Biostimulant Application as a Tool to Improve Rooting of Olive Tree Cuttings in Brazil

In Brazil, the olive tree (Olea europaea) is propagated by cuttings using indole-3-butyric acid (IBA) for rooting and sand as the substrate. Auxin-producing microorganisms may enhance this process when applied together with IBA. This study evaluated the rooting capacity of cuttings from four olive cultivars—Arbequina, Maria da Fé, Ascolano 315, and Koroneiki—treated with commercial products based on microorganisms, plus IBA. The biostimulants used were Bacillus amyloliquefaciens, Bacillus licheniformis, Bacillus subtilis, Trichoderma harzianum, and the commercial product Bioraiz® (a mixed mineral fertilizer) in liquid formulation. Trichoderma harzianum and Bacillus spp. improved the quality of rooted cuttings, promoting the formation of more roots per cutting (about 10) and longer roots, on average of 8.1 cm in the cultivars Maria da Fé, Ascolano 315, and Arbequina. Cuttings treated with Trichoderma harzianum, Bacillus subtilis, and Bacillus licheniformis pro- duced higher percentages of rooted cuttings, over 50%, and more developed root systems. Conversely, the control and Bioraiz® showed weaker rooting performance, producing fewer than seven roots per cutting. Overall, the results highlight the potential of biostimu- lant applications, such as Trichoderma and Bacillus subtilis, as promising tools to optimize the rooting of olive tree cuttings, whereas the fertilizer showed limited effectiveness in promoting rooting.