Appearance, impact and biology

Light leaf spot (LLS) is a disease of oilseed rape and some vegetable brassicas, caused by the fungus, Pyrenopeziza brassicae. The disease appears, after a long period of symptomless growth, as speckles of white spore pustules on leaves, stems and pods. These spore pustules are only visible after a period of dry weather. As infections mature, the fungus can also cause bleaching and stunting of leaves in affected positions causing leaf distortion and by spreading to the growing point, can cause stunting of the whole plant. The disease typically reduces yield by a third of the incidence (90% plants affected at early stem extension causes 30% yield loss) and on average, UK yield losses equate to £100-140M per year. The disease has increased in southern Britain (AHDB east and west regions) in recent years due to favourable weather, changes in fungicide use and cultivars grown. For disease-prone regions (see LLS forecast at http://www.rothamsted.ac.uk/light-leaf-spot-forecast ) it is recommended to use relatively resistant varieties (and see fungicide information below).

The fungus survives over summer on crop debris, volunteers and some weeds, from which it can spread short distances (less than a few metres) by rain-splashed spores called conidia. It can also develop structures called apothecia, which release wind-dispersed sexual spores (ascospores) that can blow long distances but mostly affect crops within 300m of the debris.

After initial infections in oilseed rape crops during the autumn, the disease intensifies by more cycles of rain-splashed spores which are produced on the infected plants. Because of this dispersal method, new infections develop around initial primary infection "foci" and the disease appears to be patchy within the field.

A second phase of wind-dispersed ascospores often occurs in the spring to spread the disease again to new crops and higher positions in the pod canopy, which can lead to early pod senescence and pod-shatter.

The amount of stem and pod infection determines the amount of inoculum for winter oilseed rape crops in the next season and along with weather records, is the basis for a forecast of LLS severity the following season – seehttp://www.rothamsted.ac.uk/light-leaf-spot-forecast . This forecast indicates the predicted disease severity in different parts of the country and a crop specific interactive forecast allows growers to input various options to assess the effect of cultivar choice, sowing date and fungicide regime.

Fungicide choice

Information on fungicide efficacy is available from the AHDB– see: http://www.hgca.com/crop-management/disease-management/fungicide-performance/fungicide-performance-in-oilseed-rape.aspx

Most currently recommended products contain triazoles and these reduce LLS when applied ahead of infection. A mid-autumn application is suggested in regions prone to high levels of disease and this is often before visible symptoms occur, although backward crops may suffer a yield reduction if autumn-applied fungicides have a growth regulatory affect. Fungicides give little control on crops that are already severely infected. In addition, a spray at the start of stem extension is recommended if the disease incidence is over 15%. The disease is hard to see following wet weather, except for symptoms of leaf bleaching and distortion. Disease assessment can be improved by taking about 20 low-mid-canopy leaves, shake most water off and incubate them in a closed plastic bag in a cool room (ideally 5-10°C) for 3-4 days before looking at them. Any sporulation should be clear on the leaves if the disease is present.

Other control options

Good crop rotation to maximise separation of new OSR fields from the previous season’s crop debris ideally by at least 300 metres and practices to avoid OSR volunteers that can act as a reservoir of the disease will help to reduce epidemics.

For information on current cultivar resistance, please see: http://cereals.ahdb.org.uk/varieties.aspx and click on the winter oilseed rape link. For 2017-18, the current resistance scores vary from 5 - 7 in the east-west region or 6 - 7 in the north region, so none are completely resistant.

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