As part of the EU emergency measures against the pine wood nematode (PWN) (Bursaphelenchus xylophilus) laid down under Decision 2012/535/EU, Portugal should perform, outside and during the flight season of the vector, surveys of coniferous trees located in the 20 km wide buffer zone established along the Spanish border, with the aim to detect trees which are dead, in poor health or affected by fire or storm. These trees shall be felled and removed, as required by the Decision, to avoid that they act as attractants for the longhorn beetle (Monochamus species), the vector responsible for the spread of PWN. The CanHeMon project tasked the Joint Research Centre with analysing a portion of the buffer zone, using remote sensing data, to support detection on the ground of declining pine trees. During the project, a 400 km2 area was imaged twice, in autumn 2015 and autumn 2016, at 15 cm resolution from aircraft, and individual declining tree crowns were detected using a MaxEnt-based iterative image analysis algorithm, the performance of which was gauged through visual photointerpretation. The scalability of the automated methods was then tested using an image mosaic of the entire buffer zone at 30 cm resolution. Finally, broad recommendations were formulated on the use of remote sensing for large-area surveys in the context of plant health emergencies.