27 March 1998

Judge risk to cut sprays

By Andrew Swallow

TWO-THIRDS of anti-sclerotinia sprays are a waste of money. But even without a robust forecasting technique growers can do much more to target their sprays.

That is the conclusion of a four- year survey by ADAS. But fungicide maker BASF reckons benefits for the current crop and the whole rotation means growers can benefit from treating crops at lower risk.

Using petal testing diagnostics the ADAS trial monitored sclerotinia levels in 251 crops. Although the test results were too late for spray decisions, they helped show how many sprays were applied unnecessarily.

Just 19.5% of crops were shown to have been at risk of economic yield loss, yet 53% received a treatment targeting the disease.

Consequently John Davies of ADAS Terrington recommends spraying only where disease risk is high. "This disease is not important unless there is a risk that more than 20% of plants will become infected."

In the absence of an accurate soil test for sclerotia, growers need to know the history of each field to identify high risk sites. "A field which has had more than 20% sclerotinia in the past, or is adjacent to such a field, is at high risk. If showery weather is forecast during flowering such a site warrants a spray," says Dr Davies.

He also reminds growers of 1991 when crops were devastated. That years legacy lingers in the soil. "In any spring, given the right conditions, sclerotia in the top inch of soil may germinate to produce apothecia. These puff out asco-spores which infect petals, the petals fall and stick to the leaves leading to infection of the crop. Accurately identifying these situations is the key to economic control."

To do this growers should assess micro-climate within the crop at the start of petal fall. "If you walk through the crop and come out soaked and covered in petals, then conditions are right for petals to stick," says Dr Davies. Combined with high inoculum levels this represents high sclerotinia disease risk.

Get the timing right

Current fungicides are protective not curative, so need applying before infection occurs, he warns. "The key is getting the timing right, at mid- to full-flower when conditions favour disease development." Systemic products applied at stem extension will have no effect on sclerotinia, he adds.

Peter Gladders of ADAS Boxworth confirms this approach. Growers should also monitor regional intelligence on apothecia development, he says. "Already at Boxworth 5% of sclerotia have germinated to produce apothecia this year, whereas in 1996 there were none until July." He considers 25% germination of sclerotia at the time of flowering as high risk.

However, BASF argues that chemical control is economic at lower levels of crop infection. Their trials show a yield increase of 7% can be obtained by spraying a crop which would have produced 17% stem infection if left untreated.

Rotational considerations may lower the infection threshold further suggests BASF fungicide product manager Tony Grayburn. "This could be as low as 1% or 2% where other host crops such as potatoes or peas are in the rotation. Oilseed rape is the only crop with recommended chemical controls of the disease," he stresses.

BASF advocates the use of Konker (mbc + vinclozolin) at 1.5 litres/ha, costing about £30/ha (£12/acre). An alternative is three-quarter rate Ronilan (vinclozolin) plus mbc. But do not rely on mbc alone, due to risk of resistance building up in the fungus, he warns.

&#8226 Rapid diagnostic testing for sclerotinia is an essential future development, adds Dr Davies. But a rapid petal test planned by the Central Science Laboratory is still three years away. Current petal tests take 8-10 days to diagnose sclerotinia levels. "This is 6 days too late to detect innoculum at late flowering," he concludes.

SCLEROTINIA

&#8226 Sixty-three % of sclerotinia sprays unnecessary.

&#8226 Better targeting possible.

&#8226 Cropping history essential.

&#8226 Seasonal factors fundamental to forecasts.

&#8226 Crop micro-climate affects petal stick.

&#8226 Rapid petal testing 3 years away.

Avoid unnecessary sclerotinia sprays by focussing on the risk factors – site history, spore release at flowering and crop micro-climate. Only where disease risk is high, are sprays warranted, says John Davies of ADAS Terrington.