While Recommended Lists (RL) resistance ratings give an indication of how susceptible different varieties are to disease, when drilling dates are brought forward, RL ratings can be misleading.
That’s according to Bayer technical manager, Gareth Bubb, who is seeing a massive difference in disease levels between early and later sown wheat varieties at Bayer’s trials site in Callow, Herefordshire.
Where wheat was sown on September 22, septoria is present at high levels in nearly all of the variety plots, even in those with RL ratings of 6 and 7, including Costello which has a lot of active septoria on the upper leaves.
Mr Bubb says: “If you had sown Costello later, you wouldn’t expect it to have so much septoria, but people grow Costello because it’s a 6; it has a good septoria rating, but it’s got quite a frightening amount of septoria when it’s drilled early.”
Meanwhile, in plots sown only 10 days later, on October 2, although crops look much more backward, the less susceptible varieties appear freer from disease.
Mr Bubb says this is because earlier drilling leads to increased exposure to inoculum, as well as denser crop canopies which create an ideal microclimate for the disease development.
“In early sown crops, the disease is in there for longer and crops tend to be denser so all the conditions are favouring septoria.”
Where crops are sown in September, Mr Bubb says growers should rely less on RL disease ratings.
“If you’ve got a variety that is a 7 for septoria, but you’re sowing it early in September, I don’t think you should treat that variety as a 7 anymore, you want to treat it as a 5 or a 6.
See also: Setting up for spraying at T0
“Whereas, If you grow something that is a 5 for septoria, and you are sowing it in November, in most seasons that’s probably going to be better than sowing a more resistant variety earlier.”
Given black-grass was slow to emerge in the autumn, many growers delayed drilling for longer than usual, which will have reduced the level of infection of diseases like septoria and eyespot, but mildew incidence will be greater as a result, he adds.
“The fact that some people delayed drilling, waiting for black-grass to emerge, will help septoria and it will help with eyespot, but actually it could make mildew worse because mildew tends to be worse in late-sown crops, where you get rapid growth later on.
“There are later sown varieties, like Revelation, which is caked in mildew and will need some sort of mildewicide either in with the T0 or the T1 to keep it under control.”
However, the potential for mildew to impact on crop yield is much lower than septoria and so drilling date and variety choice should be largely targeted towards septoria control, according to Mr Bubb.
See also: Stay on the front foot against disease
Ultimately, it is about managing risk. “When we talk about risks, we are convinced it’s all down to susceptibility of the variety, but drilling date makes a massive difference.
Whether septoria can be effectively controlled in some of the earlier sown varieties this season, will depend largely on weather.
“If we get high rainfall in April, I think we could throw all the chemistry we want at those early sown crops and we still won’t be able to keep the septoria out with this much inoculum present early on,” he says.