How to manage phoma in oilseed rape

Phoma leaf spot and stem canker is an important disease in oilseed rape. Through understanding the drivers of risk, treatment thresholds and fungicide efficacy, it is possible to lessen the level of yield losses.

The following factors are associated with elevated phoma risk in winter oilseed rape:

• Crop residues, which are the main source of spores
• Region, with the disease most damaging in Central, Southern and Eastern England
• Warm, wet late-summer/autumn weather, which encourages spore release, infection and spot development
• Varieties, as stem cankers are more severe on susceptible varieties
• Higher temperatures during autumn/winter – at low temperatures, the pathogen grows as slow as 1 mm per day, which helps the plant out-grow infection
• Small plants, as the pathogen moves from the leaf to the critical stem area faster

Notes:

Early phoma epidemics are the most damaging to yield.

These typically put 0.5 t/ha of yield at risk, although rapid re-infection in the autumn can also reduce yields.

Plants sown by late August are usually well-grown before the onset of phoma leaf spot.

Late epidemics, in February/March, can be very damaging if plants are small in late autumn or winter.

As early drilling can increase light leaf spot risk, it is important to monitor all crops regularly, irrespective of the drilling date.

Integrated pest management (IPM) is essential:

• Bury infected debris*
• Extend the rotation**
• Increase the distance between previous and current season’s oilseed rape fields to reduce disease pressure – by 200 to 500 m, if possible
• Select varieties in the Recommended Lists (RL) with strong resistance to diseases, including phoma
• Check the AHDB phoma leaf spot forecast for a regional indication of seasonal risk
• During autumn, monitor oilseed rape for phoma leaf spots – prioritise susceptible varieties and small crops***
• Check AHDB fungicide performance data for information on product efficacy against phoma
• A fungicide applied as close as possible to a threshold helps maximise its effect****
• Treat varieties with lower resistance ratings for stem canker, 7 and below, and backward crops first, when 10 to 20% of plants have phoma leaf spot
• Consider treating varieties with high resistance ratings for stem canker, 8 to 9, if more than 20% of plants have phoma leaf spot
• When reinfection occurs, based on the presence of new lesions, consider a second spray – typically, four to ten weeks after the first spray
• Varieties with high resistance ratings do not usually require a second spray
• Adjust spray programmes to account for any late-autumn fungicide required for light leaf spot control – consider efficacy against both phoma and light leaf spot
• Note that December to February leaf infection or re-infection is unlikely to warrant control, unless plants are relatively small
• Use a range of mode of actions to maximise control and reduce fungicide resistance risks

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 phoma and light leaf spot, as well as soil-borne disease, such as clubroot and sclerotinia stem rot.

***Look on the underside of leaves; if white tufts of mycelium and spores are present, the symptom is downy mildew, not phoma.

****Fungicide treatment timing is critical. Most crops usually start to breach treatment thresholds in October.

Although phoma leaf spot is detected in some Scottish crops, local conditions mean it seldom develops to the damaging canker-forming stages.

As a result, light leaf spot is the main target of sprays in Scotland.

There are two types of resistance to phoma in oilseed rape.

Firstly, single R genes that act in the leaf, which can be very effective.

Secondly, multiple genes that act in petioles and stems, which can be effective, especially when combined.

The RL stem canker resistance ratings use a standard 1 to 9, susceptible to resistant, scale.

Updated annually and based on final assessments of stem canker severity in early summer, varieties with strong levels of resistance are far less likely to require multiple fungicide applications.

Note:

Although some resistant varieties may exhibit high levels of spotting, they only develop low levels of canker.

Two sprays at half the recommended label rate are sufficient to achieve good control of phoma leaf spot and stem canker, according to AHDB fungicide performance data.

Although complete control is possible with three sprays, it is not economical to use such an intensive fungicide programme.

When applied prior to infection, all approved fungicides offer protection.

However, infected small plants will require consideration of curative and plant growth regulator activity.

There is now a range of modes of action available, including azole and non-azole options, for the control of phoma leaf spot/stem canker.

No resistance to fungicides has been recorded in the UK phoma population.

However, mechanisms for azole resistance have been identified in other countries.

Using a range of modes of action, in alternation or as mixtures/co-formulations, throughout the fungicide programme is necessary as part of a robust fungicide resistance management strategy to prevent the selection for fungicide insensitive strains.