Effect of Environmental and Host Factors on Biological Control of Fusarium Wilt by Non-Pathogenic Fusarium oxysporum in Tomato

Abstract

This study was focused on the effect of environmental and host factors on the antagonism of Fusarium wilt of tomato, caused by Fusarium oxysporum f. sp. lycopersici (Fol), by non-pathogenic strains of F. oxysporum. Seven non-pathogenic strains of F. oxysporum were isolated and screened for antagonism of Fusarium wilt. Strains F1 and F4 were chosen for further experiments as they reduced disease severity more than other non-pathogens. The best method for applying non-pathogens and pathogens was to inoculate soil with conidial suspension. The non-pathogens reduced seed germination and growth of tomato plants in the absence of the pathogen. In a split root system, non-pathogens F1 and F4 induced resistance of tomato plant against Fusarium wilt although there was no direct contact between the pathogen and non-pathogens. Iron at high and standard concentration in the nutrient solution stimulated induced resistance. However, direct antagonism of Fol by F4 was greatest at low level of iron.<br/> Tomato root exudates increased in the vitro antifungal activity of non-pathogens toward pathogens and also increased spore germination of both non-pathogens and pathogens. The components of root exudates including sugars and organic acids influenced the antagonism of non-pathogens against pathogen in vitro. However sugars and organic acids had little effect on disease suppression in pot trials.<br/> In dual culture, using NaNO3 as source of N, the inhibition of Fol by F1 and F4 was decreased at high level of N, whereas at high level of NaNO3 as source of N the antibiotic production increased. Using NH4Cl as source of N at high and low level, the inhibition of Fol by F1 and F4 was increased and the antibiotic production of non-pathogens also increased. In pot trials, the disease severity was less at low N compared with high level of N.<br/> In dual culture, at low level of K the antagonistic activity of s F1 and F4 against Fol was improved. However, at high level of K, the antibiotic production of non-pathogens increased. In glasshouse pot trials, non-pathogens improved plant health at low level of K, whereas the growth of non-pathogens was decreased at high level of K.<br/> At high level of Ca, the inhibition of growth of Fol by F1 and F4 was decreased. Also at low level of Ca the inhibition of growth of Fol by antibiotic production of F4 was increased. However, the inhibition of growth of Fol by antibiotic production of F1 was increased at high level of K.<br/> The inhibition of growth of Fol by F1 and F4 was decreased with high level of iron. The antibiotic production of F1 and F4 inhibited growth of Fol at low level of iron more than at high level. Biological control did not work well at high levels of iron.<br/> Further work is needed on the effect of non-pathogens on the growth of plants. More tests should be done on the effect of root exudate on antagonism. Biocontrol agents should be found that can work well at low nutrient levels.