How to manage light leaf spot in oilseed rape

Understanding the factors that influence disease risk is the first step in light leaf spot management. Through cultural control, monitoring and carefully considered fungicide applications, it is possible to suppress the threat from this major foliar pathogen.

 The following factors are associated with elevated light leaf spot risk in winter oilseed rape:

• Higher levels of stem and pod infection in the previous summer
• High temperatures in the previous summer
• Presence of infected crop residues
• Close proximity to previous oilseed rape fields
• Use of home-saved seed from heavily infected crops
• Varieties with a low disease resistance rating, especially under 6
• Earlier-sown crops
• Severe winters that limit crop growth

Notes:

Although early drilling increases light leaf spot risk, it can decrease phoma risk.

For example, plants sown by late August are usually well-grown before phoma leaf spot onset.

This slows pathogen movement from the leaf to the critical stem area.

Therefore, it is important to monitor all crops regularly, irrespective of the drilling date.

There are numerous cultural control options available. Although none in isolation provides complete control, a combination of techniques can substantially reduce disease risk:

• Cultivate/plough oilseed rape stubbles to bury infected crop debris*
• Ensure sufficient break between susceptible crops in the rotation**
• Site new crops as far as possible from fields previously used to grow susceptible crops – by 200 to 500 m, if possible
• Drill later, especially in higher-risk situations
• Avoid saving seed from heavily infected crops
• Grow a variety with a good level of resistance***

Notes:

*Crop residues are an important source of inoculum for many oilseed rape diseases, including phoma leaf spot and stem canker, light leaf spot and dark leaf spot.

**Extended rotations will reduce trash-borne disease, such as light leaf spot and phoma, as well as soil-borne disease, such as clubroot and sclerotinia stem rot.

***The AHDB Recommended Lists (RL) includes disease ratings for light leaf spot. As part of an integrated management approach for this disease, select varieties with a disease rating of at least 6.

It is important to inspect crops on a field-by-field basis, with prioritisation given to early-sown susceptible varieties.

Since light leaf spot often occurs in distinct patches, whole-field monitoring is required to assess disease severity.

To help confirm the presence of light leaf spot, incubate potentially infected leaf samples to bring out symptoms.

1. Take crop leaf samples that show light-green circular lesions.
2. Place leaves in a plastic bag with damp kitchen roll.
3. Mark where the sample – field/part field – was taken.
4. Leave samples in a warm room for two to three days.
5. Look for classic light leaf spot white spore masses around the lesion.

Note:

Sporulation may not occur in recently sprayed crops.

Across the growing season, one or two light leaf spot fungicide sprays are usually applied.

In a high-risk situation, apply the first spray in November.

This may coincide with the first or second fungicide application for phoma leaf spot, which will require the selection of a product with efficacy against both diseases.

Resistance management and plant growth regulation (PGR) requirements also drive product choice.

Continue to inspect crops throughout winter.

If light leaf spot is present, apply a fungicide as soon as possible, where conditions allow.

This may mean a follow-up treatment in January or February, which will be more effective than treating heavily diseased crops at the stem extension stage.

Recent fungicide experiments indicate that good control of light leaf spot can appear difficult to achieve, however, we still see yield responses to fungicides.

Some sites have shown benefits from using application rates above half dose, but others have not.

Appropriate fungicide dose is site and situation specific and depends on variety resistance rating, regional location and disease pressure.

For increased efficacy at high-disease-pressure sites, relatively high doses may be necessary.

Notes:

Sclerotinia fungicides can also have activity on light leaf spot, if the disease is present during flowering.

Products with PGR activity may reduce yields, when high doses are applied to stressed crops.

In laboratory tests, some light leaf spot isolates have decreased sensitivity to azole fungicides.

Currently, these shifts are not large and azoles continue to be as effective as non-azoles against light leaf spot in the field.

However, whenever a pathogen is present, even if it is not the target, the use of fungicides will drive selection for strains with decreased sensitivity to them.

Consequently, it is important to use various modes of action, in alternation or as mixtures/co-formulations, throughout the fungicide programme.