High levels of yellow rust are being observed in winter wheat crops this season in areas where it is not usually problematic and, in some cases, in varieties with high resistance ratings, reports Marianne Curtis and Alice Dyer.
A mild winter which failed to kill fungus, followed by a warm spring and overnight dews have created the perfect storm for a yellow rust epidemic, according to Catherine Garman, crop health and protection scientist for diseases at AHDB.
She says: “Conditions have been perfect. Untreated crops or where spray timings have been missed or delayed are faring worst, but there have also been reports of disease in untreated varieties with good resistance ratings for yellow rust.”
This means there could be a different strain of the disease, with history showing the yellow rust pathogen population is changeable and varietal resistance can break down rapidly.
Ms Garman adds: “This is not confirmed yet. The United Kingdom Cereal Pathogen Virulence Survey (UKCPVS) is looking into it. In 2016 we had a similar situation and a lot of disease was seen in varieties with a high resistance rating because there was a race change and a new group of isolates coming in.
“This led to big changes to the Recommend List disease ratings and we released them in October rather than December, so growers were more aware early on of the vulnerability of their varieties.”
However, Dr Sarah Holgate from NIAB and UKCPVS says so far there is no evidence to indicate particular varieties are behaving differently from what their resistance scores for the disease would suggest.
She says: “People are saying there is lots of yellow rust but not that such-and-such a variety is not doing its job anymore.”
With a number of unconfirmed reports of yellow rust affecting KWS Zyatt, which has a rating of 8 for resistance to the disease, Dr Holgate says: “We would expect to see a bit of yellow rust.
We have had a couple of samples of Zyatt which we are looking at in more detail but it is not on the same scale as the Reflection situation in 2016 when we were getting a sample every day.”
So far Dr Holgate has received 69 samples for all varieties.
She says: “This is fairly modest. We have had quite a few in the last two weeks but this is normal. In terms of changes of race, as far as we can see there is nothing too much to worry about.
“In 2016 we saw a lot more samples consistently from the same varieties, such as Reflection, Myriad and Zulu. So far we have mostly had a good spread of varieties. If in the next week or two we get a huge influx of samples of any particular variety, we would get concerned.”
While Norfolk-based independent agronomist Philip Simons, of Prime Agriculture has not seen yellow rust at higher levels than normal, he suggests some reported cases may be due to failure to apply fungicides such as tebuconazole at T0.
“If you don’t apply T0 early on, a rust-susceptible variety, it may come back to bite you.”
He also points out prothioconazole is slightly weaker on yellow rust than epoxiconazole and tebuconazole.
Kent-based Agrii agronomist Neil Harper says he has seen more than in previous years, with it tending to occur in older varieties.
“I’ve seen it in Cordiale and a little more in Skyfall than previously. In untreated plots and spray misses, there is slightly more than I would expect at this time.”
Most growers will have applied T2 fungicides by now, but it is not too late to combat the disease, says Ms Garman.
“With T3s, depending on whether ear diseases are being targeted, growers can apply a fungicide with yellow rust activity at GS59.”
However, if T3s are applied at GS59, they will not have an effect on ear disease, warns Ms Garman.
Any growers who have unexpected yellow rust in their crop should send infected leaf samples to Freepost UKCPVS.