Abstract
Olive knot (OK) is a widespread bacterial disease, caused by Pseudomonas savastanoi pv. savastanoi (Pss), which currently has not effective control methods. The use of naturally occurring microbial antagonists, such as bacteria, as biocontrol agents could be a strategy to manage this disease. The objective of this work was to select bacteria from olive tree phyllosphere able to antagonize Pss using in vitro and in planta experiments. The elucidation of their modes of action and the potential relationship between antagonism and bacteria origin has been investigated, as well. To this end, 60 bacterial isolates obtained from the surface and inner tissues of different organs (leaves, twigs, and knots), from two olive cultivars of varying susceptibilities to OK, were screened for their in vitro antagonistic effect against Pss. A total of 27 bacterial strains were able to significantly inhibit Pss growth, being this effect linked to bacteria origin. Strains from OK-susceptible cultivar and colonizing the surface of plant tissues showed the strongest antagonistic potential. The antagonistic activity was potentially due to the production of volatile compounds, siderophores and lytic enzymes. Bacillus amyloliquefaciens P41 was the most effective antagonistic strain and their capacity to control OK disease was subsequently assayed using in planta experiments. This strain significantly reduces OK disease severity (43.7%), knots weight (55.4%) and population size of Pss (26.8%), while increasing the shoot dry weight (55.0%) and root water content (39.6%) of Pss-infected olive plantlets. Bacterial isolates characterized in this study, in particular B. amyloliquefaciens P41, may be considered as promising biocontrol candidates for controlling OK disease.
Highlights
Olive knot (OK) disease, caused by Pseudomonas savastanoi pv. savastanoi (Pss), is a serious threat to olive production worldwide, especially in Mediterranean countries (Quesada et al, 2012)
This study aims to answer the following questions: (i) Is the antagonistic effect displayed by bacteria against Pss linked to their origin in terms of host, plant organ and/or microbial habitat? (ii) Which mechanisms are involved in the antagonistic effect displayed by native bacteria against Pss? (iii) What is the potential of native bacteria in controlling OK disease development and in reducing Pss population on olive phyllosphere? By combining the mechanisms of antagonistic bacterial agents with host plant features, we expect to increase the likelihood of finding more effective biocontrol agents
The bacterial isolates previously obtained from the phyllosphere of two olive cultivars (Cobrançosa and Verdeal Transmontana, more tolerant and susceptible to OK disease, respectively) and plant organs (Mina et al, 2020a,b) were further studied (Supplementary Table S1)
Summary
Olive knot (OK) disease, caused by Pseudomonas savastanoi pv. savastanoi (Pss), is a serious threat to olive production worldwide, especially in Mediterranean countries (Quesada et al, 2012). Savastanoi (Pss), is a serious threat to olive production worldwide, especially in Mediterranean countries (Quesada et al, 2012) This disease is characterized by the formation of overgrowths (tumorous galls or knots), mainly on the olive tree branches and twigs (Quesada et al, 2010, 2012). For a most successful identification of biocontrol agents, the performance of both in vitro and in planta experiments has been recommended (De Silva et al, 2019) Such a screening process should include microorganisms, which are already adapted to the crop, as well as resident microbiota in the same environment where the biocontrol approach will be used (Ozaktan et al, 2012). As far as we know, no studies have examined if the antagonistic effects of a specific bacterial strain is related to the origin of the isolate, in terms of host cultivar and/or plant organ
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