Spray OSR now to protect yield against phoma

12 February 1999

Spray OSR now to protect yield against phoma

By Andrew Blake

MOST oilseed rape crops need spraying now to stop phoma stem canker sapping yield.

Light leaf spot also seems more widespread than expected.

Monitoring of disease in unsprayed winter oilseed rape plots at seven NIAB trial sites around the UK highlights the risk. Heavy phoma leaf spot at most sites and unexpected light leaf spot in Gwent show the need for spraying, says NIAB pathologist Jane Thomas, who oversees the work for Du Pont.

Backward crops with small plants are most vulnerable and should take priority, she advises. "The earlier you can get it on the better." Even autumn-sprayed crops need a top up treatment without delay, such is the disease pressure after prolonged wet weather encouraged both diseases over-winter, she believes.

At Swavesey, Cambs, nine in ten plants on unsprayed plots were infected with phoma leaf spot by Jan 29. Even at sites such as Morley Research Centre and ADAS Bridgets, where levels have slipped since November, there is little room to relax, says Dr Thomas. Infected leaves may have dropped off, but plenty of inoculum remains to reinfect new growth, she warns.

With yield responses to a comprehensive disease control programme of up to 19% in 1997 and 54% in Cambs last season, the return on fungicide investment is beyond question, she calculates.

Light leaf spot infection levels of 20% on Apex in Gwent is quite a surprise, adds colleague David Kenyon. The disease is normally much more troublesome further north. "The levels in Gwent are certainly worth treating," he says.

How many crops were treated in the autumn is hard to tell, says Du Ponts Andy Selley. But growers who applied a half-rate fungicide then should clearly follow up with another half rate now, he says. For crops which have had no fungicide yet the sooner they are treated the better. "Do not wait for the growth regulator timing."

ADAS echoes that view, saying profitable responses to full-rate February sprays have been achieved where phoma is obvious in trials. But time is running short for cost effective control, it notes. &#42


&#8226 Phoma levels high.

&#8226 Light leaf spot in SW.

&#8226 Early treatment advised.

&#8226 Dont wait for pgr timing.

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