetics. “The problem is that it’s not as obvious as an active ingredient losing its approval. You feel encouraged to take your foot off the septoria control, and that’s when varietal resistance becomes exposed and the pathogen finds a way round it. Before you know it, lesions are appearing on a variety that should have a septoria score of 7.4.”
It’s a problem growers are all too familiar with when it comes to yellow rust, he points out. “There are a lot of varieties currently in commercial use that rely on Hereford or Timaru for their apparently high scores. Yet the Hereford yellow rust race is already making inroads into the UK population.”
But Bill doesn’t dismiss the value of varietal resistance. “Breeders are generally doing a good job of bringing stronger disease resistance into their lines without compromising yield, and that’s not easy. You’re dealing with a Rubik’s cube of traits and every time you try to bring another in, you multiply up the muddle which makes it even harder to line up the ones you want.
“It’s up to the grower and agronomist to look behind the headline figures and question the agronomics of a variety on its pedigree before making a choice.”
Another strategy he favours for those growing feed varieties is to plant a mixture. “It’s a good idea to spread your risk, and growers who make their choice carefully have found they get a number of benefits.
“Three is a good number, and avoid those with the same or similar pedigrees. Graham, Costello and Theodore might make a good mix for example, although you can run into problems with varieties that don’t ripen at the same time or have different heights. So try a small area first,” Bill advises.
“But the crucial aspect is not to rely on varietal resistance for disease control — protect the genetics just as you protect the chemistry. It used to be the case that many growers ignored varietal disease resistance and relied too heavily on the chemistry for control. Now we’re in danger of going too far in the other direction. The wise grower makes good choices with both their wheat varieties and the chemistry they use to ensure they perform,” he concludes.