Three new legume diseases in The Netherlands are described: Wisteria vein mosaic, pea necrosis, and pea leafroll mosaic. In particle size and morphology and in host reaction the virus isolates resembled bean yellow mosaic virus (BYMV), but they were readily distinguishable in several test plants.In recent years several new legume viruses related to BYMV and bean common mosaic virus have been described. Besides, more and more viruses of the potato virus Y group are proving to be naturally infectious to legumes, e.g. lettuce mosaic virus, beet mosaic virus, watermelon mosaic virus, and even turnip mosaic virus, all of which are somehow related to BYMV. To investigate the nature and degree of these relationships, the virus isolates causing the three new legume diseases were compared with a normal strain of BYMV and with pea mosaic virus, clover yellow vein virus, cowpea aphid. borne mosaic virus, two isolates of beet mosaic virus, and lettuce mosaic virus.They were all found to have several hosts and symptoms in common. The differences observed showed a range of gradations only. Unexpectedly, BYMV was found to infect 17 out of 20 non-legumes tested. The Wisteria isolate and lettuce mosaic virus did not produce inclusion bodies, whereas all others did. Often nueleoli were very much enlarged or contained crystals. The pea necrosis isolate produced many nucleolar crystalline needles.Cross-protection tests were of little help in determining mutual relationships.Antisera prepared against the Wisteria isolate, the pea necrosis isolate, and BYMV, and an antiserum against bean common mosaic virus, revealed definite relationships, but also substantial differences. By using the electron microscope the three new isolates were indistinguishable from BYMV, whereas the particle lengths of two isolates of beet mosaic virus were considerably shorter. The isolate of pea mosaic virus had much longer (840 m~x) and more rigid particles.

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Beijerinck's entirely new concept, launched in 1898, of a filterable contagium vivum fluidum which multiplied in close association with the host's metabolism and was distributed in phloem vessels together with plant nutrients, did not match the then prevailing bacteriological germ theory. At the time, tools and concepts to handle such a new kind of agent (the viruses) were non-existent. Beijerinck's novel idea, therefore, did not revolutionize biological science or immediately alter human understanding of contagious diseases. That is how bacteriological dogma persisted, as voiced by Loeffler and Frosch when showing the filterability of an animal virus (1898), and especially by Ivanovsky who had already in 1892 detected filterability of the agent of tobacco mosaic but kept looking for a microbe and finally (1903) claimed its multiplication in an artificial medium. The dogma was also strongly advocated by Roux in 1903 when writing the first review on viruses, which he named 'so-called "invisible" microbes', unwittingly including the agent of bovine pleuropneumonia, only much later proved to be caused by a mycoplasma. In 1904, Baur was the first to advocate strongly the chemical view of viruses. But uncertainty about the true nature of viruses, with their similarities to enzymes and genes, continued until the 1930s when at long last tobacco mosaic virus particles were isolated as an enzyme-like protein (1935), soon to be better characterized as a nucleoprotein (1937). Physicochemical virus studies were a key element in triggering molecular biology which was to provide further means to reveal the true nature of viruses 'at the threshold of life'. Beijerinck's 1898 vision was not appreciated or verified during his lifetime. But Beijerinck already had a clear notion of the mechanism behind the phenomena he observed. Developments in virology and molecular biology since 1935 indicate how close Beijerinck (and even Mayer, Beijerinck's predecessor in research on tobacco mosaic) had been to the mark. The history of research on tobacco mosaic and the commitments of Mayer, Beijerinck and others demonstrate that progress in science is not only a matter of mere technology but of philosophy as well. Raemaekers' Mayer cartoon, inspired by Beijerinck, artistically represents the crucial question about the reliability of our images of reality, and about the scope of our technological interference with nature.

Twelve virus isolates from pea, broad bean, red clover and yellow lupin have been compared with the B25 strain of bean yellow mosaic virus (BYMV-B25), the E198 strain of pea mosaic virus (PMV-E198) and the pea necrosis virus (El 78), which were described earlier (Bos, 1970).On the basis of host ranges, symptoms and bean and pea varietal reactions most isolates could be classified into three groups, representatives of which did not differ appreciably serologically. These groups were considered to be typical bean yellow mosaic virus isolates (E212, L1, B25), pea yellow mosaic strain isolates of BYMV (E198, E204, Kow28) and pea necrosis strain isolates of BYMV (E197, E199, E221). From these results and from a survey of literature it is concluded that PMV is only a strain of BYMV.The pea necrosis virus (EI78), described earlier as a distinct entity, is still considered a different virus. A severe pea necrosis isolate (Kow14) resembled E178 in many respects and was also more distantly related serologically to the BYMV isolates tested. Four other virus isolates from pea and broad bean (E196, Vfl 5, Vf18 and Vf30) could not yet be identified. Lettuce mosaic virus (LMV) was found to be serologically rather closely related to BYMV.Results of cross-protection tests were erratic, and particle length measurements were no help in differentiating the strains and viruses studied.

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In plant pathology , terminological confusion still reigns despite national attempts at standardization. Terminological agreements reached within the crop protection community in The Netherlands are elaborated here and presented as an endeavor toward international consensus. Much of the on-going terminological disconcert derives from differences in outlook between academically oriented biologists (including biologically trained pathologists) and pathologists working in and for agricultural institutions where disease and harm have anthropocentric connotations. The name crop protection science more realistically covers and marks the field dealt with by most plant pathologists, and adoption of the FAO-defined term pest to encompass all biotic factors that are harmful to plants and their products is advocated. The effect of pests on plants and the interrelationships between pests and plants in dependence upon the environment, topical in resistance breeding, are especially dealt with. A diagrammatic model is used to better describe these relationships and to define the terms that denote the phenomena and mechanisms involved.