Induced resistance in tomato plants by IAA against Fusarium oxysporum lycopersici.

The phytohormone IAA (indol-3-acetic acid) was tested in vitro on growth of tomato wilt pathogen Fusarium oxysporum lycopersici. The hormone reduced spore germination, mycelial dry weight and protein content. Such reduction was matched with the elevation in the hormone concentration. The in vivo application of IAA to soil of the uninoculated plants (controls) improved growth and yielded longer shoot and root, particularly at low concentrations. Moreover, the hormone could prevent completely any chance for disease incidence by soil pathogens. Presence of IAA in soil of inoculated plants not only reduced the infection rate but also increased plant growth, causing that they appeared healthy and normal. Disease suppression in tomato plants, exerted by application of IAA, was achieved through either increasing plant growth, exerting a direct harmful effect on the target pathogen and/or inducing resistance in host tissue. The induced resistance was correlated with induction of certain secondary metabolites which may have a role in increasing tolerance in tomato plants to the pathogen.