Light leaf spot is now being reported from crops in all regions, reports ADAS in its latest Crop Action bulletin. The Rothamsted Light leaf spot forecast has been updated to take account of the winter weather (www.rothamsted.ac.uk/Content.phpSection=Leafspot&Page=llsforecast2) and shows increased risks particularly in the east (now 46% crops at risk compared with 27% last autumn) and south (42% crops at risk compared with 32% last autumn). Note these forecast percentages relate to crops with 25% of plants affected, and a lower threshold of 15% of plants affected at early stem extension is justified at current crop values. Some affected crops have received a fungicide spray recently.
It is difficult to diagnose light leaf spot in the field until you can find symptoms on new growth. Be prepared to incubate samples in a polythene bag for a few days as the diagnostic white spore droplets of light leaf spot can often be found on apparently healthy green leaves. Where light leaf spot pressure is high, there are usually distinct patches and symptoms will be present on the young, partially expanded leaves.
The priority for fungicide treatment now changes to light leaf spot as yield responses can exceed 1 t/ha from late winter/spring treatments. Recent HGCA Fungicide Performance experiments have shown that standard two spray programmes (November + March) are only partially effective and may not be improved by using doses greater than half dose. Consider using a two spray approach in late winter and spring with the first spray applied when symptoms are found before stem extension and the second treatment about 4 weeks later. If crops have been defoliated by pigeons it is better to wait for some growth so that there is a target for spray uptake.
There have been more reports of high levels of phoma leaf spot in crops. Phoma A leaf spots of various sizes are present on several leaves per plant in untreated crops and where fungicide protection has not been maintained. Over 90% plants are affected in some crops in the east, with early January sprays providing good control. A few Phoma B leaf spots are also present, mainly at low levels. It is now rather late to expect fungicides to give good control of phoma cankers so make decisions based on light leaf spot activity in the crop.