Early control vital to avoid OSR loss

21 September 2001

Early control vital to avoid OSR loss

By Charles Abel

EARLY oilseed rape disease control is the key this autumn, to avoid a repeat of the £61m lost to phoma stem canker and light leaf spot in 2000.

That is the message from an industry-wide team working on a novel forecasting package to help growers combat oilseed rape pests and diseases.

A light leaf spot forecast is already available, a phoma forecast is expected next autumn and a pest forecast is being evaluated, all under the umbrella of the DESSAC decision support programme.

Getting sprays on in good time this autumn is vital, says Andy Selley of Du Pont, which is involved in the HGCA/DEFRA-funded PASSWORD project, along with CSL, ProCam, ADAS and Syngenta.

Last autumns wet weather saw oilseed rape fungicide use drop by a third, allowing disease into crops and hitting yield hard, NIAB data showing a 0.6-1.1t/ha yield penalty where sprays were missed.

"The key is to monitor crops from emergence onwards and use a well timed fungicide programme," says Peter Gladders of ADAS. Wet weather encourages spore release from rape trash to infect adjacent crops, typical leaf spot symptoms appearing 14-25 days later.

He favours a two spray programme, starting once a threshold of 20% infected plants is reached. Punch C (flusilazole + mbc) and Plover (difenoconazole) offer similar phoma control, but Punch offers better light leaf spot control, he notes.

Young crops with small leaves are at most risk, Dr Gladders adds, because the disease develops faster in warmer conditions and is closer to the vulnerable stem base where it forms yield-sapping cankers.

CSL data shows a lot of sprays fail to deliver a yield response, mainly due to poor timing.

By next autumn the PASSWORD project should be able to provide an early warning of phoma, so sprays can be applied before infection is seen or field-walking targeted to vulnerable crops, says consortium chairman David Ellerton, of ProCam.

The project is already throwing up useful pointers, CSL work showing distinct phoma hot-spots, notably in Oxon, Northants, Cambs, Suffolk, Sussex and Kent.

Local climate, cropping intensity, farm size and local trends in fungicide use mean those areas are at highest risk from phoma and regularly suffer the worst losses, says the CSLs Judith Turner.

When canker incidence in the previous year is also taken into account a forecast can be made for the coming season. Assuming winter weather is normal, phoma risk across the UK is likely to be similar to last year, says Dr Turner.

Another new finding is that there are two types of phoma. Type A is most prevalent, hitting crops early and causing most yield loss. Type B, which has a darker lesion and fewer black-dot pycnidia, moves onto crops later and causes less yield loss. If only type B is present spraying is unlikely to be worthwhile, says Bruce Fitt of IACR Rothamsted.

&#8226 This years interactive web-based light leaf spot forecast (www3.res.bbsrc.ac.uk/leafspot/) has been improved to take account of variety and drilling date. &#42


&#8226 £61m loss in 2000.

&#8226 Spray early for phoma.

&#8226 Wet weather raises risk.

&#8226 Economic response likely, especially if light leaf spot risk.

&#8226 Two sprays usually worthwhile.

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