The soilborne fungus Fusarium oxysporum f. sp. radicis-lycopersici causes tomato 
foot and root rot (TFRR), which can be controlled by the addition of the 
nonpathogenic fungus F. oxysporum Fo47 to the soil. To improve our understanding 
of the interactions between the two Fusarium strains on tomato roots during 
biocontrol, the fungi were labeled using different autofluorescent proteins as 
markers and subsequently visualized using confocal laser scanning microscopy. 
The results were as follows. i) An at least 50-fold excess of Fo47over F. 
oxysporum f. sp. radicis-lycopersici was required to obtain control of TFRR. ii) 
When seedlings were planted in sand infested with spores of a single fungus, 
Fo47 hyphae attached to the root earlier than those of F. oxysporum f. sp. 
radicis-lycopersici. iii) Subsequent root colonization by F. oxysporum f. sp. 
radicis-lycopersici was faster and to a larger extent than that by Fo47. iv) 
Under disease-controlling conditions, colonization of tomato roots by the 
pathogenic fungus was significantly reduced. v) When the inoculum concentration 
of Fo47 was increased, root colonization by the pathogen was arrested at the 
stage of initial attachment to the root. vi) The percentage of spores of Fo47 
that germinates in tomato root exudate in vitro is higher than that of the 
pathogen F. oxysporum f. sp. radicis-lycopersici. Based on these results, the 
mechanisms by which Fo47 controls TFRR are discussed in terms of i) rate of 
spore germination and competition for nutrients before the two fungi reach the 
rhizoplane; ii) competition for initial sites of attachment, intercellular 
junctions, and nutrients on the tomato root surface; and iii) inducing systemic 
resistance.