16 October 1998

Phoma makes an early move

WATCH OUT, phomas about. The damaging oilseed rape disease is already apparent in some crops. But growers should wait for threshold levels before spraying.

Some forward crops on Yorkshires east coast are already showing 15% infection, warns independent Yorkshire consultant Andrew Fisher.

"Its the mild, moist conditions that have prompted such an early infection. There have also been plenty of rape stubbles left unploughed. These allow spore release over an extended period."

He urges growers to spray once 15% of plants exhibit symptoms, particularly where plants are small. Infection can move to the stem more rapidly in those, to form yield-sapping cankers next summer.

Timing is more important than product rate, adds Mike Lickman of Novartis. Trials by ADAS last autumn with Plover (difenoconazole) showed a half rate spray did as well as full rate, provided it went on as symptoms were seen.

Where fast growing plants dilute fungicide concentrations a follow up spray is likely to be needed. "Where infections start early you will almost certainly need a follow-up spray before Christmas. Crops should be checked four weeks after the first spray for fresh lesions," he says.

ADAS phoma expert Peter Gladders agrees the disease needs hitting earlier than last years optimum timing of mid-November. But he urges growers not to over-react.

"There could be a slow trickle of one or two infected plants for a while." For best results spray once 10% of plants are affected, he advises.