Apply fungicides early before sclerotinia gets established. That’s the clear message from experts, who warn that another bad year for the disease could be on the cards if optimum spray timings are missed.

“There’s potentially a lot of inoculum around,” says the Scottish Agricultural College’s Simon Oxley, who reminds growers that the disease can survive in the soil for many years as sclerotia. “Sclerotia can be ploughed back up from previous years, so fields with a history of the disease and tight rotations could well be at high risk.”

Spores can also travel easily on the wind, so even if there is no sclerotinia history on one field, growers should be aware of any incidence in surrounding crops, ProCam’s Dave Ellerton advises.

“We’d always perceived crops around here [East Anglia] as low risk, but last year proved that it can be a major disease in all areas. We even found it in fields where rape had never been grown before, despite hardly any rain last April.”

There are no sclerotinia-resistant varieties and fungicides only offer protectant activity, so it is vital growers know the risk and treat crops early, Dr Oxley says.

Predicting the exact timing and severity of infection is very difficult, he acknowledges, although weather during flowering as well as stem rot history are useful indicators. “Sclerotia generally produce spores in early May, which coincides with flowering. Showery weather at flowering increases the infection risk, because if the crop is wet, petals infected with the fungus stick to stems and allow sclerotinia to attack.”

Such “petal stick” events are favoured by light rain, whereas heavy rain washes petals off the leaves, ADAS Boxworth’s Peter Gladders adds. “High-risk conditions occur when several dry days that have provided good conditions for ascospore release are followed by a day with light showers.”

But Dr Ellerton says 2007 proved that heavy dews can be enough to cause sclerotinia infection and growers should be prepared to spray at-risk crops even if there is little or no rainfall.

Fungicide choice and timing

Because fungicides only offer protectant activity, it is important to spray high-risk crops at early flowering to ensure petals are well protected before infection occurs, Dr Ellerton says. “Ideally you need to get on with the first spray as soon as you see the first flowers forming.”

In some bad sclerotinia areas – such as parts of Scotland last year – these early sprays could run out of steam and allow secondary infection lower in the canopy, he says. “You need to play it by ear a bit, but if you’re in a high-risk area and got caught out last year, a second spray at mid-flowering could be worthwhile.”

In terms of product choice, Dr Ellerton agrees with the results from the second year of the HGCA’s dose-response project (www.hgca.com), which found that robust (0.75-1.0) rates of Compass (iprodione + thiophanate-methyl), Filan (boscalid) and Proline (prothioconazole) were the most effective products against sclerotinia.

“The main thing is to keep doses up. If you’re in a low-risk site, half-rate is fine, but on high-risk sites you really need to be at three-quarter to full dose for best effect.”

Two-thirds to three-quarter rate Charisma (famoxadone + flusilazole), mixed with half-rate Filan is also worth considering, he suggests. “We’ve seen good results from this mix, but growers need to beware there could be shortages of Filan across Europe this season.”

Azoxystrobin is another alternative, which gives useful greening effects alongside sclerotinia control, he says.

Water volumes

For any sclerotinia spray, good coverage of the crop – even lower down into the canopy – is essential, so water volumes need to be kept up, Dr Ellerton says.

For both early and mid-flower sprays, 200 litres/ha is the minimum you should be looking at and ideally water volumes should be nearer 300-400 litres/ha. 100 litres/ha is definitely the wrong way to go.”

paul.spackman@rbi.co.uk

•See HGCA Oilseed Rape Sclerotinia Decision Guide