Native Endophytic <i>Pseudomonas putida</i> as a Biocontrol Agent against Common Bean Rust Caused by <i>Uromyces appendiculatus</i>

This study aimed to evaluate the efficacy of endophytic bacterium to control common bean rust disease under greenhouse conditions. Endophytic bacterium <i>Pseudomonas putida</i> ASU15 was isolated from fresh asymptomatic common bean, identified using biochemical and molecular characteristics. In vitro, the inhibitory effect of different concentrations of <i>P. putida</i> (1 × 10⁴, 1 × 10⁵ and 1 × 10⁶), as well as fungicide ortiva (0.01%) on uredospores germination of <i>Uromyces appendiculatus</i> were tested using water agar medium. The concentration showing the highest reduction of uredospores germination was at 1 × 10⁶, while there was complete inhibition of uredospores germination associated with using ortiva. Scanning electron microscope exhibited the ability of <i>P. putida</i> cells to attack the cell wall of the fungal uredospores germ tubes of <i>U. appendiculatus</i>, causing obvious cell wall breakdown. The activities of chitinase, lipase, and protease produced by <i>P. putida</i> ASU15, in vitro, were evaluated spectrophotometrically. Chitinolytic, proteolytic, and lipolytic activities were exhibited, contributing 55.26, 3.87, and 26.12 U/mL, respectively. Under greenhouse conditions, treated plants with <i>P. putida</i> ASU15 (two days before pathogen inoculation or at the same time of pathogen inoculation) or fungicide reduced the disease severity, compared to the control. Applying <i>P. putida</i> ASU15 at the same time of pathogen inoculation showed reduction in disease severity (69.9%), higher than application before pathogen inoculation (54.9%). This study is considered the first report that demonstrates the mycoparasitic strategy of <i>P. putida</i> for controlling <i>U. appendiculatus</i>. In conclusion, our results revealed that <i>P. putida</i> ASU15 affords a significant disease reduction that may be attributed to direct suppression of pathogen spores germination.