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2 Introduction
to Classical
Crossprotection
Ron S. S. Fraser 1. History In the first three decades of the 20th century, it was shown that a number of plant diseases could be transmitted by mfectious sap that had been passed through a bacteria-proof filter Plant virus parttcles had yet to be identified and charactertzed, and much of the research effort of plant virologists went mto descrtbmg 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 Wingard (I) found that, m tobacco and other hosts infected with tobacco rmgspot, new growth appeared that dtd not show any signs of disease It was not possible to cause rmgspot symptoms on these leaves by a further direct maculation Nevertheless, sap from these symptomless leaves caused rmgspot when maculated to healthy plants. McKmney (2) noted that tobacco plants infected wtth a light green mosaic (now known to be tobacco mosaic virus) did not develop further symptoms when maculated with a yellow mosaic form In contrast, plants infected with a mild, dark green mosaic form did develop yellow symptoms when remoculated with the yellow form. Further work on such mteractions between viruses was facilitated by the developing ability of plant virologists to discrimmate between different vu-uses, or isolates of the same virus, using differential hosts and the emerging techmques of serology and virus particle characterization. It was recognized early that interference occurred prtmarily between closely related vn-uses, and the term “crossprotection” was applied to Indicate this relatedness Indeed, crossprotection was used as one diagnostic test for relatedness between virus isolates (3,4). However, more modern approaches using nucleic acid sequencFrom Edlted<