16 March 2001

OSRneeds fungicide lift

Oilseed rape crops have

struggled this winter. So

how best can you give them

a boost this spring? Apart

from early N a timely

fungicide is also likely to

pay, as we report in our

latest OSR Adviser article

MANY oilseed rape crops need spraying urgently for light leaf spot control, says Dr Peter Gladders of ADAS Boxworth.

Phoma control may also benefit, but in most cases canopy-regulating applications will need a separate spray in April.

A reluctance to use spring and summer sprays last year, together with very wet and cold conditions this year which prevented autumn treatments, means there is more of the disease around this spring.

The disease can cause yield losses of up to 1.5t/ha, costing growers as much as £30m a year, despite spending more than £5m a year on fungicides.

"This is a bad year for light leaf spot and a March spray could still achieve an extra 1t/ha," says Dr Gladders. "Cold weather favours the development of the disease, so if it is well established on growing points, spray as soon as you can."

Growers wondering about the economics of a March spray should remember that yield loss is related to the number of infected plants. "Crops with 25% infection at early stem extension (GS3,3) see a yield benefit of 8% from a fungicide spray. If infection is more severe, the rewards will be greater," he stresses. "Do not forget that secondary spread by spores in wet weather takes 3-4 weeks to express itself.

"So 25% infection and wet weather is a good threshold for this year. And where there are strongly established patches, move up from half to three-quarters rates."

Crops in the north of the country are treated most years for the disease. But there are higher levels in all areas this year, so growers should look in the fields. Du Pont trials report an £80/ha return from a spring light leaf spot spray.

To improve control a light leaf spot forecast has been developed with HGCA funding and is now offered through the PASSWORD project. See following web-site: www.res.bbsrc.ac.uk/leafspot.

Product choice

The best fungicides for light leaf spot control are Folicur (tebuconazole) and Punch C (flusilazole). Caramba (metconazole) also has activity against the disease.

Choice will depend on other diseases to be controlled and spray timing. "Both Caramba and Folicur have a good growth regulator effect on the crop, but growers will loose this benefit with an early spray," points out Dr Gladders.

Where light leaf spot is severe, they should spray as soon as they can and then re-appraise the crop when it is knee-high to see if it needs a PGR. "Do not compromise effective disease control by waiting for the PGR timing."

Phoma

Ideally, phoma control should be out of the way by January, advises Dr Gladders. "The economic returns from spraying have gone by mid-March."

But more consistent yield effects have come from two phoma sprays and as the year has been so extreme, there may be reason to spray for the disease this spring.

"Late drilled crops are more at risk from phoma than light leaf spot," he notes.

"Where both diseases are present, a spring spray is justified. But at this stage of the season, most growers are chasing light leaf spot."

Best products for phoma are Punch C (flusilazole) and Plover (difenoconazole). Punch C controls both light leaf spot and phoma very effectively and is preferred where both diseases are active.

Sclerotinia

Sclerotinia will only be a concern if crop development and spore activity coincide, says Dr Gladders. Although it was in greater evidence last year, at double the level of the previous season, conditions are not usually in its favour, he notes.

"The first signs of sclerotinia development come at the end of March. Soil temperatures and moisture are important factors in determining when they germinate."

The key stage for decision-making on fungicides is early to mid-flowering. Compass (iprodione+thiophanate-methyl) and Folicur (tebuconazole) can be used.

PGR timing

Getting the full benefit of the growth regulating properties of some fungicides means applying them at the yellow bud stage.

Tim Nicholson of Bayer says that a yield response of 0.6t/ha was recorded in trials at ADAS Rosemaund when Folicur (tebuconazole) was applied in early April. "And that was not including the disease control response."

Early green bud applications will have a good effect on reducing lodging, but will not give the best yield response. His advice is not to spray before the plant is 30cm tall, as the benefit will be lost before then. "When you see the first bit of yellow bud, the timing is right."

This year, he advises that disease control and PGR timing should be split where necessary. "It has been so cold and wet that crops are not moving."

The aim for an oilseed rape crop is to achieve a canopy size of 3-3.5 units of green area index after flowering. Normal and forward crops will justify a spring treatment for canopy manipulation this year, believes Clare Tucker of BASF.

"Using Caramba (metconazole) at stem extension will help get the best balance between canopy size and light interception," she explains. "In practice, any crop in excess of 25cm in early March is likely to benefit. It is only the backward crops which wont respond." &#42

Making the most of oilseed rape as a break crop is a key goal for UK arable farmers. That is why plant breeder PBIC Seeds is sponsoring this series of articles in farmers weekly. Over the coming months we will examine key issues involved with the crop, keeping you up to date with the latest advice to help ensure oilseed rape is a profitable part of your crop rotation.