The term Multiline Genetics is used for a variety is a composite of genetically identical lines, except that each line possesses a different gene for resistance to the disease. This is also called Multiline Varieties.
In the event of occurrence of a new race, some plants may be susceptible, but not all. This mix of susceptible and resistant plants should provide a buffering effect against rapid disease development and thus extend the life of resistance genes.
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).
Multi-lines can improve significantly the control of any disease that has an airborne 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.
Pest and disease develop slower 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 neighbors – 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.
