Phoma leaf spot and stem canker

Phoma leaf spot/stem canker is one of the most important diseases of oilseed rape. This page includes information about the disease and its management.

Pathogen

Leptosphaeria maculans and L. biglobosa

Hosts

Oilseed rape

Symptoms

Phoma stem canker is caused by two closely related fungal plant pathogens, Leptosphaeria maculans and L. biglobosa. The former causes tawny-colour leaf spots with dark specks (asexual fruiting bodies, known aspycnidia), while the latter causes darker leaf spots (that contain few, if any, pycnidia). Cankers caused by L. maculans are, generally, associated with the stem base and are considered to be more severe than the upper stem lesions often associated with L. biglobosa.

Life cycle

Phoma stem canker goes through one complete disease cycle each season (i.e. it is monocyclic). In the autumn, airborne ascospores are released from infected stubble. Spores infect the leaves of susceptible plants, from emergence onwards, and this results in the characteristic leaf spots. From the leaf, the disease grows along the petiole to the plant stem without causing any further visible symptoms. Once at the plant stem, the fungi invade and kill plant cell tissue, resulting in the formation of phoma stem cankers. The disease reduces yield by restricting water and nutrient transport through the stem, resulting in premature senescence. In severe cases, phoma stem cankers may sever the plant stem, killing the plant.

Importance

Phoma stem canker is one of the most important diseases of winter oilseed rape in England. The disease is predicted to result in losses of about £100M* each season, despite fungicide treatment. Early phoma epidemics are the most damaging to yield and typically put 0.5 t/ha of yield at risk, although rapid re-infection in the autumn can also reduce yields. Late epidemics (occurring in February/March) can be very damaging, if plants are small in late autumn or winter.

*Estimate based on annual survey data presented through CropMonitor.

Risk factors

  • Crop residues – residues are an important source of inoculum
  • Region – the phoma pathogen population makeup varies across the UK. The disease is frequently the most important in southern and eastern England
  • Weather – phoma pathogens develop fastest in warm, wet conditions. Localised showers that re-wet debris favour pathogen development. Although development is not limited by low temperatures, it is slowed in cold seasons
  • Crop growth stage – the pathogen grows quickly from leaf to stem on small plants. Strong, early vigorous growth can help the plant grow away from disease. Early drilled crops (sown by late August) are often well-grown prior to the onset of the disease.
  • Varieties – cankers are more severe in susceptible varieties

Management

Varietal resistance

In oilseed rape there are two types of resistance against the phoma pathogens: R-gene (single gene) and multiple gene (quantitative). R-genes act within the leaf and can be very effective. However, phoma populations can change rapidly and influence the effectiveness of any particular R-gene. AHDB-funded work has found regional differences in the effectiveness of different R-genes. Many plant breeders agree that varieties should aim to combine effective R-gene resistance with background quantitative resistance (that acts within petioles and stems).

The stem canker resistance ratings in the AHDB Recommended List provide information on varietal resistance in winter oilseed rape on a 1 (susceptible) to 9 (resistant) scale. These annually updated ratings are compiled from final assessments of stem canker severity in late spring and do not include phoma leaf spotting data, nor do they distinguish between phoma species. Where phoma leaf spot is a target for management, varieties with a stem canker rating of 5 or above should be selected. Varieties with good resistance may only require a single fungicide application at threshold to acheive control.

Chemical control

The timing of fungicide applications is important to achieve phoma control. Use fungicides in response to disease forecasts, crop monitoring and thresholds.

The AHDB phoma forecast uses temperature and rainfall information (1 July to 26 September) to simulate the development of L. maculans. Accounting for subsequent crop infection, the forecast predicts the date when 10% of oilseed rape plants could potentially show symptoms of phoma leaf spot*. This level of infection relates to a treatment threshold for varieties with lower disease ratings for stem canker.

Action points

  • Monitor crops for phoma leaf spots
  • Look on the underside of leaves, if white tufts (mycelium and spores) are present, the symptom is downy mildew, not 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
  • Only treat varieties with high resistance ratings for stem canker (8 to 9) if more than 20% of plants have phoma leaf spot

*It is important to note that localised rain events can mean that nearby sites may have predicted dates of 10% leaf spotting that differ by a month or more. 

AHDB’s fungicide performance work shows that good control of phoma leaf spot and stem canker can be achieved with two sprays at half the recommended label rate. The second treatment is applied when re-infection is observed. Typically, this is 4 to 10 weeks after the first treatment. It should be noted that all azoles offer protection when applied prior to infection, though product choice will also be influenced by requirements for curative activity when small plants are infected. PGR effects of some actives should also be taken into account where plants are small. Some varieties are also likely to require an autumn fungicide (November) for light leaf spot control (if there is a risk) and this should be considered when planning autumn programmes.

Further information

Phoma leaf spots can be caused by two closely related pathogens. Image (C) Yongju Huang

Phoma stem canker symptoms in oilseed rape. Image (C) Neal Evans.

Phoma forecast

Phoma leaf spot forecast 2019-20

Click on the site "pointer" symbols to reveal phoma forecast information for that location. 

NOTE: Date ranges are displayed Month/Day/Year (US format)

Raw data (updated 10 October 2019)

1 = No symptoms. 2 = Infection started. 3 = 10% incidence. 4 = No data.