Vertical and Horizontal Resistance

VERTICAL AND HORIZONTAL RESISTANCE Specific or Vertical Resistance. This resistance is controlled by a single gene or multiple genes of a cultivar which are responsible for resistance, often this is effective only for certain races of the pathogen and does not provide resistance to other races. This type of resistance is called specific resistance or vertical resistance.Incorporating specific genes into the host to confer resistance only provides protection to the plant in the short term. Plants that have this kind of resistance break easily, in a relatively short time, after being used commercially. This breakdown in resistance is the result of the development and selection of pathogenic races that are able to attack cultivars that were previously resistant. The cultivar genotype does not change, it still remains resistant to the pathogen strains present at the time of its release. So the host resistance working for these particular strains is specific resistanceIdentification of various pathogenic races is carried out by inoculating selected cultivars to differentiate pathogenic races based on the reactions that arise. The use of different cultivars to identify races of this pathogen must be carried out under controlled conditions because changes in environmental factors such as temperature and light can influence the symptoms that occur in the host. Gene-by-gene hypothesis The genetic relationship between plants and pathogens is not yet clearly known. But some researchers have concluded that for each genetic locus there are those that regulate host resistance or sensitivity and there are loci that regulate whether a pathogen is virulent or not. This relationship is called the gene-to-gene hypothesis, which states that every gene in the host is capable of mutating to provide resistance and there are genes in the pathogen that are capable of mutating to deal with this resistance. This hypothesisalso provides examples that explain the occurrence of cultivar resistance breaking down in field conditions.This gene-by-gene hypothesis can explain the host-pathogen relationship in a number of "biotropical pathogens" such as: rust, smut, powdery mildew,Phytophthora infestansAndVenturia inaequalis. Evidence suggests that the hypothesis can be extended to a number of fungal host-pathogen combinations and perhaps also to other groups of parasites including insects.In general, host resistance to fungal pathogens is dominant,on the other hand, pathogen virulence is usually recessive and can be inherited freely.The gene-for-gene hypothesis provides an example for studying parasitehost systems in situations where genetic relationships are not yet fully known. Characteristics of Specific or Vertical Resistance: 1. Vertical resistance mechanisms are usually inherited as single genes or multiple genes. So this is relatively easy to manipulate in plant breeding programs 2. Vertical resistance is usually equated with the gene-by-gene hypothesis 3. Vertical resistance usually provides a high degree of resistance but is easily lost if a virulent pathogen strain is present. Once this vertical resistance is lost it will then be completely broken, and the plant will be completely sensitive to pathogenic strains capable of attacking it. 4. Vertical resistance acts almost entirely after the pathogen penetrates the plant and is often described as a hypersensitive reaction in the host 5. Vertical resistance often causes a delay in the start of an epidemic. Non-Specific or Horizontal Reactions Some experts emphasize that there is another form of resistance called horizontal resistance. This term is generally used to denote resistance that works on a large number of pathogenic races, although not one hundred percent effective. Horizontal resistance is broader in nature and will not encourage the selective increase of certain races of a pathogen, as happens with vertical resistance. In such circumstances, the composition of the pathogen population will stabilize and the overall number of diseases caused will decrease or decrease. Determining and using horizontal resistance in a plant breeding program can overcome the problem.Horizontal resilience is quantitative resilience, thus quantitative data is needed to measure it. This quantitative data is obtained through epidemiological procedures either on relatively large populations in the field or on limited populations under supervised conditionsThe mechanisms that confer plant horizontal resistance have not been fully studied. When selection for disease resistance is so intensively carried out, varieties often show a more stable disease response and rarely suffer severe attacks, due to the presence of new pathogen strains. Selection of varieties with horizontal resistance is mostly based on natural infections in the field. Many old varieties have horizontal resistance because they have been selected accidentally over a long period and reached equilibrium with the pathogen population in the field.Horizontal resistance is (1) regulated by a number of genes, (2) does not show differential reactions to different races of the pathogen, (3) the variety in question is susceptible to the disease, but the development of the epidemic is inhibited.It is generally said that varieties that show horizontal resistance are "slow disease fomer" due to mechanisms: reducing the number of lesions produced per unit of inoculum, increasing the length of the pathogen generation time and shortening the reproductive life of the wound.Horizontal resilience can be measured using various epidemiological methods which can be classified into three types of measurements, namely: measuring the effects of disease; measuring epidemic development and measuring resilience components Characteristics of Non-Specific or Horizontal Resistance According to the definition, horizontal resistance works against all strains of a pathogen so that it does not break easily.Other characteristics are as follows: 1. Usually provides a lower level of resistance than vertical resistance, rarely providing high immunity or resistance 2. Usually inherited polygenically with several participating genes. The consequences of plant breeding to obtain resistant types are difficult and are usually perpetuated by plant breeders 3. Horizontal resistance mechanisms operate before and after the pathogen colonizes the host. Nothing to do with hypersensitivity reactions 4. Since horizontal resistance causes a decrease in spore production, its effect is demonstrated by a decrease in the rate of epidemic development. 5. Almost all cultivars have horizontal resistance to infection. Many researchers state that vertical resistance alone cannot protect plants against disease on geological time scales. For example, the resistance of two potato cultivars to 16 racesPhytophthora infestans. Both cultivars show vertical resistance to pathogen races (complete resistance to disease)