Screening of potential biocontrol endophytes and epiphytes against olive knot disease

Abstract: Pseudomonas savastanoi pv. savastanoi (Psv) is an important pathogen of olive tree, causing the olive knot disease. Its control is very difficult, requiring preventive bactericide applications. Here, five endophytes and epiphytes previously isolated from olive tree were screened for the suppression of Psv growth by using a bioassay with detached olive branches. The effect of protein extracts from olive trees elicited by some of these microorganisms in inhibiting Psv was also evaluated. The isolates were identified by sequencing a portion of the16S (for bacteria) or ITS (for fungi) of rDNA region, as Beauveria bassiana, Aureobasidium pullulans and Vishniacozyma victoriae, Alcaligenes faecalis and Bacillus amyloliquefaciens. In detached olive twigs, the fungi B. bassiana, A. pullulans and V. victoriae, inhibited completely the development of tumors after 15 days of inoculation with the pathogen. At the same time, 50 % of the branches inoculated with Psv had tumors. However, only the protein extracts from the olive tree inoculated with A. pullulans, and at the concentration of 0.625 and 0.3 mg protein/ml, significantly inhibited the growth of Psv (2.6 times), when compared with protein extracts from non-inoculated plants. Overall, A. pullulans seems to be a promising biocontrol candidate for controlling olive knot disease. Further experiments are needed to determine its effectiveness under field conditions and to identify protein compounds responsible for its antimicrobial activity.