Very early phoma onset forecast

Watch out for phoma leaf spot in winter oilseed rape, as current risk could be higher than you think.

According to the phoma leaf spot forecast (updated yesterday), the unsettled spells of weather during August and the middle of September have accelerated the development of the fungus.

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 per cent of plants affected.

In fact, the phoma season appears to be so early that many of the sites monitored have already passed this threshold. Although forecasts can only ever estimate what is happening in the field, AHDB has received reports of phoma spots showing in the field. This confirms the predictions made by the phoma forecast model.

Early phoma epidemics are the most damaging to yield, although rapid re-infection in the late autumn can also reduce yields. Making a spray application as close as possible to the threshold timing can help maximise the effect of fungicides. Varieties with a relatively low resistance ratings (3 to 4) and backward crops should be the priority for treatment.

A second spray should be made when re-infection is evident – typically four to ten weeks after the first spray. As some varieties are also likely to require a late-autumn fungicide (November) for light leaf spot control, this should be considered when planning autumn programmes.

Fungicide performance information can be accessed via ahdb.org.uk/knowledge-library/fungicide-performance

The phoma leaf spot forecast can be accessed via ahdb.org.uk/knowledge-library/Phoma-and-light-leaf-spot