http://pdf.usaid.gov/pdf_docs/PNAAC914.pdf
While most plant breeders recognize the importance of utilizing genetic diversity in breeding programs, much of the breeding efforts in the past has largely been based on small segments of locally adapted cultivars. Only in recent years has it become apparent that plant breeding programs with a broad genetic base could deter rapid and widespread epidemics of plant diseases and outbreaks of insect pests and thus sustain a high level of crop yield. To meet the continuously expanding needs of varietal improvement, the assemblage, evaluation, and preservation of the entire existing germ plasm are essential to more rewarding breeding efforts. This manual discusses these aspects as they relate to rice.