In this section

• Recombination and selection
• Resistance is the rule
• Breaching the defensive barriers
• Breeding for resistance
• Recombination and selection strategies
• Characteristics of resistance
• Potential for pest variability with respect to resistance
• Deployment of resistance genes
• Incorporation of host plant resistance into an IPM program

Recombination and Selection

For 10,000 years humans have been selecting the best of the seeds of their crops to save for planting in the next cropping cycle. Over thousands of generations they have been improving their crops without any scientific understanding of what they were doing.

With the rediscovery of Gregor Mendel's principles of heredity early in the 20^th century, the science of plant breeding began. Plant breeders developed improved cultivars by selecting the desired plants and making planned crosses in repeated cycles over many generations, often taking advantage of characteristics found in wild relatives. Half a century later, plant breeders began to use mutagenic chemicals and radiation to create genes that could not be found in existing breeding populations or readily transferred from wild relatives. The desirable mutant genes were incorporated into cultivars using the principles of classical plant breeding.

Toward the close of the 20^th century, knowledge of the universality of the genetic code and the development of recombinant DNA technology allowed plant breeders to transfer into plants genes from distantly related organisms. In a continuum of scientific and technological development–from simple selection of the best to the creation of a plant genetically modified by human design–plant breeding has reached a level where it can truly be called "genetic engineering."

Early plant breeding efforts to create cultivars resistant to diseases and insects focused solely on the crop itself and its wild relatives. Only when the resistance (after a period of effective control of the pest) appeared to "fail," did scientists begin to look at the genetic changes in the pest population and appreciate the impact of inadvertent human-directed evolution of the pest. Indeed, it looked as if plant breeders were destined to struggle on a "genetic treadmill" in which they would forever be obliged to seek new resistance genes and incorporate them into new cultivars as fast as the pests could overcome them. These days, the strategies for deployment of resistance genes to sustain their effectiveness over the long term have become as important as the breeding strategies themselves.

In this unit, we use "genetic manipulation of the crop" to cover the gamut from traditional plant breeding to genetic engineering and gene deployment.