Incidence of Viruses in 2007 and 2008

Potato Leafroll Virus

Model.  A predictive model has been developed at SASA and can be used to estimate the incidence of leafroll in the current Scottish seed potato crop.  The model uses the numbers of flying Peach-Potato aphids (Myzus persicae) caught in suction traps and the incidence of leafroll in seed crops recorded during crop inspection to estimate the expected incidence of leafroll in the crop during the following season.  Two calculations have been made, one using aphid data from the Dundee suction trap, and the other using data from the Edinburgh trap. 

2007.  During growing crop inspections in 2007, plants affected by leafroll were found in seed crops covering an area of 180 ha out of a total of 11,143 ha, i.e. leafroll was present in 1.6% of the seed area, a decrease from 1.7% in 2006.  Using Dundee and Edinburgh aphid data from 2006, it was estimated that leafroll would be present in 3.3% and 2.2% respectively of the total area entered for classification in 2007.  The observed incidence of leafroll symptoms present in 1.6% of the area of the entered crop area is in line with the level of control achieved in most years since the introduction of the aphid monitoring programme. 

2008.  Using the virus data and Dundee aphid data collected in 2007, the model estimates that in 2008, 4.2% of the total area entered for classification will be found to contain leafroll affected plants.  A slightly lower figure of 3.2% is predicted using the Edinburgh aphid data.  If virus control measures, including aphid monitoring during 2007, were as effective as they had been in the earlier years of aphid monitoring, we may expect these levels to be reduced to less than 50% of the expected value, i.e. around or below 2%.

Summary.  The area of seed crops planted in 2008 that will contain at least one plant affected by leafroll is likely to be similar to the 2007 figure of 1.6%. If growers employ good virus management practices, including the early roguing of infected plants and effective aphid control, then leafroll can be effectively managed.  This is particularly important following the relatively mild 2007-2008 winter, which presents a higher risk of virus transmission in growing crops because they greater likelihood of early-season aphid activity and consequent greater risk of virus spread.

Mosaic viruses

In 2007, plants affected by mosaics were observed in 12.6% of the seed crop area (up from 6.5% in 2006).  The dominant causal virus is PVY, estimated as being present in 8.6% of the seed crop.  Predictions based on the abundance of cereal aphids during 2007, suggest that this value will fall to below 4% for the 2008 crop.  The next most important causal virus is PVA, estimated as being present in 4.3% of the seed crop.  Although the epidemiology of this virus is not yet understood, work carried out at SASA during 2007 indicates that the incidence of PVA in the 2008 growing crop is likely to be higher than in 2007.