Phoma infection in oilseed rape crops could return if the weather turns milder, so growers must remain vigilant, Syngenta has advised.

The recent cold spell will have slowed disease development and movement down the leaf, but it will not kill phoma lesions already on the leaves, warned the firm’s technical manager, Beth Hall.

“With more open crop canopies, plants tend to hang on to older leaves for longer, so there is a greater risk of established phoma infection spreading; thick crops shade out the infected older leaves, which die back and drop off,” she explained.

Crops which were slow to be treated in the autumn are already seeing plants dying as a result of early phoma infection, added ADAS’s Peter Gladders. “It’s too late to do anything about early infection that has reached the stem, but smaller plants and susceptible varieties are still more vulnerable to late infection.”

Mrs Hall suggested applying Plover (difenconazole) as soon as new lesions are identified, to control the disease before it reaches the stem and causes cankers.

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