The aim of the present research was to assess the relative importance of systemic induced resistance in the suppression of Fusarium wilt of tomato by non-pathogenic Fusarium oxysporum (AvrS). Effects of one isolate of nonpathogenic F. oxysporum (Avr5) were tested in vitro and in vivo for their efficacy in controlling Fusarium wilt of tomato. Two methods were applied including root-dip-inoculation and soil's infestation method with the F. o.xysporum f. sp. lycopersici (FOL). Isolate of nonpathogenic F. oxysporum in vilro did not inhibit radial growth of FOL and had no significant effect on reduction growth of pathogen. At the same time in vivo nonpathogenic F. oxy~porum (Avr5) with concentration of 0.1 ml conidial suspensions of I 06 conidial/ml per gram soil (I 00 ml conidia per pot) significantly reduced incidence and severity of Fusarium wilt (62% reduction) compared to the control pathogen. F. oxysporum (Avr5) only colonized the epidermis of tomato's root but was not found in vessels of stems, while the FOL colonized inside of root cells and stems of inoculated plants. Plants inoculated with FOL showed disease symptoms after 3 weeks, whereas plants inoculated with F. oxysporum (Avr5) or a mixture of both fungi remained symptomless for 60 days. In order to induce systemic resistance in tomato plant, three bioassays including split-root, benomyl and cutting systems were used. Results of three bioassays indicated that F. oxysporum (Avr5) is able to induce resistance in tomato plants against FOL. Non-pathogenic F. oxysporum (Avr5) remained spatially separated from the pathogen in plants in the split root bioassay, but in the cutting and benomyl systems F. oxysporum (Avr5) and FOL were separated in time, suggesting that these effects were systemic in nature. It has been concluded that application of nonpathogenic F. oxysporum (Avr5) can be considered as a potential biocontrol agent in next studies.
