Factfile:
Phoma stem canker is caused by two species of fungi: Leptosphaeria maculans and L. biglobosa.
In autumn, fungal spores are released from infected crop stubble. The spores infect young susceptible leaves which then develop the characteristic leaf spot.
Fungi grow from the leaf into the leaf petiole and then infect the plant stem – the smaller the plant, the quicker this step can occur.
Once in the stem, the fungi kill tissue causing visible phoma stem cankers in the late spring and through the summer.
Cankers caused by L. maculans are generally associated with the stem base and are considered to be more severe than the upper stem lesions often associated with L. biglobosa.
This warning comes in the October update of its leaf spot forecast, hosted by Rothamsted Research.
It uses temperature and rainfall data from the summer to early autumn period to provide information on potential disease pressure and guide spray decisions against phoma.
With plenty of rain throughout August and unsettled conditions predicted for the foreseeable future, all UK regions are forecast to be at risk from a mass eruption of airborne spores during early to mid-October.
But due to the importance of localised rain/wetting events, the timing of visible leaf spot symptoms is forecast to vary tremendously across the UK.
Growers are advised to keep an eye on the phoma leaf spot forecast website to track local risk.
The forecast project’s Dr Neal Evans said: “The UK forecast map is based on more than 80 sites and is updated daily to show when each site is predicted to have 10% leaf spotting, which is the established threshold for treatment.
“Thus, the map can guide spray decisions but, as localised rain events play such an important role, nothing beats walking the crop and inspecting the leaves.”
The forecasting project also receives funding from Bayer CropScience.