Sources and Spread
There are two main sources by which a cereal crop might be infected
1. By non-migrant wingless aphids already present in the field and which colonise newly-emerging crops. This is known as "green-bridge transfer".
2. By winged aphids migrating into crops from elsewhere. These then reproduce and the offspring spread to neighbouring plants.
Read more about this topic: Barley Yellow Dwarf
Famous quotes containing the words sources and/or spread:
“The sources of poetry are in the spirit seeking completeness.”
—Muriel Rukeyser (1913–1980)
“[Allegory] should ... be very sparingly practised, lest, whilst the writer plays with his own fancies and diverts himself by cutting the air with his wide spread wings, he should soar out of view of his readers, leaving them in confusion and perplexity to explore his viewless track.”
—Sarah Fielding (1710–1768)