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Disease resistance through varieties of wheat

Because purebred (genotypically identical) plant varieties often only have one or a few major genes for disease resistance, and plant diseases such as rust are continuously producing new races that can overcome a pure line's resistance, multiple linear lines varieties were developed. Multiline varieties are mixtures of several phenotypically similar pure lines which each have different genes for disease resistance. By having similar heights, flowering and maturity dates, seed colors, and agronomic characteristics, they remain compatible with each other, and do not reduce yields when grown together on the field.

In 1953, Borlaug extended this technique by suggesting that several pure lines with different resistance genes should be developed through backcross methods using one recurrent parent. Backcrossing involves crossing a hybrid and subsequent generations with a recurrent parent. As a result, the genotype of the backcrossed progeny becomes increasingly similar to that of the recurrent parent. Borlaug's method would allow the various different disease-resistant genes from several donor parents to be transferred into a single recurrent parent. To make sure each line has different resistant genes, each donor parent is used in a separate backcross program. Between five and ten of these lines may then be mixed depending upon the races of pathogen present in the region. As this process is repeated, some lines will become susceptible to the pathogen. These lines can easily be replaced with new resistant lines.

As new sources of resistance become available, new lines are developed. In this way, the loss of crops is kept to a minimum, because only one or a few lines become susceptible to a pathogen within a given season, and all other crops are unaffected by the disease. Because the disease would spread more slowly than if the entire population were susceptible, this also reduces the damage to susceptible lines. There is still the possibility that a new race of pathogen will develop to which all lines are susceptible, however.