Oilseed rape growers should be prepared to treat crops for phoma over the next few weeks as the onset of an epidemic is imminent, Rothamsted Research has warned.

Updated predictions from its phoma leaf spot forecast suggested that the spray threshold of 10% plants infected with the disease would be reached between 17 October and 7 November – depending on location.

Once this threshold was reached, a fungicide treatment should be applied and growers should pay special attention to smaller crops where emergence was delayed due to dry seed-beds earlier in the autumn, Rothamsted’s Neal Evans advised.

“Backward crops will be extremely susceptible to phoma and we may see some autumn plant death from this disease this season. Growers should check the forecast and inspect crops for leaf lesions and damage.”

Some of the first lesions had already been found in more forward crops, according to Andrew Blazey from Prime Agriculture in Essex. “The most responsive conditions for the control of the disease are when it is found in late September/ early October in relatively backward crops,” he said.

“Therefore it is likely that in established crops two applications of fungicide will need to be applied this autumn. The first treatment should be made when there is 10% incidence in susceptible varieties and 20% in resistant ones.”

See FWi’s weekly Crop Watch reports for more in-field reports from agronomists around the country.

The interactive phoma leaf spot forecast can be found here