How disease risk forecasting helps prioritse fungicides

Growers wanting to ensure effective disease control in the event of limited spraying opportunities this spring should identify fields to prioritise for treatment.

“For large cropping areas, there can be a temptation to treat everything as one,” says Dave Howard, head of integrated crop management for Hutchinsons. “But even relatively small differences in drilling date can make a big difference to disease risk.” 

This is particularly true for wheat varieties with medium Septoria resistance drilled in late September and early October. “They can neither be approached as a resistant variety or a susceptible variety so other risk factors become highly important.”

Temperatures fluctuate quite a lot at that time of year, says Mr Howard, so a variation of 7-10 days in drilling date can make a big difference to the speed of disease movement and risk going into spring.

Rainfall in April and May has long been associated with increased Septoria incidence. But last year demonstrated that it can take off rapidly later, even from a low base disease level, given the right conditions.

Omnia’s wheat disease risk forecasting model provides a baseline risk assessment for any crop location. It considers factors including recommended list variety scores, drilling date and critically historic and forecast weather data.

“This gives growers a visual representation of where the main risks are and strategically plan control programmes accordingly,” says Mr Howard. The model is being further developed to forecast Septoria severity based on weekly rainfall data.

Early sowing and a largely mild, damp winter means many cereal crops are at relatively high disease risk going into spring, making early fungicides a worthwhile investment, says Hutchinsons technical development director David Ellerton.

“We saw a lot of rust, mildew and Septoria on lower leaves at the start of the year, and although January frosts slowed disease down, generally thick, early-sown crops will be at higher risk if conditions are conducive,” says Dr Ellerton.

Yellow rust control should be the focus of the first fungicide spray, usually applied to winter wheat around growth stage 25-30. “If you’ve got yellow rust, it’s important to knock it out early with a tebuconazole or metconazole-based fungicide.”

Modes of action

Including a strobilurin such as azoxystrobin or pyraclostrobin, can bring extra persistence – but strobilurins should not be used alone. “Resistance management guidelines state strobilurins must be mixed with a fungicide with a different mode of action.” 

Spray programmes should prioritise susceptible varieties rated 3 or 4. These include Skyfall, KWS Kinetic, KWS Zyatt or RGT Wolverine, which were hit particularly hard by yellow rust last year, but will give greatest yield response to treatment.

“Varieties with higher Recommended List ratings cannot be ignored though, especially if it is based on adult resistance, which may not kick-in until late in the season,” says Dr Ellerton.

“Varieties with reasonable adult resistance, such as KWS Barrel or RGT Gravity can still be vulnerable to disease as young plants. If disease establishes early, it can cause significant damage before adult resistance takes effect, so needs controlling.”

Early treatment is paramount for yellow rust control. But Dr Ellerton advises growers check product labels carefully and consult their agronomist, as some products cannot be applied before GS 30.

Nitrogen strategy

“Straight tebuconazole or azoxystrobin cannot be used pre-GS 30, whereas some mixes containing tebuconazole can. Also, remember nitrogen strategy affects disease risk, with stressed crops, or those with excess nitrogen, potentially more prone to yellow rust.”

Last year’s emergence of Septoria races with virulence to Cougar makes early protection particularly important for varieties with Cougar parentage, such as RGT Saki or KWS Firefly, adds Dr Ellerton.

Following the withdrawal of multi-site chlorothalonil, which was the go-to option, folpet is now the main fungicide for Septoria protection where risk is high. Where mildew is a threat, prothioconazole offers reasonable early season control.

The strongest mildewicide, cyflufenamid, cannot be used until the beginning of stem elongation. “The biostimulant Scyon can play a useful role within integrated disease control programmes,” says Dr Ellerton.

“Its combination of naturally occurring, signalling metabolites strengthens the plant’s natural defences and maximizes nutrient uptake, making plants healthier and better able to fight off disease,” he explains.