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Causal organism - Barley yellow dwarf virus
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Barley yellow dwarf virus became very much more important and widespread
with the increase in early-drilling of winter cereals. The virus can be transmitted
by several species of cereal aphid. The bird-cherry aphid (Rhopalosiphum
padi) 
is the principal vector in the south of England. In the north of England and
in the Midlands the grain aphid (Sitobion avenae) is
usually more important.
In the autumn, BYDV can be introduced into cereal crops in two ways:-
Direct transfer by wingless aphids living on grass or on volunteer cereals, which survive cultivation, move through the soil, and colonise the following cereal crop. (This is much more common in coastal areas of the south west where cereals more commonly follow grass and winters can be mild).
Indirect transfer by winged aphids migrating into newly emerged crops from grass or volunteer cereals elsewhere. BYDV introduced by winged aphids flying into crops is generally more common and important than BYDV resulting from direct transfer.
Importance
With the now common practise of sowing winter cereals very early, BYDV has increased in importance in many areas of the UK. The frequency of very mild winters has also meant that, for many farms, BYDV is now a regular problem.
