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

As usual, integrated pest management (IPM) is essential for disease-management success. The timing of any fungicide treatment is particularly critical.

1. Bury infected debris and increase the distance between previous and current season’s oilseed rape fields to reduce disease pressure.
2. Select varieties with strong resistance to diseases, including phoma (see the AHDB Recommended Lists).
3. Check the AHDB phoma leaf spot forecast for a regional indication of seasonal risk (e.g. if early onset is likely).
4. During autumn, monitor oilseed rape for phoma leaf spots (prioritise susceptible varieties and small crops).
5. Look on the underside of leaves; if white tufts (mycelium and spores) are present, the symptom is downy mildew, not phoma.
6. Note that most crops usually start to breach treatment thresholds in October.
7. Check AHDB fungicide performance data for information on product efficacy against phoma.
8. A fungicide applied as close as possible to a threshold helps maximise its effect.
9. Treat varieties with lower resistance ratings for stem canker (7 and below) and backward crops first, when 10–20% of plants have phoma leaf spot.
10. Only treat varieties with high resistance ratings for stem canker (8 to 9) if more than 20% of plants have phoma leaf spot.
11. When reinfection occurs (based on the presence of new lesions), consider a second spray – typically, four to ten weeks after the first spray.
12. Adjust spray programmes to account for any late-autumn fungicide (November) required for light leaf spot control (consider efficacy against both phoma and light leaf spot).
13. Note that late (December–February) leaf infection or re-infection is unlikely to warrant control, unless plants are relatively small.
14. Use a range of mode of actions to maximise control and reduce fungicide resistance risks.

There are two types of resistance to phoma in oilseed rape. Firstly, single genes (R-genes) that act in the leaf, which can be very effective. Secondly, multiple genes (quantitative) that act in petioles and stems, which can be effective, especially when combined.

The stem canker resistance ratings in the AHDB Recommended List use a standard 1 (susceptible) to 9 (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. It is important to note that although some resistant varieties may exhibit high levels of spotting, they will 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 (e.g. Metconazole and tebuconazole) activity.

Although variation in fungicide sensitivity in the two phoma pathogens has been observed, it is not large enough to warrant separate management strategies under field conditions.

No resistance to fungicides has been recorded in the UK phoma population. However, mechanisms for azole resistance have been identified in other countries. Consequently, it is important to use various modes of action, in alternation or as mixtures/co-formulations, throughout the fungicide programme.