Light leaf spot lurking invisibly in OSR

OSR crops across the UK could be infected with Light Leaf Spot (LLS), but there may be no obvious symptoms. If crops are left unchecked the disease could significantly reduce yields warns oilseed rape breeder NK.

"LLS is often most visible following a frost as infected plant cells collapse and die producing dead patches on leaves," explains Nigel Padbury, NK's technical manager. "The lack of severe frosts across the UK, especially in Scotland, means identification in the growing crop is proving to be difficult because symptoms are 'invisible' or are being confused with other plant diseases or nutrient deficiencies."

LLS is frequently regarded as only a real problem in the north of the UK, but the disease is often seen in the south. If the country continues to have mild winters with fewer frosts, invisible LLS could prove to be a UK-wide problem.

"I think there are higher levels of LLS in the south than growers realise. Tests on samples from NK's trial site in Cambridgeshire show between 10 and 30% LLS infection where none was visible in the field," says Nigel.

He advises growers to carry out their own simple test by selecting 5-10 leaves at random, placing them into a sealed plastic bag and leaving it for 3-5 days in a cool room. The leaves will develop typical symptoms if the disease is present. The economic threshold for treatment is 25% of plants infected.

"Over the past few years LLS resistant varieties like NK Grace have regularly come top in untreated trials across the UK, despite low levels of any visible disease in the plots. 'Invisible' LLS may have been a problem for longer than we realise," says Nigel Padbury.