Plant resistance to pathogens is the most efficient form of disease control. However, identification of resistant genotypes is often difficult, especially when the genetic basis for the host-pathogen relationship is unknown. This study was carried out to test a methodology capable of providing, in a simple way, information about host vertical and horizontal resistance as well as pathogen virulence. A simulation using twenty, ten, and five pathogen races and twenty hosts was carried out. Host reaction was controlled by ten genes with two alleles each. Eight genes had little effects, one had medium and the other strong effects. Genetic control of pathogenicity was identical to that of the host. Only homozygous genotypes were used for pathogens and hosts in this simulation, with no epistatic effects. Simulation was based on the expected disease severity with the inoculation of twenty hosts with twenty pathogen races, according to additive and interactive models proposed by Parlevliet and Zadoks (Euphytica 26: 5-21, 1977). Data were analyzed by model IV of Griffing, using a partial diallel scheme. A high correlation was found between general reaction ability (GRA) and potential host resistance, which proved to be an indicator of horizontal resistance. A high correlation between general aggressivity ability (GAA) and potential pathogenicity of the race also proved to be an aggressivity indicator. Specific interaction ability (SIA) is an indicator of host vertical resistance and pathogen virulence. Simulation with a lower number of races (ten and five) showed similar results. SIA was significant in both the interactive and additive models.
