Biological control of Pseudomonas savastanoi pv. savastanoi causing the olive knot disease with epiphytic and endophytic bacteria

The aim of this study was to determine the biological control possibilities of Pseudomonas savastanoi pv. savastanoi (Psv) by using epiphytic and endophytic bacteria isolated from the roots and shoots of healthy olive trees. A total of 336 bacterial isolates (196 epiphytes and 140 endophytes) were recovered and diagnosed by using Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF-MS). In dual culture tests, 81 bacterial strains inhibited pathogen development at different degrees. The highest inhibition value (A-index) was determined in Bacillus megaterium HZEP7 with a 5.24 A-index value, followed by B. subtilis HZEN1 (4.85) and Pseudomonas koreensis HZEN27 (4.83). Mechanisms of antagonist bacterial strains for siderophore and protease production were also investigated by in vitro tests. The strains to be used in vivo tests were determined according to high antagonistic effects and B. subtilis HZEN1, B. megaterium HZEP7, P. koreensis HZEN27, and B. pumilus HZEP29 strains were selected for in vivo tests. In the carrot slices inoculation, the highest inhibition value was detected in B. subtilis HZEN1 with 98.78%, followed by B. megaterium HZEP7 (94.49%), P. koreensis HZEN27 (92.09%), and B. pumilus HZEP29 (90.26%) isolates. Similar to carrot slices in sapling inoculation tests, all bacterial isolates inhibited gall formation. The highest inhibition value was determined in the B. subtilis HZEN1 isolate with 78.72%, followed by B. megaterium HZEP7 (60.87%), P. koreensis HZEN27 (46.09%), and B. pumilus HZEP29 (44.87%) isolates. The obtained results showed that B. subtilis HZEN1, B. megaterium HZEP7, P. koreensis HZEN27, and B. pumilus HZEP29 isolates could be used against Psv, the causal agent of the olive knot disease, as a biocontrol agent.

Automated summary

This study aimed to determine the biological control possibilities of Pseudomonas savastanoi pv. savastanoi (Psv) by using epiphytic and endophytic bacteria isolated from healthy olive trees. Among the bacterial isolates, Bacillus megaterium HZEP7, B. subtilis HZEN1, Pseudomonas koreensis HZEN27, and B. pumilus HZEP29 showed high antagonistic effects and could potentially be used against Psv as biocontrol agents.