avirolence

could someone please define this for me? the book is very cryptic. It says it is a pathogen that can only mildly harm the host plant. How is this important for gene-for-gene recognition?
Thanks

In really broad terms, avirulence is a property of plant pathogens (plant disease-causing killers) which happens when a plant happens to have a corresponding resistance gene to that pathogen. For example, a pathogen produces protein A, which when secreted into the plant, will cause the plant to produce correspondingly protein B, which will give the plant immunity and resistance to the pathogen.

That's how the gene-for-gene recognition works: genes in the plant, which produce immune proteins, will make the pathogen "avirulent" (i.e. harmless), because genes in the pathogen produce proteins that are easily recognized by the proteins of the plant, produced by the plant's genes. and thus Gene-for-gene recognition!