Pathogenic oomycete (species of Phytophthora) belong to some of the most dangerous pathogens of plants in the world. Exist in forestry, but also in agriculture and horticulture. They come mainly from Asia, and therefore must be treated as invasive foreign organisms. In forestry, as new pathogens, they cause the dieback processes of many species of forest trees in nurseries, on crops and in the stands. Phytophthora species found so far on the oaks, beeches, alder trees, and lately on the ashes. This last species, due to the bad state of health, has been periodically excluded from the sylviculture. Phytophthora species also occur on coniferous trees, in Poland on spruce and fir, but in the UK there has been a mass occurrence on larch. At the moment, a few million trees die down on several thousands of hectares. The threat is serious for other European countries. Probably, the species of Phytophthora give off characteristic odors (especially in artificial culture on Petri dishes). The possibility of the application of the method of the electronic nose to detect them will be particularly useful in container nurseries, greenhouse crops and on border control, whether coming to Polish or other European countries, to see if the plant material is free of pathogens. Early detection of pathogens in nurseries is of crucial economic importance; when they get out on crops, their eradication is very difficult. It is estimated that in Poland, as in Europe, produced seedlings are infected by pathogens of fine roots in a high degree (60 to 80%). Taking into account the annual production (approx. 500 thousand cuttings only in Poland) the problem is growing and in the range of Europe concerns millions of seedlings per year.