Since the Green
Revolution is likely to increase the rate at
which resistance genes are overwhelmed by virulent
races and since in some crops there is already
a shortage of new strong genes, we need to find
how best to deploy our limited supply of resistance
genes. As part of a possible best strategy,
the growing of "multiline" varieties
has been suggested as a means of bringing the
crop into a stable equilibrium with the disease
races.'° A multiline variety is a mixture
of genetic types (lines) of a crop similar in
growth characteristics but which differ in the
resistance genes which they carry. Such varieties
have already been produced for wheat (in Colombia)
and oats (in Iowa, USA).
Multilines can
improve significantly the control of any disease
that has an air-borne dispersal phase (rusts,
mildews, septorioses, helminthosporioses, Rhynchosporium
and even Pseudocercosporella herpotrichoides),
often to the extent that the use of fungicide
becomes uneconomic. Because of this and other
interactions among the components, mixtures
provide a buffer against environmental variation
so that yield is stable among environments.
Disease develops
slowly in the multiline, but, inevitably, one
of the lines in the multiline will be more infected
than the remainder. Because of the infection,
this variety may use less space and fewer resources
(light, water, nutrients) than its neighbours
– which can then use those surplus resources.
So, the reduced yield of the more susceptible
plants is compensated by the increased yield
of the more resistant plants. Such compensation
is less likely in a monoculture where all plants
are equally susceptible. This is part of the
reason why mixtures are more stable than monocultures
in terms of yielding ability in different environments.
