Light leaf spot threatens Ireland's expanding oilseed rape area

Light leaf spot can cost growers up to 1t/ha in susceptible winter oilseed rape varieties — and Ireland's crop area has expanded significantly this season, according to Teagasc, which is urging growers to start monitoring now.

Early sowing, available soil mineral nitrogen and strong responses to organic manures have produced large canopies in many fields this autumn. Well-established crops are welcome. Big canopies, less so, because they raise the disease risk that comes with mild, wet weather.

Light leaf spot (LLS) is the most damaging disease of winter oilseed rape in Ireland. The good news is that resistance in recommended list varieties has improved. The ratings tell the story:

• Ambassador — rating 6
• Aurelia — rating 7
• LG Aviron, LG Auckland — rating 8
• LG Armada (provisionally recommended) — rating 8

Good resistance, however, does not remove the need for monitoring and control. Teagasc is explicit on that point.

How do you spot LLS in autumn?

Symptoms are hard to see at this time of year. Growers should look for small white spots, resembling grains of salt, around affected areas of the leaf. As infection develops, leaves can curl, become brittle and crack.

Where identification is difficult, there is a low-tech trick: place leaves in a plastic bag in a warm room for one to two days to encourage sporulation. The fungus does the diagnosing for you.

Early sown crops with large canopies warrant particular attention. Mild, wet conditions favour disease development, with rain splash spreading infection within the crop. Cold weather slows it down, but disease can increase rapidly once conditions turn favourable again.

What's the spray plan?

For at-risk crops, Teagasc recommends a two-spray programme targeting late October/November and early spring. The autumn treatment aims to suppress infection and reduce the number of disease cycles.

A prothioconazole-based fungicide at 75-100% of the recommended label rate, depending on disease pressure, will provide good control and will also target phoma. That dual activity matters when you're planning one pass against two diseases.

Tebuconazole offers only limited LLS control, and any plant growth regulatory effect fades as temperatures drop and growth slows. Not the tool for this job.

Where canopies are excessively big — the result of early sowing or organic manure — an application of Caryx at 0.7L/ha can help manage the canopy. But Teagasc notes the best results from Caryx come in spring, when growth takes off.

Growers should keep checking crops through the winter to assess the need for a follow-up treatment in February, applying as soon as LLS is present.

What about phoma stem canker?

Phoma produces greyish leaf spots containing small black dots called pycnidia. The real damage happens when infection moves through the leaf stalk into the stem, causing cankers that restrict water and nutrient movement and lead to premature ripening.

Size matters here, and in growers' favour. Larger plants have an advantage because infection takes longer to reach the stem. Small crops need closer attention, particularly in mild weather.

The treatment threshold is 10-20% of plants affected. Work towards the lower end for small crops; larger canopies can justify the higher end.

One timing note: where phoma reaches the threshold before the planned light leaf spot spray, bring the treatment forward. Don't wait for the calendar.

What does this change for growers? Mainly this: with more rape in the ground and bigger canopies than usual, the cost of skipping the autumn walk doubles. Scout now, bag a suspect leaf if unsure, and have the prothioconazole lined up for late October. February decisions will depend entirely on what you find between now and then.