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Phoma alert as wet weather puts pressure on oilseed rape crops

The onset of phoma has started early this autumn at some locations following days of repeated heavy rainfall. Some oilseed rape may already have 10 per cent of the crop affected, according to monitoring reports.

The first phoma leaf spot symptoms have already been reported on mid-August sown crops in south Lincolnshire and Cambridgeshire, says ADAS plant pathologist Philip Walker.

 

“Subsequent rainfall will further aid the release of air-borne spores and encourage leaf infection. Depending on temperatures, first lesions usually appear two to three weeks after spore release so we may see some crops reaching threshold around mid-October.

 

“Crops should continue to be monitored for signs of infection with the aim to apply fungicides at 10 to 20 per cent plants affected.”

 

Effective

 

Fungicide applied in late October/early November has been shown to be an effective strategy to provide early protection for both phoma and light leaf spot, he adds.

 

“A programme based on two autumn sprays at half the recommended rate of products has been shown to be a robust strategy for control.”

 

Key fungicide application timings help to prevent leaf surface lesions developing and the invisible spread of infection to the plant’s petioles and stem, which later prevents stem cankers forming, says Georgina Wood, Syngenta technical manager.

 

Delayed

 

With many crops still at small growth stages after delayed emergence as a result of dry conditions in August, there is also a serious risk of rapid infection spread in susceptible plants this season, she says.

 

“Plants currently with small leaves are more susceptible and a priority for an early fungicide treatment such as Plover, since it takes less time for the infection to move to the stem, compared to plants with larger leaves,” she says.

 

“Syngenta trials in recent seasons have shown initial phoma thresholds were reached an average 92 days before the first signs of light leaf spot, which highlights the crucial importance of the early sprays to specifically target phoma,” she says.


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Action points for phoma control

  • 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 10–20 per cent of plants have phoma leaf spot
  • Only treat varieties with high resistance ratings for stem canker (8 to 9) if more than 20 per cent of plants have phoma leaf spot
  • When reinfection occurs, consider a second spray – typically, four to ten weeks after the first spray
  • Adjust spray programmes to account for any late autumn fungicide (November) required for light leaf spot control
  • Access the forecast and fungicide performance data via ahdb.org.uk/phoma

Source: AHDB

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