13 November 1998

Leaf spot: north OSR most at risk

OVER half of oilseed rape crops in northern England are forecast to need treatment for light leaf spot, yet in East Anglia only one-in-10 crops will justify action, says the HGCA.

An economic threshold for treatment is reckoned to be 25% or more of plants infected at early stem extension. Yield losses of 1.5t/ha (0.6t/acre) can result and in practice fungicide is best applied as soon as the disease is found in the autumn.

A repeat spray in late winter or early spring is recommended if disease pressure is high. Forecasts will be updated at these times.

Early identification

Crops in areas with infection in previous seasons are most at risk and early identification is important particularly in susceptible varieties, says ADASs Peter Gladders. Incubation of leaves with suspected light leaf spot lesions at 10-15C (50-59F) in a sealed polythene bag will aid diagnosis.

Visible white spores around the lesions after 2-3 days confirms infection. Phoma fungicides are generally effective against light leaf spot, too, he says.

&#8226 For phoma stem canker control alone there is little to be gained by increasing the initial dose in a split autumn/spring treatment of half rate fungicide.

That is Mr Gladders message to winter oilseed rape growers prevented from applying their first spray by bad weather.

Timing rather than dose is the key to dealing with phoma, he says. Once leaf spotting is well established, its related stem canker can be hard to control. However phoma had not reached top levels by Tuesday.