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:
“My profession brought me in contact with various minds. Earnest, serious discussion on the condition of woman enlivened my business room; failures of banks, no dividends from railroads, defalcations of all kinds, public and private, widows and orphans and unmarried women beggared by the dishonesty, or the mismanagement of men, were fruitful sources of conversation; confidence in man as a protector was evidently losing ground, and women were beginning to see that they must protect themselves.”
—Harriot K. Hunt (18051875)
“Isolate city spread alongside water,
Posted with white towers, she keeps her face
Half-turned to Europe, lonely northern daughter,
Holding through centuries her separate place.”
—Philip Larkin (19221986)