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UBC Theses and Dissertations

Mechanisms of biological control of crown and root rot in tomato by a nonpathogenic Fusarium oxysporum strain Sircom, Katharine M.


The biological control of crown and root rot in tomato was studied in a sterile system in which tomato seedlings were grown on water agar in petri plates or on filter wicks in test tubes containing fertilizer solution. The biological control agent, nitBl, was a nonpathogenic strain of Fusarium oxysporum which had been mutated to a nitrate non-utilizing form in order to distinguish it from the pathogen, Fusarium oxysporum f. sp.radicis-lycopersici (FORL). Four different approaches were used to deduce possible mechanisms of control. One approach was to study the effect of inoculum timing and inoculum density on biological control. Biological control was demonstrated when nit B1 was inoculated to the roots as much as 21 days before FORL, and persisted for at least 23 days after the FORL inoculation. When nit B1 was inoculated to seedling roots at least 3 days before FORL, there was good disease control even when the initial inoculum density of nit B1 was 60 times lower than that of FORL. As the lag between inoculations decreased, higher densities of nit B1 were needed to bring about control, and when the two fungi were inoculated simultaneously the inoculum density of nit B1 had to be at least 10 times that of FORL for there to be any reduction of disease symptoms. The second approach was to observe the colonization of seedling roots by the two strains. Both nit B1 and FORL colonized the outer layers of the root. However, when nit B1 was inoculated to the root 4 days before FORL, the rate of increase of nit B1 was greater than the rate of increase of FORL. The third approach was to test possible elicitors of a defence reaction in tomato for their biological control ability. Sterile filtrates from cultures of nit B1 grown in nutrient broth, sterile exudates from nit B1-infested germinating seeds and seedling roots, heat-killed nit B1 spores, and the cell wall fraction from nit B1 all failed to protect seedlings against crown and root rot induced by FORL. The fourth approach was to test nutrient competition by adding an excess of nutrients that might otherwise be limiting. Biological control by nit B1 was not affected when excess glucose or iron were added to the growth medium. In a related experiment, FORL caused severe disease symptoms (in the absence of nit B1) even when iron availability was artificially decreased by adding a strong iron chelator to the growth medium. The conclusion from all these experiments was that nit B1may elicit a defense response in tomato roots, possibly dependent on the prior colonization of the roots by this strain, which makes the roots resistant to subsequent infection by FORL.

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