Abstract
The approaches utilized to obtain host plant resistance can be categorized by the methods used to manipulate (in the broadest sense) plant genetics: the conventional approach utilizing Mendelian genetics, the biometrician's approach and more recently the approaches of the biotechnologists. The Mendelians utilize the inheritance of characteristics from plant germplasm that are qualitatively variable and can be transferred from a source plant(s) to the recipient plants by a process of breeding and selection. The biometricians utilize the inheritance of characters that are quantitatively variable and controlled by many genes (i.e. polygenic characters). The biometricians developed methods of plant breeding (population breeding) that involve changes in gene frequency for a particular character. The biotechnologists are more akin to the Mendelians but the approach is based on techniques to transfer single genes from unrelated sources in crop plants. Both the Mendelians and the biometricians manipulate only the primary and secondary gene pools of the cultivated species for crop improvement while the biotechnologists, through advances in tissue culture and molecular biology, have made it possible to introduce genes from diverse sources such as bacteria, viruses, animals and unrelated plants into crop plants. The relative merits of the different approaches are addressed throughout this chapter. Case studies are presented, dealing with the use of vertical resistance in wheat/rust (leaf brown rust, stem or black rust and yellow (striped) rust caused by Puccinia recondita, P. graminis and P. striiformis, respectively) pathosystems; horizontal resistance to tropical rust (Puccinia polysora) in Africa; directed mass selection for leafhopper (Empoasca fabae), alfalfa [lucerne] aphid (Therioaphis maculata [T. fiscellaria somniaria]) and diseases (including rust, common leafspot, bacterial wilt and anthracnose) in alfalfa; the efficiency of visual assessment of grain yield and its components in spring barley rows; and Gardner's grid system and plant selection efficiency in cotton.