Chapter 19 - Breeding for Disease and Insect Resistance

Abstract

Resistance or tolerance to pathogens and arthropod pests is crucially important for breeding successful horticultural cultivars. A pathogen is a type of parasite that is associated with a specific phenotype, known commonly as a disease. The specific disease will always depend on the interactions of three factors: host, pathogen, and environment. Disease is a phenotype, as contrasted with “healthy,” associated with an individual (host) interacting with a pathogen. Each of the three factors must be present within a certain range for the disease phenotype to be manifested. Plant pathogens possess structural and physiological modifications that enable them to gain access to the vascular elements and inter- and intra-cellular fluids that contain the host's energy and nutrient reserves. As the pathogen navigates and feeds within the host, it follows a characteristic path that is manifested by the host as a set of progressive symptoms. Horticultural crops are particularly affected by diseases since the value is usually defined by a narrow set of qualitative features. Two broad categories of host genetic disease resistance have been defined. Vertical resistance is characterized by host race-specific pathogen interactions and disease manifestations that are “all or none” depending on the compatibility of the host and pathogen. In contrast, horizontal resistance is not pathogen race-specific and is manifested in varying levels of disease manifestations on different host genotypes or cultivars. The “gene-for-gene” theory has been developed to explain the host-pathogen dynamics involved with vertical resistance: resistance (R/r) genes in the host interact with specific corresponding avirulence (Avr/avr) genes in the pathogen to enable or disable the disease phenotype. Races of a pathogen occur commonly and are distinguished based on a characteristic and predictable pattern of compatible and incompatible host interactions, called a host differential. Horizontal resistance, in contrast, is not controlled by major R genes and Avr pathogen interactions, but rather an interaction of many genes. Therefore, the backcross method is effective for breeding new cultivars with vertical resistance, but pedigree or recurrent selection is preferable for horizontal resistance. Polyploid derivatives often exhibit higher levels of disease resistance than corresponding diploids. The plant breeder often embarks on the development of new techniques to distinguish among genetically resistant and susceptible individuals within a population, known as disease screening methods. MAS is providing a way to select more effectively for disease resistance alleles by dramatically increasing the heritability of the disease phenotype. Three fundamental strategies for breeding insect pest resistance have emerged from experimental studies: (i) introducing resistance transgenes into the host genome, (ii) exploiting natural variation in resistance already present in the crop gene pool, and (iii) introgressing resistance from sexually compatible wild relative species. With regard to resistance or tolerance to arthropod pests, plants have evolved diverse ways to cope with damage from herbivory. Scientific studies of the molecular genetics and transcriptional background of this variation have facilitated the identification of resistance genes and processes that lead to resistance against insects. For example, plant features such as cuticular hairs or trichomes and excretions thereof have been demonstrated to play a role in host preference to insect pests. While the implementation of MAS for host insect pest resistance is in progress, applications are fewer and less impressive than applications of MAS for host disease resistance. The successes of Bt-based insect pest resistance strategies have been dampened by reports of the spread of transformed Cry DNA sequences by pollen and virus. The consensus from many independent experimental studies on non-targeted effects of insect resistance, and especially of transformed Bt genotypes, on crop and natural ecosystems is, however, that impacts are minimal.