8 March 1996

MAFFreckons plenty of room for improved OSRtreatment

Recent HGCA/MAFF work is painting a clearer picture of the importance of oilseed rape diseases and the need to apply fungicides selectively, reports Andrew Blake

"WHY are you suddenly saying that fungicide spraying is necessary this year?" That was a common query from oilseed rape growers at the recent HGCA roadshows, says ADAS pathologist Dr Peter Gladders.

One answer could be that in 1994 losses from disease are thought to have cost growers £65m. And yet 74% of crops are believed to have been sprayed with fungicides at a cost of £6.5m.

The conclusion, says Dr Gladders, is that in most cases those fungicides could have been better used. "Light leaf spot affecting 20% or more of the leaf area in spring and canker lesions affecting more than 90% of plants at harvest are sure signs of scope for improvement."

His comments are based on a three-year HGCA/MAFF study using a Sportak (prochloraz)/ Compass (iprodione + thiophanate-methyl) mix on sites from Aber-deen to Dorset. One hurdle is that although disease problems have grown over the past two to three years, different parts of the UK face very different disease pressures.

Light leaf spot is more common in the north but there are still severe attacks in all parts of the UK, even on supposedly resistant varieties like Apex and Express. "It is no good thinking that because you chose a resistant variety your disease problems are over," warns Dr Gladders.

ADAS has found that for eastern England at least, the presence of light leaf spot on pods in July is a good sign of problems next spring. "It remains to be seen whether these guidelines apply to other parts of the UK."

He hopes further HGCA work will allow light leaf spot infection periods to be pinpointed as autumn progresses. "Sprays can then be timed more accurately and at an appropriate dose rate." Under the high disease pressure of 1994/95 it was clear that a combination of autumn plus spring sprays, both at half dose, gave a "substantially higher" yield response than a single full dose in the autumn.

Canker is largely driven by September/October rains, says Dr Gladders. Early MAFF findings are that fungicides to tackle canker have relatively little "kick-back" activity. "This means it is hard to get good control if leaf spotting is already widely established."

Single sprays in spring are likely to give little or very variable canker control, he warns. A split dose in the autumn should give far better results, and there seems to be some scope for a "repeat low dose" approach over winter.

Early flowering is the critical time for spray decisions on sclerotinia. Alternate wet and dry spells and rising temperatures are needed to trigger production of infective fruiting bodies (apothecia) from sclerotinia in the soil. "Farmers can monitor fields, ideally winter wheat after rape, to see if there is a risk on their own farms," he says. Many have a history of sclerotinia, but again the picture is not straightforward. "In 1993-95 there were some high risk farms where sclerotinia did not develop but also some low risk ones where they experienced substantial attacks."

Petal testing to spot sclerotinia spores can help detect such unexpected deviations, and UK thresholds are being established, he explains. "But at present the test take about seven days. So you need to sample at early flowering to get the result in time to affect spray decisions."

Many growers continue to use MBC fungicides against sclerotinia with "excellent" results, he says. Others use broad-spectrum treatments, such as Compass, Calidan (iprodione + carbendazim), Folicur (tebuconazole), Konker (vinclozolin + carbendazim) and Sportak Alpha (prochloraz + carbendazim), to combat late season pod diseases such as alternaria.

"Recent trials show some of these mixtures are giving larger yield responses than the Ronilan standard treatments used for sclerotinia." But growers should be careful to follow the label. "Some fungicides cannot be mixed with pyrethroid insecticides during flowering because they increase their toxicity to bees," he warns. &#42


&#8226 Cost of disease is high – estimated £65m in 1994.

&#8226 Fungicides could be better used – timings need more attention.

&#8226 Pod disease good pointer to next seasons light leaf spot.

&#8226 Canker levels driven by rain in September/October.

&#8226 Early flowering key time for sclerotinia spray decisions.

&#8226 Petal testing promising as fine-tuning technique.

&#8226 Take care when mixing broad spectrum products with insecticides.


CONTROL TACTICS

Light leaf spot (inset) sprays become better timed as a result of HGCA/MAFF-funded work intended to forecast disease more accurately.