Horizontal, uniform, race-non-specific or stable resistance can be discerned according to Van der Plank, from vertical, differential, race-specific or unstable resistance by a test in which a number of host genotypes (cultivars or clones) are tested against a number of pathogen genetypes traces of isolatest. If the total non-environmental variance in levels of resistance is due to main effects only differences between cultivars and differences between isolates) the resistance and the pathogen many (in the broad sense) are horizontal in nature. Vertical resistance and pathogenicity are characterized by the interaction between host and pathogen showing up as a variance compenent in this test due to interaction between cultivars and isolates. A host and pathogen model was made in which resistance and pathogenicity are governed by live polygenic loci. Within the host the resistance genes show additivity. Two models were investigated in model I resistance and pathogenicity genes operate in an additive way as envisaged by Van der Plank in his horizontal resistance. Model II is characterized by a gene-for-gene action between the polygenes of the host and those of the pathogen. The cultivar isolate test in model I showed only main effect variance. Surprisingly, the variance in model II was also largely due to main effects. The contribution of the interaction to the variance uppeared so small, that it would be difficult to discern it from a normal error variance. So-called horizontal resistance can therefore be explained by a polygenic resistance, where the individual genes are vertical and operating on a gene-for-gene basis with virulence genes in the pathogen. The data reported so far support the idea that model II rather than model I is the realistic one. The two models also revealed that populations with a polygenic resistance based on the gene-for-gene action have an increased level of resistance compared with the addition model, while its stability as far as mutability of the pathogen is concerned, is higher compared to those with an additive gene action. Mathematical studies of Mode too support the gene-for-gene concept. The operation of all resistance and virulence genes in a natural population is therefore seen as one integrated system. All genes for true resistance in the host population, whether they are major or minor genes are considered to interact in a gene-for-gene way with virulence genes either major or minor, in the pathogen population. The models revealed other important aspects. Populations with a polygenic resistance based on a gene-for-gene action have an increased level of resistance compared to populations following the addition model. The stability, as far as mutability of the pathogen is concerned, is higher in the interaction model than in the addition model. The effect of a resistance gene on the level of resistance of the population consists of its effect on a single plant times its gene frequency in the population. Due to the adaptive forces in both the host and the pathogen population and the gene-for-gene nature of the gene action an equilibrium develops that allows all resistance genes to remain effective although their corresponding virulence genes are present. The frequencies of the resistance and virulence genes are such that the effective frequencies of resistance genes tend to be negatively related to the magnitude of the gene effect. This explains why major genes often occur at low frequencies, while minor genes appear to be frequent. It is in this way that the host and the pathogen, both as extremely variable and vigorous populations, can co-exist. Horizontal and vertical resistance as meant by Van der Plank therefore do not represent different kinds of resistances, they represent merely polygenic and oligogenic resistances resp. In both situations the individual host genes interact specifically with virulence genes in the pathogen. Van der Plank's test for horizontal resistance appears to be a simple and sound way to test for polygenic inheritance of resistance. The practical considerations have been discussed. The agro-ecosystems should be made as diverse as possible. Multilines, polygenic resistance, tolerance, gene deployment and other measures should be employed, if possible in combination.
Parlevliet, J. E., & Zadoks, J. C. (1977). The integrated concept of disease resistance; a new view including horizontal and vertical resistance in plants. Euphytica, 26(1), 5-21. https://doi.org/10.1007/BF00032062