Results from the phoma risk forecast show that a dry September has led to an unusual pattern of phoma development this autumn so growers need to pay close attention to get spray timings spot on.

The forecast predicts when 10% of the crop will be affected by phoma as 10-20% infection is the threshold for the first fungicide treatment. It is based on weather data from a number of sites in the UK so growers can use the information from sites close to them to support decision-making on-farm.

"It's been an unusual year for phoma, many sites are delayed in comparison with last year and the main limiting factor has been rainfall," explains Neal Evans of Weather INnovations Consulting LP who puts together the forecast.

"September has been warmer than normal which would encourage phoma, but it has been much drier too which has limited its development. This has meant that localised rainfall has had a big effect on the overall forecast."

For example, Petham in Kent is predicted to have already passed the threshold on 22 September whereas Manston, less than 20 miles away, is predicted to pass it on 30 October. Looking at the whole UK, one site in north-east Scotland is already past the threshold which is quite unusual for a site so far north.

"With such a mixed picture, it emphasises the importance of walking crops and assessing the conditions on-farm so that sprays are right," continues Dr Evans.

Phoma control is, typically, based on a two-spray programme of triazole fungicides, with the correct timing good levels of control can be achieved.

Fungicide performance information against phoma will be updated by HGCA in the coming days to take account of the latest information; however, indications are that there have been no major changes in product efficacy.

Get fungicide performance and disease management info at hgca.com/disease control

View the phoma risk forecast at http://www.rothamsted.ac.uk/phoma-leaf-spot-forecast