Not all viruses are bad. In a few cases the effect of a virus on
the host may be prized. For example, variagation of flowering maple
leaves and tulips is caused by infection with specific viruses (Fig.).
Also, it has been known for many years that a mild infection of
a plant can protect it from damage caused by more severe isolates
of the same virus. This phenomina is known as ‘cross protection’.
An example is the control of very damaging citrus tristezia disease
in Brazil by inoculation of young trees with mild isolates of citrus
tristezia virus. In the case of dahlias, cross protection is at
present problematic because a virus strain may be mild in one cultivar
but severe in another. Also some dahlia plants may already be infected
by more than one virus and there are many examples of different
viruses interacting in unexpected ways to cause severe damage. The
cross protection mechanism is not well understood we do not know
enough to employ it safely. It should be noted that cross protection
does occur naturally sometimes.