By Professor Fiona Burnett,

head of crops and soil systems, SRUC

Managing light leaf spot in oilseed rape is challenging because the pathogen adapts so readily to new varieties and fungicides.

The disease is worst in cool, wet areas and has historically been more of an issue in Scotland – although it is now prevalent in the south of the UK as well.

It is carried on trash and is splash-borne but also moves between crops in waves of ascospores throughout the season and spikes of ascospore production over summer and harvest are often noted.

In regions at high risk, the first port of call has to be resistant varieties. This is recognised in the Recommended List for the north, where a minimum standard of 6 is set. Commercially, though, varieties of lower ratings are sometimes used.

The use of a robust varietal rating gives some scope to reduce light leaf spot risk, although where varieties are widely used they do decline. Cracker, for example, started with a ‘good’ rating that quickly eroded, while newer varieties like Alizze or Barbados have ‘7’ ratings so are more robust.

The disease is spread upwards and outwards through the canopy by rain splash and causes the classic scorched type symptoms that can easily be mistaken for other forms of damage. Incubating a few leaves overnight will reveal the classic spores which look like salt grains around the edge of the lesion.

The disease will cause puckering of the leaves and distortion of the pods if it gets that high up the canopy.

Yield losses can be severe, especially where it gets into side branches and racemes and are often 30% or more in SRUC trials.

Fungicides are the second line of defence after varietal resistance. Control has been heavily reliant on the azole group of fungicides, with the inevitable consequence that efficacy, particularly to some of the older azoles, declined.

Light leaf spot has been shown to have developed similar mutations to those that have been found in septoria to the azoles. Ideally, to manage fungicide resistance risk, we would mix or alternate fungicides to make it difficult for the pathogen to adapt – but when only azoles were available, that was impossible.

The recent introduction of products that contain strobilurins and SDHIs to aid with light leaf spot control helps. Efficacy-wise they are similar to the existing azole chemistry, like tebuconazole and prothioconazole, but what they do bring is the chance to mix and alternate.

It is really important that this principle of mixing and alternating is continued through to the flowering sprays that are applied to the crop. Of course, at that point we are targeting different diseases but the light leaf spot in the crop still gets another blast of the chemistry used to target sclerotinia and then moves off to the new crop, with that extra selection pressure applied.

Planning programmes across your whole system means that you can spot opportunities to limit each individual active to no more than two applications and to mix where possible. Some products impose this on the label but at the moment this is inconsistent.

Fungicides are most effective as a protective tool and do little to control established disease.

The disease often appears in crops in December/January, which is why a protectant spray in late October or the start of November can help, with a follow up spray at stem extension.

In years where the epidemic is late, then a spring spray will suffice. The danger there is you don’t know in advance which years those are and where disease gets established early, there is no going back.