• Different conditions to last season
• Challenge once rust builds in crops
• Wider application window can work
Wheat growers are being urged to assess disease risk and protect crops to avoid compromising yield potential or stoking problems later in the season.
The first T0 fungicide is often where some growers look to make savings. But this could be false economy if yellow rust pressure is high, suggests a trial last season by Hutchinsons and BASF at Terrington, Norfolk.
The trial compared a full programme (T1-T3) with and without a metconazole-based T0. It found that a T0 applied to KWS Kinetic – rated 4 for yellow rust – under significant disease pressure could generate a 1.5t/ha yield uplift.
“Considering the historical average yield response from a T0 is nearer 0.1t/ ha, the Terrington result is unexpected,” says David Howard, head of integrated crop management.
“But it highlights the variability that long-term averages can mask – and how challenging rust management can be once it builds in crops.”
High risk
Last season was particularly high risk for yellow rust, acknowledges Mr Howard. This was due to a mild winter and monthly temperatures about 2°C above average from January onwards.
Weather data shows most areas experienced 70-80 days between January and July where temperatures were optimal for yellow rust (10-15°C).
“Under such conditions, disease cycles faster, multiplies and reinfects crops more rapidly, keeping pressure high, resulting in clear benefits from reducing inoculum early.”
This was highlighted clearly in the Terrington trial, with less disease infection on all the main leaves, culminating in 8-10% less yellow rust on the flag leaf after a T0 was applied.
Conditions so far this season have been somewhat different to 2021/22, with cold weather during December and January reducing the yellow rust risk by killing pustules and infected lower leaves.
Research shows frosts below -5°C will affect dormant mycelium. A large area of relatively early-sown wheat last autumn may further help to counter yellow rust risk.
But growers should maintain their guard, adds Mr Howard. “Changing pathogens and increasingly unpredictable weather strengthen the case for T0 sprays to become standard for early disease – and expecially rust management.”
Lincolnshire-based Farmacy agronomist Charles Wright agrees.
“Rust pressure has been generally lower than last year, but don’t get complacent. Relative yellow rust risk is increasing in the UK because of the weather patterns we’re having, so saying it’s lower risk now is somewhat different to 10 years ago.”
Many popular varieties are susceptible to yellow rust. They includei Skyfall (rated 3), KWS Zyatt (3), and Gleam (5). But even varieties with higher AHDB Recommended List scores, such as KWS Extase (8) and KWS Dawsum (9), should be managed carefully because previous seasons have shown the potential of new races to overcome genetic resistance.
Later-sown crops, such as those after roots or vegetables, or where drilling was delayed due to blackgrass management, may be at greater risk of yellow rust, says Mr Writght.
“We need to be proactive about managing the disease.”
Application timing
Traditionally, many growers target growth stage 30 for applying a T0 spray. But Mr Howard says there is greater flexibility with a slightly wider application window of Growth Stage 25-30 where pressure is high.
This reduces the chances of weather delays rolling T0 sprays into the T1 slot. It also provides the much-needed treatment gap to reduce inoculum pressure – and allow T1 sprays to be more protective and less curative.
Metconazole or tebuconazole-based products are preferred where rust needs rapidly knocking out, while strobilurins, such as azoxystrobin or pyraclostrobin, offer longer-lasting protection, but less curative activity.
Susceptible varieties
In the most susceptible varieties, where risk is very high or yellow rust has become established over winter, there may be a need for a fungicide before the main T0 (from post-tillering, GS 20+).
But Mr Wright says growers should check with their agronomist because only a limited range of products can be used at this early timing.
He favours a straight tebuconazole pre-T0, allowing the true T0 to be applied at GS 30. Again, this will help maintain a tight 2.5-3 week interval between the T0 and T1 treatments.
“The keys to rust management are to not let it get established, and keep the intervals between active fungicides tight.”
While yellow rust is the focus of most T0 programmes, early-sown wheats high biomass will be at greater risk of Septoria, so this should not be overlooked at T0, especially in more susceptible varieties or regions.
“Some growers may have turned away from using T0s to manage Septoria after chlorothalonil’s withdrawal, but there are benefits from keeping Septoria in check and buying flexibility towards T1,” says Mr Howard.
Metconazole has some Septoria activity, although folpet offers greater protection, he says.
Don’t overlook Septoria
“Avoid primary azoles as they are likely to feature in later fungicide applications. Generally, rainfall in April and May really drives Septoria pressure, but early drilled crops with more biomass are at greater risk.
“For growers trying to be proactive against Septoria at T0, then the multisite folpet is probably my preferred option, as you’re hopefully protecting some of the new growth from getting Septoria.
“With high input prices and still decent commodity prices, any element of risk management you can bring into the situation is worthwhile.”
