Breeding for highly fertile isolates of Nectria haematococca MPVI that arehighly virulent on pea and in planta selection for virulent recombinants

The heterothallic ascomycete Nectria haematococca mating population VI (ana morph Fusarium solani) is a broad host range pathogen. Field isolates of th is fungus that are pathogenic on pea tend to be female sterile, of low fert ility, and the same mating type (MAT-I), whereas female fertile isolates of either mating type that are highly fertile tend to be nonpathogenic on thi s plant. To facilitate genetic analysis of traits that may be important in the ability of N, haematococca to parasitize peas, a breeding project was u ndertaken to produce hermaphroditic isolates of each mating type that are h ighly fertile and highly virulent on peas. Although the association of high virulence on peas with female sterility and the MAT-I mating type was not completely broken, isolates with high fertility and high virulence on peas were bred within two generations. Highly virulent progeny were also isolate d by an alternative method in which pea plants were inoculated with a mixtu re of ascospores from a cross between two moderately virulent parents. Wher eas all ascospores isolated without selection in planta had lower virulence than the parents, many isolates recovered from diseased tissue were more v irulent than the parental isolates. Some of the recovered isolates were sho wn by restriction fragment length polymorphism analysis to be genetic recom binants of the parents, demonstrating that the pea tissue selected virulent recombinants. All highly virulent isolates tested had the ability to detox ify the pea phytoalexin pisatin, again showing a link between this trait an d pathogenicity on the pea.