There is a category of palm diseases that raises many investigation problems: lethal diseases caused by phloem-restricted parasites and transmitted by vector insects. The difficulties encountered lie in the impossibility - or at least great difficulty - of growing the pathogen in vitro, and difficulties in identifying the vector. These obstacles to aetiological definition and an understanding of the disease cycle hold back the identification of resistant species or varieties. Since the 1970s, CIRAD has endeavoured to solve some enigmas surrounding these diseases classed "of unknown origin" on two major oil crop species for tropical countries: coconut (Cocos nucifera L.) and oil palm (Elaeis guineensis Jacq.). In the 1970s, use of the electron microscope for aetiological research on diseases of unknown origin became the norm and it resulted in numerous advances. It led to the discovery of the specific association of MLOs (Mycoplasma-Like-Organisms) and lethal yellowing like syndromes of coconut (CLY). MLOs are small wall-less bacteria. They are only located in the phloem (in the sap). The electron microscope also led to the discovery of flagellate protozoa - Trypanosomatidae - associated with wilt diseases in coconut ("Hartrot") and oil palm ("Marchitez sorpresiva") in Latin America. The Hartrot syndrome is similar to the CLY syndrome and the trypanosomes responsible for it are located, like MLOs, in the sap. This decisive aetiological advance was not followed by immediate effects as these two types of microorganisms could not be grown in vitro. That is still the case for all MLOs, known today as "phytoplasmas". But the rapid progress made in molecular biology in the 1990s revealed that there did not exist just one CLY, but several CLYs caused by different phytoplasmas. However, at CIRAD, we have succeeded in culturing trypanosomatids associated with Hartrot or Marchitez. That success has led to major scientific advances on these microorganisms. Vector identification