The worst yellow rust outbreak for years has been seen in susceptible wheat varieties this spring, but growers should not panic, experts have advised.
Robigus was the main variety affected, Farmacy’s Pam Chambers told Farmers Weekly on Tuesday (17 April). “It’s probably the worst outbreak we’ve seen since the days of Brigadier and Slejpner.”
The disease was not just restricted to traditional “hotspots” such as Lincolnshire, she said. “Even though we’ve had the recent hot spell, we’ve still had cooler nights and coastal mist, which has probably kept the disease going.”
Daytime temperatures over 25C were needed to slow the epidemic substantially, noted ADAS plant pathologist Neil Paveley. “15-20C is just perfect for yellow rust.” The lack of hard frosts in many areas had contributed to the high incidence this spring, he said.
But CPB Twyford’s Simon Francis said growers should not assume Robigus would be as badly affected as older varieties. “Robigus is quite susceptible [rated 3] to yellow rust, but it’s nothing like Brigadier and Slejpner, which were rated 1 and had no resistance whatsoever.”
Yellow rust would progress steadily if left untreated, but growers had time to spray, he said. “There are some good yellow rust products out there and it’s a relatively easy disease to clear up.”
Ms Chambers reminded growers that chlorothalonil was ineffective against yellow rust and, where it was used alone at T0 or no T0 was applied, a triazole plus morpholine would be needed at T1. “Morpholine fungicides give a quick knock-down, but have very short persistence. Mixing them with triazoles improves eradicant control.”
If the T0 included a triazole and yellow rust could be found, triazole would still be needed at T1, she advised. “Strobilurins also have very good protectant activity against yellow rust, but they need to be used in combination with a good triazole.”
BASF’s Rosie Bryson said rust would be a key target, but fungicide programmes needed septoria control as their backbone. TAG trials in Robigus last year found that Tracker (boscalid + epoxiconazole) at 1 litre/ha plus Opus (epoxiconazole) gave over 40% better control of yellow rust eight weeks after treatment than 0.6 litres/ha of prothioconazole. Straight Opus at 0.75 litres/ha gave 30% better control over the same eight-week period, she said.
Keeping spray frequency tight and carefully matching it to crop growth stage would be vital, Ms Chambers said. In some cases, where the T1 had been applied early to forward crops, there might be a need to go in earlier when leaf two emerges, she said.
“You can’t compensate for poor timing by simply upping the dose,” added Dr Paveley. “There’s only a window of about 3-4 days after infection when you can eradicate yellow rust, so timing is crucial.”
Frontier agronomist Brian Ross emphasised the importance of accurate timing. He had not seen any yellow rust in Robigus so far this season around the Norfolk/Suffolk border.
“All our T0s included a triazole, so it’s possible that’s why we haven’t got it. Unlike the old Slejpner days, if you catch yellow rust early enough at T0, it’s not a problem.” The disease had been found in other parts of Suffolk and around the Fens, but was limited to forward (early September-drilled) crops only, he noted.