Use of an extract of at least a part of rocket plants, preferably chosen from the group of rocket plants of the Eruca (Eruca sativa; Eruca vesicaria, etc.), Diplotaxis (Diplotaxis erucoides, Diplotaxis tenuifolia, Diplotaxis muralis, etc.), Bunias (Bunias erucago, Bunias orientalis, etc.), Erucastrum (Erucastrum nasturtiifolium, Erucastrum incanum, etc.) and Cakile genuses, in order to stimulate the defenses of plants or trees and reduce the effects of: - Xylella fastidiosa bacteria on myrtle-leaf milkwort, grape vines, olive trees, citrus trees, oleander, almond trees, coffee trees, peach trees and stone fruit trees, oak trees, lavender, rosemary, or broom, - Pseudomonas syringae pv actinidiae bacteria on plants of the Actinidia genus, - Xantomonas arboricola pv juglandis bacteria on walnut trees, - Xanthomonas arboricola pv. Pruni bacteria on Prunus spp., and preferably the following group of fruit/nut trees: apricot trees, almond trees, cherry trees, peach trees, plum trees, P. salicina, cherry laurel and other exotic or ornamental Prunus species, including P. davidiana and P. laurocerasus, - Pear Decline Phytoplasma bacteria or Candidatus Phytoplasma pyri on pear trees, - Candidatus Phytoplasma solani bacteria on grape vines, lavender, potato plants, tomato plants, aubergine plants, pepper plants and tobacco plants, - Plasmapora viticola fungus on grape vines, or Phytophtora infestans on potato plants and tomato plants, or phytophtora citrophtora on citrus trees, or Phytophtora cactorum on pear trees and apple trees, or Bremia lactucae on artichokes, or - oidium-type fungi such as Podosphaera pannosa on rose bushes, and Erysiphe necator, formerly Uncinula necator, on grape vines, and oidia on tomato plants, lettuces, cucumbers, strawberry plants, raspberry plants, currant bushes, peach trees, pear trees, privet, carnations. - Xylella fastidiosa bacteria on myrtle-leaf milkwort, grape vines, olive trees, citrus trees, oleander, almond trees, coffee trees, peach trees and stone fruit trees, oak trees, lavender, rosemary, or broom, - Pseudomonas syringae pv actinidiae bacteria on plants of the Actinidia genus, - Xantomonas arboricola pv juglandis bacteria on walnut trees, - Xanthomonas arboricola pv. Pruni bacteria on Prunus spp., and preferably the following group of fruit/nut trees: apricot trees, almond trees, cherry trees, peach trees, plum trees, P. salicina, cherry laurel and other exotic or ornamental Prunus species, including P. davidiana and P. laurocerasus, - Pear Decline Phytoplasma bacteria or Candidatus Phytoplasma pyri on pear trees, - Candidatus Phytoplasma solani bacteria on grape vines, lavender, potato plants, tomato plants, aubergine plants, pepper plants and tobacco plants, - Plasmapora viticola fungus on grape vines, or Phytophtora infestans on potato plants and tomato plants, or phytophtora citrophtora on citrus trees, or Phytophtora cactorum on pear trees and apple trees, or Bremia lactucae on artichokes, or - oidium-type fungi such as Podosphaera pannosa on rose bushes, and Erysiphe necator, formerly Uncinula necator, on grape vines, and oidia on tomato plants, lettuces, cucumbers, strawberry plants, raspberry plants, currant bushes, peach trees, pear trees, privet, carnations.