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 wo unded seedling hypocotyls and on fruit mesocarp tissue. These tests de tected different capacities in both the pre-and post-penetration phase s of infection. The isolates were grouped, based on their mating behav iour, as non-mating, or members of either mating population I (MPI) or MPV of Nectria haematococca. The MPI isolates readily infected unwoun ded hypocotyls whereas MPV and non-mating isolates were either incapab le of infecting or were very weakly pathogenic on this issue. In contr ast all isolates were capable of colonizing both wounded hypocotyls an d fruit mesocarp tissue, although there was a wide range in the size o f lesions. Segregation for pathogenicity on unwounded hypocotyls was s tudied in the progeny from seven crosses between MPI isolates of diffe ring pathogenicity. Pathogenicity was a polygenically controlled chara cter with estimates of the number of effective factors, k, involved ra nging from 3 to 15 for the seven crosses. In three of the crosses ther e was non-additive genetic interaction, and in the other four crosses the genetic variation was predominantly additive. There was evidence f or a weak association between female sex and pathogenicity in several crosses.