Chapter 4 - Phenomenology of plant-parasite relations

Abstract

From the phenomenological point of view, plant pathologists discovered at least two types of resistance to many diseases: vertical and horizontal resistance. Variations in any trait of an organism, including its response to infection, depend both on its genotype and conditions of its life. The contribution of the environmental variance is estimated from the behavior of the same genotype in different life conditions. Resistance to fungi is usually dominant resistance, while resistance to viruses is often recessive. The principal genetic difference between horizontal and vertical resistance does not contain a number of genes that control resistance but the ratio of interacting and noninteracting effects. In the presence of only the additive variance, even heterozygosis in one resistance locus can result in a continuous series of phenotypic segregation corresponding to the horizontal resistance. Qualitative and quantitative manifestation of vertical and horizontal resistance influences the signs of diseases in the plants. The chapter explains the horizontal and vertical resistance assessment of systemic diseases, which is even more complex. The relations of resistances with parasites are explained with the help of separate subheadings, including the phenomenology of relations. Investigation of the genetics of plant–parasite relations includes a parallel genetic analysis of the plant resistance and virulence of its parasite. The most important conclusion that is drawn from the parallel studies of resistance and virulence genes is that the interaction occurs between partners on the genomic level and not the gene level. Hence, there is the necessity to study the genetics of partners only in interaction.