Soilborne pathogens, especially Fusarium oxysporum, are responsible for damping-off and root necrosis in
Eucalyptus nurseries. New technologies are increasingly considering strategies for plant disease control other than
chemical fungicides. Among these, natural fungal antagonists, which are colonizers of the root cortex, are potential
biocontrol agents. An in vitro system was used: (1) to test the pathogenic effects of F. oxysporum strain Foeu1
which was recovered from a forest nursery soil; (2) to explore the potential of the nonpathogenic F. oxysporum
strain Fo47, which is known for its efficiency in biological control, to suppress damping-off of Eucalyptus
seedlings; (3) to compare the patterns of root colonization and host response to invasion by the two Fusarium
strains inoculated separately in a time-course study. Root inoculation of E. viminalis with F. oxysporum strain
Foeu1 caused damping-off in young seedlings in vitro, whilst disease symptoms were not visible in plants
inoculated with F. oxysporum strain Fo47 or when both strains (Foeu1 + Fo47) were inoculated simultaneously.
Each strain showed similarities in patterns of root tissue colonization, and in the processes of root penetration and
initial colonization. Differential effects on root tissue were observed with fungal development within the cortex:
ingress of strain Foeu1 was accompanied by severe host-cell alterations whilst no tissue damage occurred with
development of strain Fo47.