Effects on growth and comparison of root tissue colonization patterns of Eucalyptus viminalis by pathogenic and nonpathogenic strains of Fusarium oxysporum

Soilborne pathogens, especially Fusarium oxysporum, are responsible for dam ping-off and root necrosis in Eucalyptus nurseries. New technologies are in creasingly considering strategies for plant disease control other than chem ical fungicides. Among these, natural fungal antagonists, which are coloniz ers 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 potentia l of the nonpathogenic F. oxysporum strain Fo47, which is known for its eff iciency in biological control, to suppress damping-off of Eucalyptus seedli ngs; (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 cau sed damping-off in young seedlings in vitro, whilst disease symptoms were n ot visible in plants inoculated with F. oxysporum strain Fo47 or when both strains (Foeu1 + Fo47) were inoculated simultaneously. Each strain showed s imilarities in patterns of root tissue colonization, and in the processes o f root penetration and initial colonization. Differential effects on root t issue were observed with fungal development within the cortex: ingress of s train Foeu1 was accompanied by severe host-cell alterations whilst no tissu e damage occurred with development of strain Fo47.