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Harold Flor's gene-for-gene model explained boom–bust cycles in which resistance ( R) genes are deployed in farmers’ fields, only to have pathogens overcome resistance by modifying or losing corresponding active avirulence ( AVR) genes. Flor understood that host R genes with corresponding low rates of virulence mutation in the pathogen should maintain resistance for longer periods of time. This review focuses on AVR gene dynamics of the haploid Ascomycete fungus Pyricularia oryzae, which causes rice blast disease, a gene-for-gene system with a complex race structure and a very rapid boom–bust cycle due to high rates of AVR gene mutation. Highly mutable blast AVR genes are often characterized by deletion and by movement to new chromosomal locations, implying a loss/regain mechanism in response to R gene deployment. Beyond rice blast, the recent emergence of two serious new blast diseases on wheat and Lolium ryegrasses highlighted the role of AVR genes that act at the host genus level and serve as infection barriers that separate host genus-specialized P. oryzae subpopulations. Wheat and ryegrass blast diseases apparently evolved through sexual crosses involving fungal individuals from five host-adapted subpopulations, with the host jump enabled by the introduction of virulence alleles of key host-specificity AVR genes. Despite identification of wheat AVR/ R gene interactions operating at the host genus specificity level, the paucity of effective R genes identified thus far limits control of wheat blast disease. Formula: see text Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .
Barbara Valent (Wed,) studied this question.