The capture of high levels of spores in several traps, combined with weather conducive to infection, indicates a high risk in these areas, according to the Brassica Alert website and SMS service.
Richardson said: "Any crops that have not received a suitable fungicide in the past 14 days should be treated as soon as possible."
He added that white blister remains a high risk as a consequence of extended periods with wet leaves. He said: "Crops must be regularly inspected and treated at the first signs of disease."
Syngenta technical manager Jon Ogborn recommended that growers use Amistar Top to control these diseases. He said: "Amistar Top has proven highly effective in controlling all the key brassica crop diseases, especially the most damaging ringspot. Amistar gives good protection against white blister, although Folio Gold is advised for best curative action if it is established in the crop."
- For details of Brassica Alert and to sign up to the service see www.syngenta-crop.co.uk or call 0800 652 4215.