Genetic analysis of variation of pathogenicity in Nectria haematococca ( Fusarium solani) on Cucurbita sp

Pathogenicity phenotypes for over 30 isolates of Fusarium solani, from Cucurbita sp., were measured as lesion sizes on both unwounded and wounded seedling hypocotyls and on fruit mesocarp tissue. These tests detected different capacities in both the pre-and post-penetration phases of infection. The isolates were grouped, based on their mating behaviour, as non-mating, or members of either mating population I (MPI) or MPV of Nectria haematococca. The MPI isolates readily infected unwounded hypocotyls whereas MPV and non-mating isolates were either incapable of infecting or were very weakly pathogenic on this issue. In contrast all isolates were capable of colonizing both wounded hypocotyls and fruit mesocarp tissue, although there was a wide range in the size of lesions. Segregation for pathogenicity on unwounded hypocotyls was studied in the progeny from seven crosses between MPI isolates of differing pathogenicity. Pathogenicity was a polygenically controlled character with estimates of the number of effective factors, k, involved ranging from 3 to 15 for the seven crosses. In three of the crosses there was non-additive genetic interaction, and in the other four crosses the genetic variation was predominantly additive. There was evidence for a weak association between female sex and pathogenicity in several crosses.