PHOMA leaf spot is already raising its ugly head in oilseed rape this autumn.

The October update of the AHDB-funded phoma leaf spot forecast shows the disease has got off to a head start. The forecast uses temperature and rainfall data to provide information on potential disease pressure.

This week’s forecast showed that treatment thresholds had already been breached, though all were south of the Border – but there are obvious implications for Scottish farmers, too.

Visual reports from fields in these areas had confirmed the forecast and AHDB has said that spray timing will be the key to success for phoma control and the forecasts can be used to focus crop walking and guide spray decisions against phoma – but spraying conditions remains ‘challenging’.

The UK forecast map uses a colour-coded system to show how long it will take for each site monitored to reach the spray threshold of 10% of plants affected.

Dr Neal Evans, Weather INnovations Consulting LLP (WIN), who puts the forecast together, said: “The mild autumn and wet August and September have really pushed the development of the fungus on this year. It’s led to the early release of spores and the appearance of leaf infections at the earliest time for several seasons.”

Recent additions to the AHDB Recommended List (RL) have added strength to the overall resistance to phoma stem canker, with several varieties having a resistance rating of 8.

There are several varieties, however, with relatively low resistance ratings (3 to 4).

These should be the priority for treatment, particular for backward crops.

Making a spray application as close as possible to the threshold timing can help maximise the effect of fungicides.

The latest information shows good control can be achieved with half rates applied as a two-spray programme.

A second spray should be made when re-infection is evident – typically four to ten weeks after the first spray.