In the first three decades of the 20th century, it was shown that a number of plant diseases could be transmitted by infectious sap that had been passed through a bacteria-proof filter. Plant virus particles had yet to be identified and characterized, and much of the research effort of plant virologists went into describing disease symptoms and studying methods of transmission. In the late 1920s, it became apparent that, when plants were deliberately inoculated with two agents causing different types of visible symptom, there could be a form of interference.