On these chromosomes are located genes that code for the resistant mechanisms we have discussed. Conversely, the pathogen has genes that code for virulence mechanism. It is, therefore, the concurrent occurrence and interaction of specific genes for virulence in the pathogen and of specific genes for susceptibility in the host that determine the initiation and development of disease. The gene or genes for virulence in the pathogen are usually specific for one or a few related kinds of host plants. Also, the genes that make a host plant susceptible to a particular pathogen are present only in that one host and possibly a few related kinds of host plants.
Virulence genes are broad spectrum or V-genes are very numerous.
Puccinia graminis var. Resistance is the rule, susceptibility the exception Plants of a particular species also have resistance genes. These resistance genes may have been bred out of a particular variety or may not exist to certain pathogens. In many cases resistance genes are found in wild accessions. A resistance gene confers resistance to the different races of a pathogen.