Although many have only just finished putting rape into the ground, growers are being warned to be alert to the Phoma risk this season. With many crops emerging later than usual they could face heightened disease pressure during early development than normal, should the weather turn wet.

ADAS pathologist Peter Gladders is already getting reports that the disease is on volunteers and warns growers to be vigilant. “There are a few spots on volunteers already. However, the greater concern is backward crops as small plants are particularly vulnerable. The smaller the plant the easier the disease can spread from leaf to petiole and then to the stem. Plant size this autumn and early risks will depend on how much rain arrives over the next three weeks,” he warns.

Bayer’s Tim Nicholson agrees and urges growers to spray as soon as thresholds are reached. “According to the Rothamsted Phoma Forecast, due to the wet weather this season the 10% threshold is expected to be reached some 7-14 days ahead of last season. In the right conditions infections can rapidly spread above the threshold.

“Although popular autumn treatments such as Proline²⁷⁵ (prothioconazole) have excellent curative properties it can only halt the spread of the disease whilst it remains in the leaf, which can be a very short space of time in small plants, giving less leeway when it comes to timing. If infection reaches the stem then it is inevitable that stem cankers will develop.”

He also advises growers to watch the gap between sprays. “Small plants will put on a lot of new foliage, so a four week gap is probably advisable,” he warns.

And Mr Nicholson also highlights the risk of Light Leaf Spot (LLS) this season. “The wet summer encouraged LLS ahead of harvest and as a result we can expect to see more coming through this autumn. No single variety has good resistance scores for both diseases so all are vulnerable one way or the other. Prothioconazole remains the most effective active against Phoma and LLS, and as both are becoming more widespread throughout the UK prothioconazole should be the base of all autumn OSR disease control programmes,” he concludes.

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