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The ability to colonize roots is a sine qua non condition for a rhizobacteria to be considered a true plant growth‐promoting rhizobacteria (PGPR). A simple screening method to detect such a potential ability of PGPR is described. Tomato seeds were surface sterilized for 30 s in 50% ethanol and this was followed by 3 min dipping in 2% NaClO. They were then washed three times in sterile water, left immersed in a propagule suspension of the rhizobacteria for 24 h, and transferred onto sterile 0.6% water‐agar in tubes. The young, developing root system shows a tendency to grow downwards in the agar‐gel column. When the rhizobacterium has a potential ability to colonize roots it is possible to visualize, by transparency, bacterial growth (turbid, milky and narrow zone) along and around roots. Testing 500 rhizobacteria isolated from tomato rhizosphere for their ability to induce systemic resistance against Pseudomonas syringae pv. tomato, 28 of them did reduce infection to less than 40% and all 28 colonized roots according to the described bioassay. Therefore the bioassay may turn into an important auxiliary tool for helping in selecting rhizobacteria with PGPR potentiality.

A total of 107 rhizobacterial isolates, obtained from the rhizosphere of eucalypt clones were tested as rooting inducers of cuttings and mini-cuttings planted in substrate composed of carbonized rice husk and vermiculite (1:1). Cuttings and mini-cuttings were planted in conical plastic tubes containing treated and untreated (control) substrate and kept under intermittent mist irrigation at 26-28ºC. After 35 days, rooting percentage and dry root matter of cuttings were evaluated. Ten isolates capable of providing gains of up to 110% in root formation and up to 250% in root biomass over non-inoculated control cuttings were selected. Gains in rooting varied according to clone and isolate tested. The greatest gains were obtained for the mini-cuttings exhibiting the lowest rooting efficiency. Among the ten isolates tested, only 3918 (code R98) and MF4 (code R87), produced 3-indole-acetic acid in vitro, at concentrations of 0.7 and 0.67 µg ml -1 , respectively. Significant increases in rooting and root dry matter of cuttings grown on rhizobacteria-inoculated substrate were found when compared to untreated or indole-butyric acid (IBA) treated mini-cuttings.

The ability of a rhizobacterium to protect tomato plants against naturally occurring diseases as well as to improve crop yield under field conditions was studied. The rhizobacterium was introduced to the plants through seed microbiolization. Treatments consisted of different frequencies of fungicide (Chlorothalonyl) sprayings (5, 10 or 20 applications) of tomato plants grown from either microbiolized or non-microbiolized seeds over a 90-day evaluation period. Treatment of non-microbiolized seeds without fungicide application was included as a control. The progress of the following three naturally occurring diseases was evaluated in the field and quantified: early blight (Alternaria solani), late blight (Phytophthora infestans), and septoria leaf spot (Septoria lycopersici). All treatments resulted in reduced disease severity when compared with the control treatment. Highest final fruit yields were found after treatment of plants grown from non-microbiolized seeds and sprayed with fungicide 20 times over 90 days, and for treatment of plants from microbiolized seeds that received 10 fungicide spray applications, although all treatments increased yield over that obtained in the control treatment. The results demonstrate that combined rhizobacterial and chemical treatments in the field may permit reducing fungicidal spraying frequency while at the same time increasing crop yields.