The present invention relates to a device for determining or studying the state of stimulation of the natural defences of plants or portions of plants, said plants advantageously belonging to the Rosaceae family. For this purpose, the corresponding device includes a means for determining, in a sample of plants or portions of plants, the level of expression of at least one target gene in each of the following groups (a) to (i): (a) PR-1, PR- 2, PR-4, PR-5, PR-8, PR-14, PR-15; (b) PAL, CHS, DFR, ANS, PPO; (c) HMGR, FPPS, Far; (d) CSL; (e) APOX, GST, POX; (f) CalS, Pect, CAD; (g) EDS1, WRKY; (h) LOX2, JAR; and (i) ACCO, EIN3. Said device preferably consists of a kit which contains a determination means in the form of pairs of primers for implementing a quantitative PCR technique
