Infection and secondary spread can continue in mild weather from autumn right through to early winter and, in severe cases, the random foci merge into much larger areas.
These changes in leaf colour, together with associated alterations in the content of plant sap, attract further aphid vectors, compounding the BYDV problem.
'A few hours is all it takes'
Aphids alight on cereal plants in late summer/early autumn and, if carrying BYDV, can cause primary infections when feeding within a matter of hours. Aphid movement to neighbouring plants and the production of wingless young result in the much more damaging secondary spread of BYDV. As a consequence, saucer-shaped depressions, or foci, of discoloured and stunted plants appear from mid-winter onwards.
Cereals can also be infected via the 'green bridge' where wingless aphids 'walk in' from green debris (cereal volunteers or grasses) onto emerging cereals. This occurs most commonly in the West and South West of Britain and is associated with the bird-cherry oat aphid. Ploughing (especially of grassland) followed too closely by drilling will not break the 'green bridge' as aphids can survive on green (non-desiccated plant material) buried in the soil for several weeks, before making their way to the surface to infect the new crop.
Coastal areas of the South, South East and South West are habitually at high risk to BYDV, but in a mild autumn/early winter, almost any part of the UK cereals area can suffer damaging BYDV infections.
As disease development is heavily weather-dependent and influenced by the amount of virus carried by invading aphids, it is very difficult to predict likely BYDV infection levels. This helps to explain why roughly 60% of the UK winter cereals area has been routinely treated with an aphicide in autumn. Crops emerging before the end of September are particularly at risk from BYDV, but aphid invasions and the consequent primary spread of BYDV can continue throughout a mild autumn/early winter. The very damaging secondary spread of BYDV generally occurs between late October and January, but in mild weather can continue into early spring.
Effect on yield
BYDV-infected cereals produce lower yields and poor-quality, shrivelled grain; the earlier infection occurs, the greater the effect. In winter barley, severe infection can reduce yield by 70-80% and in winter wheat by 25-30%.