Emerging oilseed rape plants are likely to be under immediate threat from phoma this season, a leading plant pathologist has warned.

Around 20 days with rain from 1 August are required to trigger infective spore release from phoma cankers in stubbles and trash, a figure that has been easily surpassed already in most areas of the country, says Peter Gladders of ADAS.

The south-east, for example, where the majority of emerged crops are located, has had 26 days with rain from 1 August to the first week of September.

Alternating days of dry weather and then rain will now trigger spore release from stem cankers. “There is a real risk that phoma spores will be landing on emerging oilseed rape plants from early emergence, which makes fungicide timing and targeting difficult,” warns Dr Gladders.

“Growers should make every effort to encourage autumn vigour and early growth in emerging seedlings, since the bigger the leaf area at the time of attack the easier the crop is to manage.

“With this season’s very early start to spore release we are also likely to see repeated infection over a protracted period of time,” he adds.

Growers should try to bury as much old oilseed rape stubble and trash as possible to reduce the source of phoma inoculum and other diseases, Dr Gladders suggests.

“There are also huge numbers of volunteer oilseed rape seedlings in harvested stubble, which pose a serious threat of spreading downy mildew to the new crop. It may cause serious losses in crops infected at the cotyledon stage.”