AN ALERT to be vigilant against phoma in oilseed rape crops has been issued after some disturbing evaluation of the disease’s effect this year.

Some winter oilseed rape may already have 10% of the crop affected by phoma, according to the leaf spot forecast for the disease, according to AHDB, which is monitoring the situation.

A monitoring site in Herefordshire (ADAS Rosemaund) has been the first site predicted to reach that level of infection, which indicated a need to consider treatment in some situations.

Crops usually start to breach treatment thresholds in October. Compared to 2018/19, a later disease onset is forecast for most sites. However, phoma onset appears to have started relatively early this autumn at some locations.

Catherine Harries, who manages disease research at AHDB, said: “Late-summer and early autumn rain speeds up the maturation of phoma spores on stubble. In some areas, it was relatively wet over this critical period – hence, the potential early onset. In fact, there have been some field reports of early spotting in crops.

“Naturally, there’s a lot of variation. For example, the South-east was very dry up to a week ago and that tends to delay development. People should check the forecast, monitor for spots and treat susceptible varieties, when thresholds are reached.”

The phoma forecast uses temperature and rainfall information to simulate the development of leptosphaeria maculans, a key pathogen responsible for the leaf spot and phoma stem canker. 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 is the 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 10–20% 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
  • When reinfection occurs, consider a second spray – typically, four to 10 weeks after the first spray
  • Adjust spray programmes to account for any late-autumn fungicide (November) required for light leaf spot control.