You can verify this concept by careful examination of the table. The interactions discussed above can also be demonstrated by the metaphor of a lock and key model, as illustrated in the class handout. Return to Outline Epiphytological perspective Vertical vs. Horizontal Resistance Vertical resistance, since it affords complete, or nearly complete protection against specific pathogen genotypes, effectively reduces initial inoculum, K 0 . As we have learned previously, this results in shift of disease progress curves , rather than a change in r . Conversely, horizontal resistance redu ces the disease rate, r , which specifies the slope of the exponential phase of the disease progress curve is altered rather than initial inoculum.
Single genes can evolve through mutation or recombination of genes to generate new virulences.
The host exerts a high selection pressure for the emergence of these new virulence genes or races. This is the main drawback to use of vertical or speci fic resistance in breeding programs. The following comments derive from comments made by Dr. Fred Cholick in guest lectures to previous PS333 classes.