Abstract
Verticillium wilt, caused by the pathogen Verticillium dahliae, is extremely devastating to olive trees (Olea europea). Currently, no successful control measure is available against it. The objective of this work was to evaluate the antifungal activity of Bacillus velezensis XT1, a well-characterized salt-tolerant biocontrol strain, against the highly virulent defoliating V. dahliae V024. In vitro, strain XT1 showed to reduce fungal mycelium from 34 to 100%, depending on if the assay was conducted with the supernatant, volatile compounds, lipopeptides or whole bacterial culture. In preventive treatments, when applied directly on young olive trees, it reduced Verticillium incidence rate and percentage of severity by 54 and ~80%, respectively. It increased polyphenol oxidase (PPO) activity by 395%, indicating an enhancement of disease resistance in plant tissues, and it decreased by 20.2% the number of fungal microsclerotia in soil. In adult infected trees, palliative inoculation of strain XT1 in the soil resulted in a reduction in Verticillium symptom severity by ~63%. Strain XT1 is biosafe, stable in soil and able to colonize olive roots endophytically. All the traits described above make B. velezensis XT1 a promising alternative to be used in agriculture for the management of Verticillium wilt.
Highlights
Verticillium wilt, caused by V. dahliae, is a soil-borne fungal disease
The objective of this study was to assess the antifungal activity of B. velezensis XT1, a well-characterized salt-tolerant biocontrol strain previously isolated in Andalusia (Spain) [34], against the fungal pathogen V. dahliae V024, a defoliating pathotype characterized as highly virulent [35]
The results obtained for P. aeruginosa PAO1, which caused a high mortality of the nematodes, were significantly different (p < 0.01) to the results obtained by strain XT1 and E. coli OP50 (Figure 1b)
Summary
Verticillium wilt, caused by V. dahliae, is a soil-borne fungal disease. First described in Italy [1], it affects around 400 plant species, ranging from herbaceous annuals to woody perennials, such as cotton, tomato, alfalfa, almond, pistachio, and peach [2,3]. Monocotyledonous plants are not affected, they may act as asymptomatic reservoirs of the fungi [4], and only dicotyledonous plants can be attacked by the disease Amongst the latter, V. dahliae is extremely devastating to olive trees (Olea europaea) and is currently considered the main soil-borne disease threatening olive production. Infection by Verticillium affects the volume of production, and the quality and properties of the oil produced [5] It affects all the olive-growing regions around the world, especially the Mediterranean area [6,7], and causes severe economic losses due to plant death [6]. The disease is a major concern in the Andalusia region of southern Spain, which produces 83% of total olive oil in Spain [9]. The mean disease incidence is 20.4% [10], this percentage could be underestimated due to the absence of more recent data
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