Very early phoma leaf spot forecast in oilseed rape

Wet and warm conditions have generated an extremely early forecast for phoma leaf spot development in oilseed rape.

Based on rainfall and temperature information, the online model forecasts the dates (at hundreds of sites across England, Scotland and Wales) when 10% of oilseed rape plants may develop phoma leaf spot symptoms (caused by Leptosphaeria maculans).

With around half of the sites forecast to have passed this date by the end of September, it is the earliest predicted disease pressure in many years.

Rachel McGauley, who helps manage the forecast at AHDB, said:

“Unusual weather was a feature during a key modelling period (15 July to 26 September), which saw one of the wettest Julys on record, a mixed August (with two named storms) and an unprecedented heatwave at the start of September, with mixed weather following. In general, summer was wet and warm, and phoma onset is likely to be very early.”

The forecast is most accurate in regions with no/limited variability in forecast dates. However, variability is very large this autumn, with few patterns, except that the sites with the latest dates (mid-to-late October) are located towards north-eastern GB areas.

Due to the variations in the forecast this autumn, crop inspections are even more important.

Guidance is to 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.

AHDB fungicide performance data shows application timing is critical, with good control achieved with two sprays at half the recommended rate. 

A second application should be considered when re-infection is evident (typically, 4 to 10 weeks after the first application).

There are several modes of action available for the control of phoma leaf spot/stem canker, including azole and non-azole options.

As part of a robust resistance management strategy, modes of action should be used in alternation or as mixtures/co-formations throughout the fungicide programme.

The Recommended Lists (2023/24) features oilseed rape varieties with high levels of resistance to phoma stem canker (disease ratings 8 or 9).

To access the forecast and phoma management information, visit: ahdb.org.uk/phoma-leaf-spot-forecast

AHDB is discussing ways to provide forecast data earlier (before 27 September), which is especially important in years associated with very early disease pressure.