Читать книгу Plant Pathology and Plant Pathogens - John A. Lucas - Страница 36

In common with all other biological characteristics, host resistance and pathogen virulence are genetically determined. However, these two properties can only be assessed in the presence of the other partner. In the majority of cases, host resistance or pathogen virulence are not obviously correlated with other phenotypic characters. Features of the pathogen, such as rapid and extensive growth or the production of cell wall‐degrading enzymes, may or may not be related to virulence. Assessments of resistance and virulence are therefore based on disease reaction types. An interaction where symptoms are clearly expressed is described as a compatible disease reaction as opposed to an incompatible reaction where symptoms do not develop and the effect on the plant is minimal (Figure 2.6).

Host resistance is controlled by one or a few genes whose individual effects may be easily detected, or by a multiplicity of genes, each of which contributes only a small fraction of the property as a whole. The practical implications of this are described in more detail in Chapter 12. In a few instances, disease reaction type has been shown to be controlled by factors inherited through the host's cytoplasm. The best‐known example of such cytoplasmic inheritance involves the reaction of maize to the leaf blight fungus Bipolaris maydis. In the past, the production of hybrid maize has involved the laborious task of detasselling by hand to avoid self‐pollination occurring. The discovery of a cytoplasmically inherited mitochondrial factor for male sterility (Cms), which meant that cross‐pollination was essential, removed the need for this operation. Because of this, cultivars possessing Cms came to predominate throughout the USA. Unfortunately, Cms was also correlated with susceptibility to a particular strain (race T) of B. maydis. As a result, the occurrence in 1970 of favorable conditions for the development of the pathogen resulted in a disastrous epidemic (see Chapter 5, Figure 5.1). In any breeding program, the possibility that the cytoplasm may be important in disease resistance must therefore be considered.

Although pathogen virulence, host resistance, and disease reaction types are genetically determined, the environment may modify the expression of any of these characters. For example, the Sr6 gene conferring resistance in wheat to black stem rust is effective at 20 °C, but is inoperative at 25 °C.